TABLE OF CONTENTS

Survey of Sensym Pressure Transducers

The Smith-Kettlewell Auditory Barometer

Assorted Smith-Kettlewell Vacuum- and Pressure-Gauge Circuits

The Jameco XY168 Solder Station

SURVEY OF SENSYM PRESSURE TRANSDUCERS

The editor came across these kinds of sensors about thirteen years ago when Smith-Kettlewell was called upon to adapt the gauging equipment of a blind mechanic at United Airlines. A circuit very similar to the barometer in this issue solved his problem. Since then, Sensym took over the pressure sensor business from National Semiconductor (who used to make these units). Over the years, Sensym has managed to bring the cost of these transducers way down; now you can make an accurate audible tire gauge for your bicycle for a reasonable amount of money.

The Sensym catalog contains many more units than are listed here. For example, there is a $60 class of precision sensors whose main disadvantage is that a minimum order of ten identical units is required. There is another set of sensors with "signal conditioning" (driving and detection circuitry) built into the package; these cost over $100 (although they would save you some building). The devices listed here are the basic "bridges" for which you must build the signal-conditioning circuits.

The cheapest of the lot are those in the "SX" series; these cost about $14. The next class presented here, the "SCX" sensors, are said to be "temperature-compensated"; costing about $26, their specifications show tighter tolerances. Both types can be gotten in two configurations: "absolute" pressure sensors measure pressure with reference to a vacuum (p.s.i.a.); "differential" or "gauge-pressure" sensors can either compare pressures or measure one value against atmospheric pressure (p.s.i.g.).

The heart of these are Wheatstone bridges of strain-gauge elements embedded in a silicone diaphragm. Sensym calls these strain-gauges "Piezo-resistive ceramic elements." I never heard of Piezo-resistive materials before, but those brilliant Italian physicists messed with just about every kind of thing, so why not.

Sensym will sell you a board containing the signal conditioning circuit if you like. It has various jumpers that allow you to correct for different polarity offsets and other fancy stuff. However, if you're building a specific instrument, you won't need some of that flexibility, so I chose not to describe their board here.

They used a rather nifty high-impedance differential amplifier--that I've never seen before--for detecting the output of the bridge. (Straightforward differential amplifiers present the source with significant impedances, and they are asymmetrical at that.) Sensym recommends a particular quad op-amp made by Linear Technologies--the LT1014. With this, you can get some nice high gains, and the input currents of the individual op-amps are very low. If you want precision, you should probably get that LT1014; the LM324's I've been using make pretty drifty instruments.

Sample Differential-Amplifier Circuit

(Note: All resistors, unless otherwise stated, are 1% units. All pots should be wire-wound for best temperature stability.) A single 15V supply is used in this example; its negative terminal is grounded. Besides the 15 volts, a stable "standard voltage" is required. This is gotten from a Linear Technologies LT1004; it looks like a zener diode and provides 2.5V. The anode of this "zener" is grounded, while its cathode goes through 10K to plus 15V. (This zener's resistor is the only 5% unit in the bunch.)

A Linear Technologies LT1014 has its pin 11 grounded, while pin 4 goes to plus 15V. One op-amp supplies the sensor with 10 volts. Thus, the non-inverting input of this goes to 2.5V. The inverting input goes through 10K to ground, and there is a 30.1K feedback resistor between output and inverting input. The output goes to the "Plus Supply" pin of the sensor, while the sensor's "ground" pin is grounded.

To compensate for "offsets" in the various sensors, another op-amp is used to get a low-impedance offset voltage. Across the zener is a pot; often, this is a 10K pot whose bottom goes through 10K to ground and whose top end goes through 10K to 2.5V. The arm of this pot goes to the non-inverting input of the op-amp. This op-amp has a gain of 2; its inverting input goes through 10K to ground, and there is a 10K feedback resistor between output and inverting input.

The differential amplifier starts by running the inverting input of A1 through 10K to the output of the above offset circuit. There is a 10K feedback resistor between the output and inverting input of A1. A1's output also goes through 10K to the inverting input of A2. Between the inverting input and output of A2 is a 10K feedback resistor; this is often shunted by a 0.1uF Mylar capacitor.

There is a more global feedback resistor which determines the gain of the differential amplifier. This is a series combination of a fixed resistor and a rheostat, and it goes between the inverting inputs of A1 and A2. The output of the system is the output of A2. The differential inputs are the non-inverting inputs; the non-inverting input of A2 is the "plus input," while that of A1 is the "minus input."

Gain Considerations

First, you decide what "span" (range) you want to cover; then, you look at the table of sensors and find out the "sensitivity" (millivolts per p.s.i.), and calculate the resultant span in millivolts that the differential amplifier will see. Verry importante, however, is to note that the figures in the table are given with a 12-volt supply on the sensors. The sample circuit given here supplies the sensor with 10 volts, so you have to multiply that sensitivity figure by 10/12. In the gauges of the next two articles, we used a 5V sensor supply, so the sensitivity figure must be multiplied by 5/12.

Next, you decide what range of output voltages you want to represent the pressures you have chosen. Dividing this figure by the number of millivolts you expect to see at the gauge head will give you a figure for the desired gain.

The gain of the differential amplifier is given by the expression:

Differential Gain equals 2 times the quantity of 1 plus the ratio of R over RF,

where R is the value of the four identical resistors (10K in this example) and RF is the adjustable feedback resistor. Typical practical values might range from below 100 ohms to 200 ohms for many applications where a voltmeter is used as a readout. As far as correcting for offset is concerned, the gain of the system is 1.

Non-inverting amplifiers which get their input signals from the 2.5V standard are used to power the sensor and to create an offset voltage. The expression for their gain is:

Non-Inverting Amplifier Gain equals 1 plus the ratio of R-Feedback over R-In.

Package Descriptions and Pin Assignments

Remember that the "N" suffix means that you get your sensor in a housing with nipples. If you forget to tack on the N when you order, you'll get a little round module with a hole in each face and with pins coming out the edge. The housing cost less than a dollar, in most cases. Naturally, the kinds of nipples provided will depend on the pressures for which the gauge is designed. Our first-hand experience is with the small units which only need smooth "slip-on" fittings; the ones on our SX and SCX sensors are 3/16th of an inch with tapered ends.

The housings for the SX units and the SCX ones are very different. They are described as follows:

The SCX----N (temperature-compensated) sensors are a squarish block with the two nipples on one face. Mounting ears are at opposite sides, and six pins come out the third side. The port nearest the pins is "Port A," with the other being "Port B." On the absolute-pressure units, Port B has been sealed in a vacuum, so only Port A is active.

