Gas station without pumps

2015 July 17

Reworking electronics class for cheap equipment

Filed under: Circuits course — gasstationwithoutpumps @ 23:09
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On of my goals for this summer (on which I’ve made little progress) is to rewrite the labs in the book so that there are three levels: a version for use with professional equipment (like we have in the circuits lab), a version for use by home hobbyists (with a lab and parts budget of a few hundred dollars), and a version for impecunious students to do in their dorm rooms (under $100 total for parts and equipment).

The equipment in the lab is really high quality and definitely overkill for a first electronics lab. The equipment for each 2-student bench is listed (on the lab-support web page) as

(1) Digital Oscilloscope, Tektronix Model 3052, 500MHz/5GS/sec; 2-Channel with IEEE488-GPIB interface
(1) Arbitrary Function Generator, Agilent 33120A, 5MHz, GPIB Interface
(2) Triple Output DC Power Supply, Agilent E3631A Digital Display
(2) Digital Multimeter, 6.5 digit, Agilent 34401A, GPIB interface
(1) Analog Oscilloscope, Kikusui COS5041, 45MHz, 2-Channel
(1) Digital Counter, Leader Model

Computers 
Dell GX-270; Pentium 4 Computer with 17in LCD (flat panel) Display

Although I initially taught the students to use the analog oscilloscopes, most of the signals we look at in the bioengineering lab are too slow for that to be really convenient, so I no longer teach the analog scopes.  We’ve also never used the digital counters, and one power-supply per bench would be plenty.We often use both multimeters, to simultaneously measure both current and voltage. Some students do use the computers, not wanting to bring their laptops or having difficulty installing software on them, but the computers are ancient—a more functional laptop can be bought used for under $200.

Few hobbyists are going to want to spend the thousands of dollars needed for even a stripped down bench with

(1) Digital Oscilloscope, Tektronix Model 3052, 500MHz/5GS/sec; 2-Channel with IEEE488-GPIB interface
(1) Arbitrary Function Generator, Agilent 33120A, 5MHz, GPIB Interface
(1) Triple-output DC Power Supply, Agilent E3631A Digital Display
(2) Digital Multimeter, 6.5 digit, Agilent 34401A, GPIB interface,

so I’m looking for serviceable replacement equipment for home use, with the understanding that a lot of the high-end functionality would be sacrificed.  I am also looking for cheaper ways to do the labs that require less equipment.

For oscilloscopes, I have two replacements:

For hobbyists with a couple hundred dollars in the budget, a USB oscilloscope with a 20M sample/sec sampling frequency is a worthwhile investment, but a lot of the labs can be done at low speed with just the PteroDAQ.

For function generators, I’ve looked at several choices, including the Elenco FG-500 kit and the JYETech FG085 kit. Although none of them are really satisfying, the FG085 kit is probably good enough for use in the applied electronics class.  The frequency can be set precisely to low frequencies (needed for the sampling and aliasing lab), the peak-to-peak voltage and amplitude can be set, and the rather nasty glitches in the waveform are not too bad for the labs in the course.  The frequency limitation is acceptable for the low-speed work in the labs, and at <$50 the function generator is within reach for hobbyists and possibly even for students. The only lab that might be hard with the FG085 function generator is the class-D power-amp lab, as the distortion introduced by the glitches might be a problem, as would the discrete steps in the triangle waveform at high frequencies.

For power supplies, I’ll be redoing all the labs except the class-D power amp to use a single power supply, so that the labs can be run off either 5V USB power or the 3.3V regulator on the KL25Z board. The power amp definitely needs an external power supply, but the cheapest way for a hobbyist to get one is to buy a regulated wall wart for $5–$10 and a barrel-jack connector to connect to the breadboard. I’ll have to think about whether to try modifying the power amp to be single-rail, or to recommend getting two wall warts and connectors.  One problem with using wall arts as that they are single-voltage supplies and lack the adjustable voltage and current limits of a bench supply.  I did recently buy some cheap step-down buck regulator boards that claim to have an adjustable output voltage (with a multi-turn trimpot), but I’ve not tried playing with them yet.

For voltmeters, a handheld Fluke 87 multimeter would be a good choice, but at about $400, they are rather expensive even for hobbyists. So I’ve been looking at cheap digital multimeters to see if they would be adequate for the labs.  Even the cheap ones are good enough for measuring DC resistance and DC voltage, but several of the labs rely on AC voltage measurements, sometimes at moderately high frequencies, so that is the main bottleneck. I started looking at the voltmeters I had already, and determined that the $5 DT-380B, which has only 200V and 600V AC scales was not really usable for the class, but an old Radio Shack pocket multimeter would be, if Radio Shack still existed and still sold that multimeter. I just bought a $10 DT-9205A multimeter and have started testing it. I’ll review it on my blog soon, but I think it would be usable for the class.  I’m also looking a whether the labs can be redesigned to use a single multimeter, instead of a pair of multimeters, though at $10 each, it might be worth recommending getting two.

Students will also need a soldering station for several of the labs.  I like my Weller WESD51 soldering station, but it is a bit expensive.  The student labs got Weller WLC100 irons, which use the same tips but are much cheaper.  They just have a power control (5W–40W), not temperature feedback control, and the highest setting runs much too hot for working with small electronics, but they are certainly usable. I think that the students may have damaged (oxidized) some of the tips by running the irons too hot.  I’ll probably have to clean and re-tin them all at the beginning of each quarter that I use them. (I’ve learned that I can’t expect the lab-support people to do any maintenance.)