Of the six pins, four of them go to the four corners on a Wheatstone bridge. However, the supply points on the bridge are not powered directly; rather, two additional pins (plus Supply and Ground) each go through temperature compensation resistors to their respective points on the bridge. Thus, the "Plus Temp Out" pin is the actual top of the bridge, while it goes through a resistor to the "Plus Supply" pin. The input resistance is said to be 4K, and the output impedance is also said to be 4K. This means that you should detect the bridge outputs with a circuit having an input impedance of 100K or more.

The SX----N (uncompensated) sensors are octagons which have mounting ears on opposite short sides, and four pins coming out another side. Stated another way, the rounded "ends" with mounting ears give these a "footprint" which is something like a TO3 power transistor--sort of. Anyhow, both nipples come out of the edge which is opposite the pins. With the printed side up (which you cannot tell by feel), "Port 1" is the upper nipple and "Port 2" is the lower. (The printing on the top face is white; to an active light probe, it looks fairly irregular and quite bright.)

These packages have four pins; they connect to four corners of a Wheatstone bridge. (It is said that the input resistance is 4.5K and the output impedance is 4.6K. This means, detect them with some circuit which has an input impedance of 100K or more.)

Pin Assignments for the SX Series

With the printed side up and the pins facing you, the pins are 1 through 4 from left to right.

• Pin 1--Ground (the negative supply point of the bridge)
• Pin 2--Plus Output (increases with positive pressure on Port 1)
• Pin 3--Plus V (the positive supply point of the bridge)
• Pin 4--Minus Output (decreases with positive pressure on Port 1)
• Port 1 is closer to the printed face, on the corner just opposite pin 4. Port 2 is lower and is on the corner just opposite pin 1.

Pin Assignments for the SCX Series

[Editor's Note: The literature is very confused as to which pin is called "Plus Output" and which port to use. The Plus Output they define for the differential units increases with positive pressure on Port B; then they turn around and say in the pin diagram that the polarity is reversed for the absolute pressure units because Port B is not used. This editor, who is a low-pressure kind of guy, will restate the pins so that pressure on Port A increases the voltage on a renamed pin "Port A Plus Out." Now, don't forget that when using an SCX15A to indicate vacuum, the "Port A Plus Out" pin will rest positive with respect to its counterpart, "Port A Minus Out," and the voltages on those two pins will approach each other as a vacuum is pulled on Port A.]

With the nipples pointing down and the pins toward you, they are numbered 1 through 6 from left to right.

• Pin 1--Temp Out Plus (not usually used by outside circuitry)
• Pin 2--Plus Supply
• Pin 3--Port A Minus Out (decreases with positive pressure on Port A)
• Pin 4--Ground
• Pin 5--Port A Plus Out
• Pin 6--Minus Temp Out (not usually used by outside circuitry) Port A is the nipple nearest the pins; Port B is farthest.

List of Sensors

There are two kinds of bridge arrangements listed here:

The SX series gauges are cheap--$13 to $17. They present a 4.5K resistance to the supply, and have an output impedance of about 4.6K.

The SCX series gauges (typically $26) are "temperature-compensated"; they accomplish this by putting resistors (with a particular temperature coefficient) in series with the supply points. (The actual "top and bottom" of the bridge are made available at pins. Apparently, you can build fancy circuitry to monitor the gauge temperature and correct for the effects of thermodynamics.) On these, both the supply resistance and output impedance are 4K. (With some network analysis, one could calculate the individual resistance values; however, this information would be of little use, since all you want to do is assure that you detect the bridge with as light a load as possible.)

Important! The figures here (the "Sensitivity" in millivolts per p.s.i. and "Offset" voltages) are given with a 12V supply to the sensors. These must be scaled down for any other supply. The sample circuit in this article supplies them with 10V, and the gauges in the succeeding articles supply them with 5V. Don't forget to correct these figures accordingly, or your choice of gains will be too low.

Note: The suffix "N" means that you want the sensor in a housing. This housing costs under $1, so who would want otherwise? Obviously, units with the "D" suffix are "differential" (they can compare pressures on two ports; these are the units one would use to measure "gauge pressure" (a pressure on one port being compared with atmospheric pressure on the other). Where possible, units with the suffix "C" are listed here; this means "commercial grade," and the specifications are slightly more lenient than those without (the non-C ones are not even listed here, as they are about $10 more expensive).

Absolute Pressure Sensors

• SX15AN--15 p.s.i.a. (over pressure of 30 p.s.i.a.)
• Sensitivity--18mV per p.s.i.a.; 270mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over Temperature range (0 to 50 Dg.C.--1.2mV
• Sensitivity Shift over Temp. Range (0 to 50 Dg.C.)--5% of full scale
• SX30AN--30 p.s.i.a. (60 p.s.i.a. over pressure)
• Sensitivity--9mV per p.s.i.a.; 270mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX100AN--100 p.s.i.a. (150 p.s.i.a. over pressure)
• Sensitivity--3.6mV per p.s.i.a.; 360mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.1% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX150AN--150 p.s.i.a. (200 p.s.i.a. over pressure)
• Sensitivity--1.8mV per p.s.i.a.; 270mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.1% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SPX100AN--15 p.s.i.a. (30 p.s.i.a. over pressure)
• Sensitivity--4mV per p.s.i.a.; 60mV full scale
• Offset Calibration--0 to 35 millivolts
• Linearity--0.1% of full scale
• Repeatability and Hysteresis--0.5% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--plus/minus 0.75mV Sensitivity Shift over temp. range (0 to 50 Dg.C.)--1.2%
• SPX200AN--30 p.s.i.a. (60 p.s.i.a. over pressure)
• Sensitivity--2mV per p.s.i.a.; 60mV full scale
• Offset Calibration--0 to 35 millivolts
• Linearity--0.1% of full scale
• Repeatability and Hysteresis--0.5% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--plus/minus 0.75mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--1.2%

The following are temperature-compensated; they have compensation resistors in series with the supply lines to the bridge.

• SCX15ANC--15 p.s.i.a. (30 p.s.i.a. over pressure)
• Sensitivity--6mV per p.s.i.a.; 90mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX30ANC--30 p.s.i.a. (60 p.s.i.a. over pressure)
• Sensitivity--3mV per p.s.i.a.; 90mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX100ANC--100 p.s.i.a. (150 p.s.i.a. over pressure)
• Sensitivity--1mV per p.s.i.a.; 100mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• Gauge Pressure and Differential Sensors
• SPX50DN--7 p.s.i.d. (15 p.s.i.d. over pressure)
• Sensitivity--8.6mV per p.s.i.d.; 60mV full scale
• Offset Calibration--0 to 35 millivolts
• Linearity--0.1% of full scale
• Repeatability and Hysteresis--0.5% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.75mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--1.2% of full scale
• SPX100DN--15 p.s.i.d. (30 p.s.i.d. over pressure)
• Sensitivity--4mV per p.s.i.d.; 60mV full scale
• Offset Calibration--0 to 35 millivolts
• Linearity--0.1% of full scale
• Repeatability and Hysteresis--0.5% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.75mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--1.2% of full scale
• SPX200DN--30 p.s.i.d. (60 p.s.i.d. over pressure)
• Sensitivity--2mV per p.s.i.d.; 60mV full scale
• Offset Calibration--0 to 35 millivolts
• Linearity--0.1% of full scale
• Repeatability and Hysteresis--0.5% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.75mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--1.2% of full scale
• SX01DN--1 p.s.i.d. (20 p.s.i.d. OVER PRESSURE)
• Sensitivity--48mV per p.s.i.d.; (48mV full scale)
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX05DN--5 p.s.i.d. (20 p.s.i.d. over pressure)
• Sensitivity--36mV per p.s.i.d.; 180mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX15DN--15 p.s.i.d. (30 p.s.i.d. over pressure)
• Sensitivity--18mV per p.s.i.d.; 270mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX30DN--30 p.s.i.d. (60 p.s.i.d. over pressure)
• Sensitivity--9mV per p.s.i.d.; 270mV full scale
• Offset Calibration--minus 80 to 0millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX100DN--100 p.s.i.d. (150 p.s.i.d. over pressure)
• Sensitivity--3.6mV per p.s.i.d.; 360mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale
• SX150DN--150 p.s.i.d. (200 p.s.i.d. over pressure)
• Sensitivity--1.8mV per p.s.i.d.; 270mV full scale
• Offset Calibration--minus 80 to 0 millivolts
• Linearity--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--1.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--5% of full scale

The following are temperature-compensated; they have compensation resistors in series with the supply lines to the bridge.

• SCX01DNC--1 p.s.i.d. (20 p.s.i.d. over pressure)
• Sensitivity--18mV per p.s.i.d.; 18mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX05DNC--5 p.s.i.d. (20 p.s.i.d. over pressure)
• Sensitivity--12mV per p.s.i.d.; 60mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX15DNC--15 p.s.i.d. (30 p.s.i.d. over pressure)
• Sensitivity--6mV per p.s.i.d.; 90mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX30DNC--30 p.s.i.d. (60 p.s.i.d. over pressure
• Sensitivity--3mV per p.s.i.d.; 90mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX100DNC--100 p.s.i.d. (150 p.s.i.d. over pressure)
• Sensitivity--1mV per p.s.i.d.; 100mV full scale
• Offset Calibration--plus/minus 1 millivolt
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• SCX315DNC--3 to 15 p.s.i.d. (30 p.s.i.d. over pressure)
• Sensitivity--6mV per p.s.i.d.; 72mV full scale
• Offset Calibration--0.3 millivolts Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale
• Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.4% of full scale
• BP01 (blood-pressure gauge)--0 to 300 millimeters of mercury (1500 mm over pressure)
• Sensitivity--2.6mV per p.s.i.g.; 15mV full scale
• Offset Calibration--plus/minus 0.3 millivolts
• Linearity--0.2% of full scale
• Repeatability and Hysteresis--0.2% of full scale Offset over temp. range (0 to 50 Dg.C.)--0.2mV
• Sensitivity Shift over temp. range (0 to 50 Dg.C.)--0.2% of full scale

Some Pressure-Unit Relationships

1 Pascal equals 1 Newton per square meter.

1 bar equals: a million Dynes per square centimeter, 100,000 Newtons per square meter.

1 atmosphere equals: 1013 millibars, 14.696 p.s.i.a., 760 millimeters of mercury, 406.8 inches of watter.

1 inch of mercury equals 13.6 inches of water.

1 pound per square inch (p.s.i.) equals: 2.036 inches of mercury, 27.68 inches of water.

Address List

[Note: Player Piano Co. has a lot of tubing and fittings ("Y adapters," reducers, and elbows). Their two smallest sizes fit the tapered press-on fittings; the so-called "tracker-bar tubing" is 5/32 inch, and "expression tubing" is 3/16 inch.]

Player Piano Company: 704 E. Douglas, Wichita, KS 67202; Phone: (316) 263-3241.

Sensym: 1255 Reamwood Ave., Sunnyvale, CA 94089; Phone: (408) 744-1500.

THE SMITH-KETTLEWELL AUDITORY BAROMETER

Abstract

This project uses the Sensym SCX15ANC absolute pressure sensor to create an instrument which indicates atmospheric pressure. Coupled with Tom Fowle's fine thermometer circuit (first in the SKTF Winter 1986 issue, then embellished in Fall 1986), the amateur meteorologist will be on his way. (Humidity detection with wet-and-dry thermometers will be something to try next; there's no reason why that cannot be taken up in the future.)

The origin of this barometer was as a part of a Smith-Kettlewell equipment demonstration. We needed something to show off the capabilities of our "Flexi-Meter." A brainchild of Tom Fowle, the "Flexi-Meter" is a dedicated computer in a box which can be "taught" (by way of firmware) to be any ol' metering device for the blind or deaf-blind. As outputs, it can send you tones, talk, send Morse Code to a vibrating shoehorn, run a Braille display, or operate a printer. (Our thanks to a deaf-blind engineer, Ray Boduch, for showing us the shoehorn trick.) The Flexi-Meter's input systems are equally as flexible. The way we chose to demonstrate the marvelous Flexi-Meter was to set up a talking and Morse Code weather station--displaying temperature, barometric pressure, wind speed and wind direction--all presentable in its various modes.

Besides programming the computer inside the Flexi-Meter, so-called "signal conditioning" (driving circuitry) for its various input sensors is always necessary. That kind of engineering puts old people to work (those who haven't built our careers around computers), and the editor offers this simplified barometer circuit as fruits of his labors.

The instrument is built around a Sensym SCX15ANC. (The "A" suffix designates that this sensor measures "absolute pressure"; i.e., one of its ports has been sealed in a vacuum. The "N" designates that the sensor is encased in a housing. "C" reflects that this buyer was too cheap to pay the extra ten bucks to get the high-quality noncommercial-grade sensor.)

The output scheme chosen here is of the most basic type, a "null-type" arrangement wherein the user adjusts a Braille-calibrated dial for a null in an audible tone and then reads the dial. That part of the circuit will look familiar; it uses the H11F3 opto-isolator as a chopper, an LM386 as an audio amplifier, and the gains are set up so that half a volt appears across the Braille-calibrated standard.