Here are labs currently in the book, with a note on what needs to be done to “hobbify” each one:

  1. Setting up:  Needs soldering station and KL25Z board.  (Could rewrite to discuss using Arduino boards as alternative.)
  2. Sampling and aliasing: Needs PteroDAQ and function generator. For FG085 function generator, would need discussion of setting period, not just frequency, since frequency resolution is only 1Hz,  but time resolution is 1ms, so 5.1Hz ≈196ms.
  3. Temperature measurement: Ohmmeter, thermometer, and PteroDAQ are all the tools that are needed.
  4. Electret microphone: measuring DC current and voltage is easiest with two meters, but can be done with one by switching probes (keep one probe on the microphone output and switch the other between power and ground). The PteroDAQ is used for the bulk of the measurements. Looking at the AC output is best with an oscilloscope and part of the intent of this lab is to teach the use of oscilloscopes, so doing without one would be tough.  One can look at waveforms with PteroDAQ and gnuplot, but the immediacy of the oscilloscope is lost.
  5. Loudspeaker modeling: Needs function generator and AC voltage meter. Because we are simultaneously measuring AC voltage and current (using just voltmeters and a current-sense resistor, not an ammeter), it is easiest to do this lab with two meters, but it can be done with one, by switching leads.  Because the resistances used are very small compared to the meter input impedances, we don’t have to worry about the voltmeters changing the behavior. Although we can go up to 2MHz with the bench equipment, going to 40kHz is probably enough to see the non-linear behavior of the loudspeaker.  The voltmeters don’t have to have absolute accuracy to 40kHz, but they need to have relative accuracy, so that ratios of readings at the same frequency are correct, even if the individual readings are way off.  We can push meters way beyond their specs if we only need relative accuracy.  Note: if someone has a USB oscilloscope, peak-to-peak readings can be done with it at frequencies as high as it can handle, which may be well beyond what a cheap hand-held meter can do, though the voltage resolution on the cheap USB oscilloscopes is rather low.
  6. Hysteresis: Measuring hysteresis voltages just requires PteroDAQ and a slowly changing voltage, which can be done with a potentiometer or with a function generator with an offset to keep the voltage in range for PteroDAQ. The capacitive touch sensor uses the KL25Z board with different software to detect the frequency (writing a version of that code for the Arduino boards is on my to-do list). It is handy to have an oscilloscope to look at the waveforms, and to see the effect the capacitive load of the oscilloscope probe has on the frequency of the oscillator, but most of the lab can be successfully done with just the KL25Z board and breadboard. I do include soldering in this lab also.  I might want to rework the hysteresis oscillator board so that students can run a loudspeaker off extra inverters or make two oscillators, but no lab equipment is needed for this lab.
  7. Electrodes: needs function generator and AC voltmeter. the impedance measurements here are very similar to the loudspeaker impedance measurements, but the impedance of the polarizable electrodes can be fairly large, so that the input impedance (particularly the input capacitance) of the meters makes a big difference. This is one I’ll have to check out with cheap meters—I know I already ran into trouble with the Radio Shack meter on this lab, so I’ll have to check it out with the DT-9250A meter. The chapter will have to be rewritten with more discussion of the choice of current-sense resistor, and why using a larger resistor causes problems at higher frequencies. Students were running into problems even with the Agilent multimeters because of the high capacitance of some of the probe sets (see Voltmeter impedance).  Using a single meter and switching between using it for measuring voltage and current would make the effect worse, as the load on the circuit would be different in the two conditions.  Using a USB oscilloscope for the measurements might work ok, though the voltage resolution is a bit low.
  8. Low-power audio amplifier: Needs oscilloscope and power supply. I plan to rework this lab to use a single power supply, so that it can be powered off of USB power. I also need to redesign the prototyping board, so that it can be used to solder up the amplifier for use as a pre-amp on the class-D power-amp lab. Using PteroDAQ for debugging the audio-amp design is possible, but would slow down debugging a lot—the students would also miss the thrill of seeing their speech waveforms appear in real time on the oscilloscope.
  9. Strain-gauge pressure sensor: can be done with just the PteroDAQ, as all signals are very low frequency. I’m probably going to eliminate soldering from this lab, which means that I no longer need the instrumentation amp protoboard, since I’ve already switched to having the students use op amps for the EKG lab.  I should design a new protoboard that is useful for both the EKG and the microphone amplifier labs.
  10. Optical pulse monitor: voltmeter and PteroDAQ suffice for bench equipment, but students will need a jig for holding the LED and phototransistor in line.  The wooden blocks that I made for the lab (similar to the ones in Failed attempt at pulse oximeter) are easy to make with a drill press (and probably with a hand-held drill), which is fine for hobbyists, but not so good for students in dorms. I’d like to come up with a better way of holding the LED and phototransistor while applying about 13kPa (100mmHg, 1.9psi) of pressure to the finger. This may be a good design challenge for the freshman design project, as I suspect that there is a solution using cheap materials like foam core and rubber bands.
  11. Class-D power amp: This is the lab that uses the most equipment. I’ll want to try reworking the lab using just hobbyist level equipment—I’ve done it before at home, but not in a clean way. An oscilloscope is really essential for this lab, to see the gate waveforms and adjust the pullups on the open-collector comparator outputs, but a decent USB oscilloscope is probably good enough.
  12. EKG: This lab needs no equipment other than a soldering station and the PteroDAQ, though an oscilloscope is useful for measuring the output of the first stage.

2 Comments »

  1. Would an inexpensive way to get +/- supply be to use a dual isolated dc-dc converter and tie the two output sides together, or would that be too noisy?

    Comment by Michael K Johnson — 2015 July 18 @ 05:11 | Reply

    • Cheaper and easier still is to use two wall warts—they are usually isolated from the line voltages, so can be connect to make a dual rail supply.

      Comment by gasstationwithoutpumps — 2015 July 18 @ 09:00 | Reply


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