A talking version could be built using the TDI voltmeter chip and the Ram-Talker. Another version would be one which, using the peak-storage circuit that embellishes the thermometer (see SKTF, Fall 1986), would store the pressure maxima and minima over time.

It was decided that the Braille scale would cover a range of from 28 to 32 inches of mercury, with 30 inches being at midscale. Three-dot markings were placed at each "inch of mercury"; five double-dot marks were placed in between these at 0.2 inch increments. Just for the flare of it, the top edge of the dial sports the words "foul" (on the low side of 30 inches), and "fair" on the high side.

The circuit requires a split supply--plus and minus 9V. However, the negative supply line only draws a couple of milliamps (just enough to supply the quad op-amp chip). Therefore, when the plus 9V battery goes dead, just trading their positions will bring the unit back to life.

Construction

My unit is built into a 5- by 7-inch box with only two controls, an on-off switch and the Braille dial. A 1.8-inch wide piece of perforated board accommodates the circuitry. (My board was too short, and caused me anxiety; make yours 5.5 or 6 inches long.)

The sensor was bolted to the component side of the board. It has two tubing nipples on its front face; matching holes in the board allow these to pass through to the wiring side. (Only one of these nipples is used; the unused one has been sealed off in a vacuum at the factory.) If you wish, the active one of these nipples can be made accessible from the outside of the box. The seal in the unused nipple--as found with a Braille stylus--turns out to be just where the nipple narrows down (perhaps 1/4 of an inch in front of the sensor housing). With a coping saw, I cut off its extra length. Thus, with my board mounted close to the front apron of the cabinet, the active nipple protrudes through a hole I made for it.

On one edge, the sensor has pins at 1/10th inch spacing; when bent at right angles (in the direction of the nipples), these can be plugged into an IC socket on the component side of the board. (The socket I used was 296 thousandths of an inch high; the pins won't reach the contacts of a socket which is much lower than this.) The sensor housing is fastened to the board with two 6-32 bolts and nuts. (This takes some careful measuring. One way of doing this is to first make elongated holes for the nipples. Then, with a trial placement of the socket, plug the sensor in and bend it over into position; a couple of trials, along with possible enlargement of the nipples' holes, will find the right position for the socket. Once the sensor is flat against the board, holes for the bolts can be drilled using the mounting fins as a guide.)

A rough statement as to layout--viewed from the component side--is as follows: The sensor is at the left end of the board with its pins facing to the right. Above and below it, along the edges of the board, are mounted the "scaling" and "offset" trim pots, respectively. On the edge toward you, just to the right of the offset trimmer, is the LM336 voltage standard. Underneath, a ground bus runs along the near edge, a plus 9V bus runs along the far edge, and a short minus 9V bus is spaced a couple of holes down from the positive one. To the right of the sensor socket is the quad op-amp--pins 1 through 7 facing the sensor. (leave perhaps 0.7 inches between those sockets; there's a lot to be done around that op-amp chip.) To the right of the op-amp is a 14-pin socket that contains both the 555 and the H11F3 chopper. The audio amplifier (LM386) is at the right end.

The device is powered by two 9V batteries. Three additional voltages find application in the circuit: A 2.49V temperature-stable source is established as a main standard. A 5V supply for the bridge of the sensor is created by an op-amp looking at the 2.49V standard. Because the "absolute" sensor has an atmospheric "bias" on it, an adjustable offset voltage is needed to bring the output of the differential amplifier down to a convenient operating point. This adjustable source consists of a section in the 324 being set up as a non-inverting amplifier (having a gain of 2), with the input of this amp looking at a pot across the 2.49V standard.

Barometer Circuit

A DPST on-off switch is used with the dual 9-volt supply. The positive side of one 9V battery goes to ground, while its negative terminal goes through one pole on this switch to the minus 9V line of the circuit. The negative side of the other 9V battery is grounded, while its positive terminal goes through the other pole on the switch to the plus 9V line.

An LM336 2.49V reference diode has its "anode" grounded; its "cathode" goes through 2.2K to the plus 9V line. (No trimmer on the 336's "adjust" pin was added.) The junction of the cathode and the resistor is the 2.49V line.

Pin 11 of an LM324 (quad op-amp) goes to minus 9V; pin 4 of this 324 goes to plus 9V. An op-amp in the 324 is used to supply the SCX15ANC gauge. Pin 10 of the 324 goes to 2.49V. Pin 9 goes through a 10K 1% resistor to ground, with a feedback resistor of 10K (1%) going between pins 8 and 9. Being non-inverting, the gain is 2 (1 plus the ratio of the two resistors, which is also 1); pin 8 gives us 5V which then goes to pin 2 of the sensor.

Pins 12, 13, and 14 of the 324 are used to create an adjustable offset voltage. Pin 13, the inverting input, goes through a 10K 1% resistor to ground. Between pins 13 and 14 is a 10K 1% feedback resistor. Pin 12 goes to the arm of a 50K multi-turn trim pot. The bottom of this pot is grounded, while its top end goes to 2.49V. (This trimmer is the "offset" adjustment.) The output of this stage is pin 14.

Two op-amps are arranged in a high-impedance differential amplifier. (All resistors in this section are 1% units.) Between pins 1 and 2 is a 10K feedback resistor. Pin 2 goes through 10K to the adjustable reference, pin 14. Pin 1 goes through 10K to pin 6, while a 10K feedback resistor goes between pins 6 and 7. Between the inverting inputs (pins 2 and 6) is 200 ohms (which I made by connecting two 100-ohm 1% units in series). (The output of this differential amp is pin 7; the inputs are the non-inverting inputs of the op-amps.)

Pin 3 of the LM324 goes to pin 5 of the SCX15ANC; pin 5 of the op-amp goes to pin 3 of the sensor. Pin 4 of the sensor is grounded, while its pin 2 goes to pin 8 of the 324 (the temperature-regulated 5V supply).

The 324's pin 7 (the output of the barometer section) goes to pin 4 of an H11F3 (or H11F1) opto-isolator. Pin 6 of the H11F3 goes through 47K to the arm of the Braille-calibrated 10K pot. The "top" (clockwise end) of this pot is grounded. The "bottom" (counterclockwise end) goes through a 10K 1% resistor, then through a 50K multi-turn rheostat to the 2.49V line. (This 50K trimmer is the "scale factor" adjustment.)

Pin 6 of the H11F3 also goes through 0.1uF to pin 3 of an LM386. Pins 2 and 4 of the LM386 are grounded. Between pins 2 and 3, located close to the chip, is a 0.01uF disc capacitor to suppress oscillations. Pin 6 of the 386 goes through 10 ohms (1/2-watt 5%) to the plus 9V line. Pin 6 is bypassed to ground by 220uF (negative of this cap at ground). Pin 7 of the 386 is bypassed to ground by 10uF (negative of this cap at ground).

Pin 5 of the 386 goes to the positive end of a 100uF electrolytic cap, with the negative end going through the speaker to ground. Between pins 5 and 4 (from output to ground) is a 0.1uF cap to suppress oscillations.

A 555 timer chip drives the H11F3 chopper. Pin 1 of the 555 is grounded. Pins 4 and 8 are tied together and go to the plus 9V line. Between pins 1 and 8, located close to the chip, is 0.1uF.

Pins 2 and 6 of the 555 are tied together and go through 0.01uF to ground. Pins 2 and 6 also go through 100K to pin 7, while pin 7 goes through another 100K resistor to plus 9V.

Pin 1 of the H11F3 goes to the plus 9V line. Pin 2 goes through 2.2K to pin 3 of the 555, which drives it.

Calibration

When the unit is first turned on, adjust the "offset pot" (the one whose arm goes to pin 12 of the op-amp chip) for a null in the tone. At this point, blowing and sucking on the active port of the sensor (the nipple nearest its pins) should audibly change the level of the tone. Rough calibration can be achieved by adjusting the "scale factor rheostat" (the trimmer in series with the Brailled pot) to get 0.5V across the calibrated pot. (You may wish to do this latter adjustment with the H11F3 unplugged; in this way, the tone won't drive you nuts, and there won't be any off-null chopper current to affect the reading.)

Correct scaling was done with a clever Alden invention, the "flexible manometer." This consists of a 4-foot piece of clear plastic tubing with perhaps 18 inches of water in it. With the two ends of the tubing held equal in height, connect one end to the nipple of the sensor. Raising (or lowering) the free end of the tube to get a measured difference in water level will give you a standard pressure (or partial vacuum, respectively), and this can be used to calibrate the instrument. A difference in height of 13.6 inches (34.5 centimeters) corresponds to 1 inch of mercury.

[Because the trapped air compresses and expands as you create this pressure or partial vacuum, you cannot just raise the tubing 34.5 centimeters and call this 1 inch of mercury; you must adjust the free end so that the tops of the water columns end up differing by this amount. Water in the closed end of the manometer may shift a couple of centimeters as you go from plus to minus 34.5cm of water. The water columns could be "seen" with my Smith-Kettlewell light probe, but visual help and a print meter stick is what we used.]

Set the "offset" so that a null occurs when the Braille dial is at an even number, say 30 inches of mercury. Try moving the tube up a foot and see what you get. (Whenever you change the "scaling," this will affect the offset, so before you make each measurement, you should re-adjust the offset so that you start from a mark on the Braille dial.) The final adjustment in scaling should be done over the longest distance your tubing permits, say from minus 34.5cm to plus 34.5cm between the tops of the water columns.)

Once scaling has been accomplished, you can phone your weather bureau and find out what the barometric pressure is at that moment ("corrected for elevation above sea level," as they say). Set the dial to the desired reading and adjust the offset trimmer for a null in the tone. Mark this offset pot well, so when you move from place to place, you won't re-adjust the wrong trimmer by mistake.

The manometer we used is so-called "medical-grade Silastic" (silicone) tubing made by the Dow Corning Company in Midland, Michigan. Bearing the number 601-335, it has an unstressed inside diameter of 0.132 inches. However, it is probably easier to buy small quantities of PVC tubing from Small Parts, Inc. They list a 1/8 inch i.d. "PVC Flexible Tubing" under catalog number CFT-24; you can buy as little as you like for $.35 a foot.

Parts List

Resistors, 1/4-watt 5%, unless otherwise specified:

• 1--10 ohms 1/2-watt
• 2--2.2K
• 1--47K
• 2--100K

Precision Resistors and Pots:

• 2--100 Ohms 1% (connected in series to get the feedback resistor of 200 ohms, although the exact value is not critical)
• 9--10K 1% resistors
• 2--50K multi-turn trim pots (wire-wound would be preferred, although we used garden-variety carbon ones)
• 1--10K panel-mount precision linear unit fitted with a pointer knob and Braille scale (Clarostat 58C1-10K or equivalent)

Capacitors:

• 2--0.01uF disc ceramic or Mylar
• 3--0.1uF disc ceramic or Mylar
• 1--1uF 10V electrolytic 1--100uF 10V electrolytic 1--220uF 10V electrolytic

Semiconductors:

• 1--LM336 2.49V zener
• 1--G.E. H11F3 or H11F1 opto-isolator (available from Digi-Key as part No. H11F1-G.E.)
• 1--555 timer chip
• 1--LM386 audio amplifier
• 1--Quad op-amp; a Linear Technologies LT1014 would be nice, although we used an LM324
• 1--Sensym SCX15ANC sensor

Miscellaneous:

• 1--DPST on-off switch
• 2--9V batteries with connectors
• 1--5- by 7-inch cabinet
• 1--4-foot length of clear 1/8th or 3/16th inch i.d. tubing (Such as Small Parts Cat. No. CFT-24)
• 1--Planet with gaseous atmosphere (Note: if a methane-oxygen atmosphere is to be used, extreme care should be taken when soldering or doing electrical work.)

Address List

Digi-Key Corp., P.O. Box 677, Thief River Falls, MN 56701, (800) 344-4539.

Sensym Company, 1255 Reamwood Avenue, Sunnyvale, CA 94089, (408) 744-1500.

Small Parts, Inc., 6901 N.E. Third Avenue, Miami, FL 33138, (305) 751-0856.

ASSORTED SMITH-KETTLEWELL VACUUM AND PRESSURE-GAUGE CIRCUITS

Abstract

Over the years, we have gotten two or three requests for vacuum gauges to be used in automotive work. The editor has a need for a very sensitive gauge to be used in player piano repair. Both these applications demand that the gauge's readout be "dynamic"; you want to know how a gauge is fluctuating, as well as being able to read it. Variations of these circuits--choosing different Sensym gauge heads--can be done to get you just about anything you need. For example, given the right "Y-connector" fitting and a 100 p.s.i.d. sensor, you could have a dynamic tire pressure gauge that would tell you how inflation was proceeding, and then tell you when you got to a preset value of air pressure. In general, this paper is a mini course on gauging, pressure or other physical quantities. The experimenter should ruminate over the section in this paper called "Other Gauging."

[The circuits in here use a dynamic meter reader--specifically, the "Fowle Gimmique" (SKTF, Summer 1982). Simpler gauges for "static readings" could be made by adapting our barometer circuit in like manner.]

The "Fowle Gimmique" meter reader is the cat's pajamas for dynamic measurement. In operation, any input signal causes an oscillator to produce a tone whose frequency is proportional to the input voltage (it is a voltage-controlled oscillator, or VCO). Beyond this, however, the "Gimmique" has a way of "chopping" the tone of the VCO when the input exceeds the setting of a Braille dial (pulsing the tone slowly enough so that you can still identify the VCO's pitch). Therefore, this readout gives you more than one version of the indication simultaneously; you can always hear relative changes (if any), and you either find out whether a desired value has been reached or you can adjust the Braille dial to measure the unknown value.

The Gimmique presented here is basically set up for a full-scale range of 1/2 volt (the same range as was arbitrarily chosen for the null-type meter reader in the barometer). The so-called "signal-conditioning" circuitry (driving and detection circuitry for the sensor) will be treated separately--a couple of gains and a couple of offset arrangements will serve as engineering notes for your own custom designs.

[Note: Our instruments use a dual 9V supply (though this may not always be necessary, given certain offset requirements). The Gimmique runs off the positive supply only. Since the negative supply has very little load on it, you can just trade the batteries around when the positive one dies.]

Because op-amps are pretty good about correcting for noise on their power supplies, no bypassing seems to be necessary, although bypassing at the far end of a 10-ohm resistor which supplies the meter reader has been done. (You'd think that noise from the meter reader would make its way onto the 2.49V point, but this LM336 seems to be quite noiseless, and by its zenerish little self, simulates effective decoupling.)

Power Supply and Voltage Standards

Two 9V batteries are used in conjunction with a DPDT on-off switch. The negative side of one battery is grounded; its positive terminal goes through one pole of the on-off switch to the plus 9V line. The positive side of the other battery is grounded; its negative terminal goes through the other pole of the on-off switch to the minus 9V line.

An LM336 is used to create a voltage standard of 2.49 volts. Its "anode" (left-hand lead) is grounded. Its "cathode" (center lead) goes through 2.2K to the plus 9V line; this "cathode" is the 2.49V point. The right-hand lead is for fine trimming of this voltage; it can be left open. (These lead orientations are given with the flat side of the package toward you and with the leads pointing up.)

One-Half Volt Fowle Gimmique Readout

The Braille calibration and the VCO frequency range have separate adjustments. The arrangement shown permits the Braille standard to cover the calibration range of from 0.36 to 1.24 volts. A rheostat which sets the pitch range of the VCO is adjusted to your liking; this has no effect on the Braille calibration.

Pin 7 of a 556 dual timer chip is grounded. Pin 14 goes through 10 ohms to the plus 9V line; pin 14 is bypassed to ground by 100uF (negative end at ground). The "Enable pins," pins 4 and 10, are tied together and go to pin 14.

Pin 5 goes through 47 ohms (a 1/2-watt unit) to one side of the speaker. The other side of the speaker goes to pin 14, the 556's plus supply.

Pins 2 and 6 are tied together and go through 0.0047uF to ground. Pins 2 and 6 also go through 10K to pin 1. Pins 2 and 6 also go through 22K to the collector of a 2N2907 PNP transistor (one of a pair to be described later).

Pins 8 and 12 of the 556 are tied together and go to the positive end of a 2.2uF capacitor, the negative end of which is grounded. Pins 8 and 12 also go through 10K to pin 13. Finally, pins 8 and 12 go through 22K to pin 7 of a dual op-amp (LM358 will work for many applications). Pin 4 of this op-amp is grounded, while pin 8 goes to plus 9V.

Two 2N2907's are employed; their emitters are tied together and go to pin 9 of the 556. As mentioned above, the collector of one already goes through a 22K resistor to pins 2 and 6 of the 556. The base of this transistor goes to both the base and collector of the other 2907 (this latter transistor being used for its base-emitter diode).

The collector of an NPN transistor (2N2222) goes to the connection of the PNP collector and both bases. The emitter of this 2222 goes through 2.2K then through a 50K rheostat to ground. This emitter also goes to pin 2 of the LM358 (the inverting input). The base of the 2222 goes to the 358 pin 1 (this op-amp's output).

Pin 6 goes to the arm of the 10K Braille-calibrated control. The bottom of this control is grounded. The top end goes through a 10K 1% resistor in series with a 50K rheostat to the 2.49V point.

The non-inverting inputs of both 358 op-amps, pins 3 and 5, are tied together; they go through 0.1uF to ground. These inputs also go through 220K to the output of the gauge circuitry to follow (pin 8 of a quad op-amp).

Gauge Circuitry

A quad op-amp is used; an LM324 will do for many applications. Pin 4 of this 14-pin package goes to plus 9V, while pin 11 goes to minus 9V.

Two of these op-amps are used as DC voltage supplies--one adjustable and one fixed. In both cases, they are in their non-inverting configuration; the gain is "1 plus the ratio of 'feedback' to 'input' resistance."

One op-amp, having a gain of 2, supplies the sensor with 4.98 volts (5V in practical sense). Pin 3, the non-inverting input, goes to the 2.49V point. The inverting input, pin 2, goes through a 10K 1% resistor to ground. As feedback, between pins 1 and 2, is another 10K 1% resistor. Pin 1 goes to the plus supply pin of the sensor (pin 3 of an SX15DN unit used as an automotive vacuum gauge, for example). The "Ground" pin of the sensor is grounded (pin 1 of the SX15DN).

Another op-amp supplies an adjustable reference. Pin 5, the non-inverting input, goes to the arm of a 50K pot; the bottom of this is grounded, while its top end goes to 2.49V. Pin 6, the inverting input, goes through a 10K 1% resistor to ground. For feedback, a 10K 1% resistor goes between pins 6 and 7.

The differential amplifier uses two op-amps. Pin 13, an inverting input, goes through a 10K 1% resistor to pin 1 (the output of the above adjustable reference). Between pins 13 and 14 is a 10K 1% feedback resistor. Pin 14, the output of the first op-amp, goes through a 10K 1% resistor to the inverting input of the other--pin 9. Between pins 8 and 9 is a 10K 1% feedback resistor. Finally, a gain-determining resistor (from 100 ohms on up) goes between the inverting inputs--between pins 9 and 13.

The non-inverting inputs, pins 10 and 12, go to their respective outputs of the gauge sensor; usually, pin 10 goes to the "Plus Output," and pin 12 goes to the "Minus Output." (On the automotive vacuum gauge, the LM324's pin 10 goes to pin 2 of the SX15DN, while the op-amp's pin 12 goes to the sensor's pin 4. This is with the idea of pulling a vacuum on Port 2, which is equivalent to applying pressure to Port 1.)

The occasion may arise where you want to have a negative offset voltage. This would be done by the following:

Negative Offset Amplifier

Pin 3 of the 324 is grounded. Pin 2 goes through 100K to the arm of the 50K pot across the 2.49V reference. There is a 200K (210.2K 1%, or something) feedback resistor between pins 1 and 2. As before, pin 1 goes to the 10K resistor off pin 13 of the differential amplifier.

Bi-directional Offset

If you really want flexibility (perhaps as a test bench version), build a gauge with this offset creation: Pin 3 of the 324 goes to 2.49V. Pin 2, the inverting input, goes through 100K to the arm of the 50K pot across the 2.49V supply. There is an adjustable feedback resistor between pins 1 and 2; you might make it the series combination of 30.1K and a 100K rheostat.

Important! In choosing gains, remember that the supply to the sensor here is 5 volts, whereas the Sensym figures were taken with 12 volts on the sensor. Thus, the sensitivity and offset figures in the data sheet must be scaled down accordingly. For example, when they tell you that the offset can be as large as minus 80 millivolts, you have to take five-twelfths of that to get a figure that would apply to this circuit--33 millivolts here. Instead of getting 18 millivolts per p.s.i. from the SX15DN, we get five-twelfths of that--7.5 mV per p.s.i.

For the automotive vacuum gauge, we wanted the full range of 15 p.s.i.g.; the sensor output--stated to be 270 millivolts with a 12-volt supply--will be 112.5 mV in this instrument. Since the Gimmique has a range of about 500 millivolts (which is quite adjustable), the differential amplifier needs to have a gain of slightly less than 5. (That gain is equal to 2 times the quantity of 1 plus the fraction of 10K over R-Feedback.) Thus, the ratio of those resistors should be around 1.5; R-Feedback works out to be 6 or 7 K. Standard 1% values of 6.65 and 6.81 K are available. (It doesn't really matter which you use, since you accomplish fine "scaling" when you calibrate the finished product.)

Calibration

There are three adjustments to be made in the Fowle Gimmique: First, you compensate for any resting offset in the sensor by turning the pot at pin 3 of the quad op-amp (LM324) so that the Gimmique's VCO ticks at very few Hertz (or maybe goes to zero). Next, with a pressure or vacuum applied, the rheostat in the emitter of the 2N2222 is adjusted for a frequency of your liking. Finally, the rheostat in series with the Brailled pot is calibrated so that, when set to the point of a known pressure, the boundary between a chopped and smooth VCO tone occurs at that point.

An Audible Tire Gauge

An SX100DN will do nicely. Except for the gain, the same circuit as is suggested for the exemplary automotive vacuum gauge is the one you want. (The simplest positive offset circuit is the appropriate one, as long as you apply the pressure to Port 1.)

In the data sheet, this device is listed with a full-scale output of 360mV (with a 12V supply). Correcting for a 5V supply, our sensor output will be 150mV for 100 p.s.i.g. To run our half-volt readout, we want a gain of 500mV over 150mV, or 3.33. (That assumes that we want our gauge to read 100 p.s.i.g. full-scale, which covers high-tension bicycle tires.)

Solving for the resistance ratio in the gain formula, we see that the ratio of 10K to R-Feedback should be 0.667; thus, the feedback resistor should be around 15K. A nearby 1% value is 14.7K. This resistor goes between pins 9 and 13 of the LM324.

As with the automotive vacuum gauge, pin 10 of the 324 goes to pin 2 of the sensor, and pin 12 of the 324 goes to the sensor's pin 4. This is with the intent of applying pressure to Port 1.

Since there are such similarities between a tire gauge and a vacuum gauge, a temptation is to try to build one common circuit which would just plug into the desired gauge head. However, if you look at the specifications for the SX sensors, you see that only the figure called "repeatability" is consistent. The resting offset and actual gain can vary to such an extent that separate offset and calibration adjustments would be necessary for each. The best one could do would be to build separate "signal-conditioning circuits" with adjustable gains, and then use a common meter reader.

My Player-Piano Vacuum Gauge

Vacuum tensions of up to 60 inches of water can be found when testing vacuum cleaners and treadle pumps by themselves. On the other hand, sensitive adjustments on fancy players require precise measurements between 5 and 40 inches of water. Therefore, I chose to give the gauge two ranges--0 to 50 and 0 to 100 inches of water.

There are about 400 inches of water per atmosphere, and this is almost 15 p.s.i. Thus, an SX05DN was chosen, covering the desired range of 3.25 p.s.i. Scaling down the figure of 36mV per p.s.i (for our 5V supply), we get 15mV per p.s.i. The full-scale range is then 3.25 times 15mV--48.75, or 50 millivolts. The 100 inches of water will give me about 50mV, and 25mV will be developed at 50 inches (the lower range). To run my half-volt readout, I want gains of 10 and 20.

A temptation is to pick two feedback resistors for the differential amplifier. (Solving for R-Feedback, gains of 10 and 20 can be had with resistors of 2.5K and 1.111K, respectively.) The flaw in this plan rests with the easily forgotten offset voltage which this sensor may or may not have. (With a 5V supply, these SX units can give you a resting offset as extreme as minus 33mV.) For each differential gain, you end up having to correct for a corresponding offset. Yucky!

The way around this is to set up the differential amplifier with the highest gain desired, then use a voltage divider off its output to present any reduced gain to the readout.

Thus, the gain of the differential amplifier should be 20. (Twenty equals 2 times the quantity of 1 plus 10K over the desired feedback resistor.) Therefore, the ratio of these resistors turns out to be 9; thus, R-Feedback should be 1.111K. While there is a standard 1% value of 1.1K, I made my feedback resistor with the series combination of 1K 1% plus 100 ohms 1% and 11 ohms 5%.

This feedback resistor goes between pins 9 and 13, the inverting inputs on the 324 quad op-amp. Now, instead of the output of the differential amplifier going to the readout's 220K resistor, the 324's pin 8 goes through 2 10K 1% resistors, in series, to ground.

The on-off switch I chose was a 3-pole 3-position rotary unit. Two of the poles handle the split battery supply. The arm of the third pole goes through 220K to pins 3 and 5 of the readout's LM358 (pins 3 and 5 being bypassed to ground by 0.1uF). Position 1 of this pole is left open (the "off" position). Position 2 (the most sensitive "50-inch" position) goes to pin 8 of the 324, the direct output of the differential amplifier. Position 3 goes to the junction on the above voltage divider off pin 8.

Mounting the SX style of sensor is not as convenient as the SCX type. However, it can be bolted in a position so that its nipples extend past the edge of the board; I then bent the pins at right angles and fit them into a socket placed just behind it. However, this case style is not as thick, and the sensor must be mounted on 1/8-inch spacers to make it sit just right.

The board was then mounted on the front apron of the cabinet. Holes for the nipples allow them to pass through the top panel and point straight up.

Other Gauging

There are a lot of transducers which require differential detection. Many of these are bridges. For example, weighing scales can be made with "load cells"; these are just strain-gauges in a Wheatstone bridge arrangement, just like these pressure sensors. Sensitive thermometers can be made with temperature-sensitive elements arranged in a bridge.

There are transformer-type position sensors which give you a differential signal that reflects the coupling to two secondaries. In these devices, a suspended (frictionless) movable core alters the coupling, and the signal of the two secondaries--when compared--tells you precisely where that core is. These transformers are the heart of nifty things: accelerometers and machinists' electronic micrometers, for example.

As for these Sensym devices, I'm not done playing yet. Upcoming is a cheap auditory blood-pressure meter. Beyond this, the editor wants to try making a special altimeter that, when calibrated to a specific building elevator, could hopefully tell you which floor you're on. Wouldn't it be fun to have a pocket Braille elevator that you can use when you visit your doctor's high-rise, or stay in a hotel with no Braille on the lifts?

* * *

Parts Lists

Dynamic Readout

(The "Fowle Gimmique")

Resistors (1/4-watt 5% unless otherwise specified):

• 1--10 ohms 1/2-watt 5%
• 1--47 ohms 1/2-watt 5%
• 2--2.2K
• 2--10K
• 2--22K
• 1--220K
• 1--10K 1%
• 2--50K multi-turn trim pots
• 1--10K panel-mount precision linear unit fitted with a Braille Dial (Clarostat 58C1-10K or equivalent)

Capacitors:

• 1--0.0047uF Mylar or mica
• 1--0.1uF disc ceramic or Mylar
• 1--2.2uF low-voltage electrolytic
• 1--100uF 10V electrolytic

Semiconductors:

• 1--556 dual timer
• 1--Dual op-amp, probably an LM358
• 1--LM336 2.49V zener (Included here, although it is common to all aspects of these gauges)

Signal-Conditioning Circuit

(Note: This list doesn't include all the options for special offset circuits, but the following are parts you know you'll need.)

• 8--10K 1% resistors
• 1--Feedback resistor to be calculated
• 1--50K multi-turn trim pot
• 1--quad op-amp; LM324 or, most desirable, a Linear Technologies LT1014 (or any good op-amp which can stand a supply voltage of 18V)
• 1--LM336 2.49V zener (shared with the above meter reader)
• 1--DPST on-off switch for the dual supply

THE JAMECO XY168 SOLDER STATION

by Tom Fowle

Abstract

This is a brief review of a newly available temperature-controlled soldering iron. Like the Weller WTCPM, it is traditional in that it is constantly powered (it is not like the Vinther iron, which is placed on the work when cool). Nonetheless, for those of us who prefer a hot iron, this unit appears to fulfill all the needed functions of the popular Weller soldering station at half or less the usual price.

A recent flier from Jameco Electronics announces a new soldering station, sold as Jameco catalog number XY168. The unit is priced at $49.95 and is temperature controlled, being adjustable from 270 to 800 degrees fahrenheit. A unit having digital temperature readout is available for a $40.00 boost in price; that model has not been bothered with as no conceivable need for such readout could be imagined.

The advertising claims that the temperature control switching is accomplished by a thyristor so that switching occurs on zero-crossings of the AC line voltage, therefore keeping spikes and transients at a minimum. (People who work with unprotected CMOS devices have reason to worry about such transients.) Temperature control and stability is claimed to be within plus or minus 10 degrees F.

To the technician, the unit appears very much like the old familiar Weller solder station. There is a base unit of a more-or-less cubical design whose top holds the spring-like cage for storing the iron between applications. The top surface also has a small parts tray, along with a well to hold the cleaning sponge. The slightly sloping front panel has the on/off slide switch--to the left of center--with a meter just above it which reads out temperature to a resolution of 50 degrees or so. To the lower right of the panel is a nice heavy 5-pin amphenol connector of the type often used for microphones on radio equipment. This accepts the plug on the end of the iron cord. This is a considerable improvement over the Weller unit, which has a strange light-duty 3-prong plug (a high-failure item).

There are two small LEDs towards the top of the front panel, the left one being power and the right one showing the on/off state of the heating element. Just above, and to the left of the iron socket, is a small pointer knob which adjusts the temperature. Setting this knob with its pointer at about the 1:30 clock position gives approximately 750 degrees, a good starting temperature.

The tips supplied are "clad"; although it is advisable that they be tinned immediately when first turned on, they require less service than the old-fashioned copper tips. Unlike copper tips, they should never be touched with abrasives; once the factory coating is damaged, they should be discarded.

The "Station" comes with a 1/16th inch tip, although other sizes are available. They list the following tips:

Note: These dimensions are in inches. The tips are not strictly conical, but are slightly blade-shaped. A flat spot on the handle conveniently enables the blind user to orient the tip's flat side as desired. These tips are $3.49 each.

• 1/8th--xy3
• 1/16th--xy2
• 3/64th--xy4
• 1/32--xy1

As shipped, the spring iron stand is not installed. It has a large wire hook which fits down into a slot near the top rear of the station base. The unit comes with a flat cleaning sponge which can be wetted and kept in the top of the base. However, the author much prefers the American Beauty "One-Pass Soldering-Iron Cleaner," (American Beauty No. 480). This is available from Marshall Industries and Mouser Electronics.

The soldering station also comes with an allen wrench whose purpose we have not yet divined.

In use, there seems to be no problem with this unit, especially to those used to the Weller. The lack of the repeated click of the Weller turning on and off is somewhat missed as a small comforting reassurance of action, but this is of no importance. There is no reason why an audible reader could not be hooked up to the temperature meter, but we have not tried this because there is a warranty period to wait through, and we see no big need for such a readout other than amusement.

In conclusion, this unit is a valuable addition to less-expensive quality tools, and we hope its existence may convince some of you, who have been reluctant to spend $100.00 on a soldering iron, to become familiar with the joys of a temperature-controlled unit. With such a tool, the rapidity of soldering and the lessened fear of severe burns brings the process of soldering to the realm of comfortable everyday work--away from a process one does with a quaver and a cringe.

Suppliers

Jameco Electronics: 1355 Shoreway Rd., Belmont, CA 94002; (415) 592-8097.

Marshall Industries: 9674 Telstar Avenue, El Monte, CA 91731; (800) 522-0084.

Mouser Electronics: 11433 Woodside Ave., Lakeside, CA 92040; (619) 449-2222.