Analog Angle Blog

Simple analog-centric circuits expand STEM perspectives

Looking at some anecdotal stories as well as some reasonably valid survey data, it seems that STEM-related subjects are getting serious interest from students at all age levels; STEM stands for science, technology, engineering, and math. There are many presumed reasons for this trend, but a large part of it appears to relate to the career prospects that these subjects likely offer.

I’ll leave any discussion of whether this is good or bad for society in general and students in particular to the talking heads and pundits who opine on these things.

For electronics, systems such as the Lego Mindstorms or low-cost platforms based on Arduino or similar kits make it relatively easy to get started and do some substantive and meaningful projects. There is also the attention to robotics via FIRST Robotics teams and competition for high-school students.

At the same time, I hope that the availability of these excellent tools and products does not have an undesired spillover effect of disconnecting students from understanding of the real analog world. By “analog” I mean it in its broadest sense of sensors, interfaces, voltage and current drive, load resistance, gain, and more—all the physical-level issues that makes functional circuits possible. Without a sense of what’s happening at this most mundane yet critical level, it’s easy to have a little grasp of why that 3-V coin-cell battery is having trouble driving a 3-V motor for more than a few seconds (hint: it’s not necessarily the limited mA-hr rating of the cell).

Transistor-only circuits

Perhaps a good place for students to start is with basic, “forgiving” transistor-only circuits which are easily built, pushed, poked, probed, and modified. There are many of these available at various sites on the web; some are a little too trivial—LED/resistor plus battery plus switch—but many actually do something interesting. In addition, many of them use standard, low-cost, widely available transistors such as the 2N2222, 2N3904, or other well-known building-block transistors.

For example, there’s a lighting-detector circuit, Figure 1, which has no critical build issues, uses three 2N4401 PNP and one 2N4403 NPN transistors (all very common and with alternates) and can be measured at internal points with a basic digital voltmeter (DVM). However, one of its drawbacks is that you have to have a lightning storm for it to demonstrate its function.

Figure 1 This four-transistor lightning detector flashes its lamp if there’s a lightning strike nearby; it’s easy to build but perhaps harder to test, since you need an available storm. Source:

A more sophisticated example is a three-transistor, 40-meter amateur-radio transceiver which uses discrete crystals rather than a synthesized tuner, shown in Figure 2 and taken from the article “A Potato-Powered Transceiver” published in QST in April 2022. It uses so little current that it can operate from a set of series-connected potatoes delivering about 0.86 V and 2 mA per potato.

Figure 2 For some genuine RF work, this crystal-based 40-meter transceiver is as simple as it gets, providing 2.1 mW RF output while drawing just 7.3 mA; in receive mode, the current drain is 2.2 mA. Reprinted with permission, April 2022 QST; copyright ARRL

While the potato-operation is a cute feature and will attract some attention, the real focus should be on the transit/receive operation. Of course, the student would need an amateur-radio license for this, but since the code requirement was eliminated, that’s pretty easy to get.

Besides these, there are countless 555-timer circuits and associated “cookbooks.” However, while those circuits are instructive in many ways, they generally don’t have a signal-flow path from an interface or sensor input to another interface or transducer output of the classic analog-signal chain. Yet it’s often at these interfaces between subcircuits or functions that the real circuit challenges must be identified and overcome.

Basic design projects

Further, there’s no need to restrict these projects to analog signals and functions. There are opportunities to build basic logic gates and flip flips—when was the last time you even thought about flip-flops?—using discrete transistors and then exercise them while observing their inputs and output states with simple LEDs or a DVM. They can also be configured to perform binary math to give the student a sense of how these modest circuits are the building blocks of processors of all types. These projects would give aspiring STEM students an opportunity to look beyond “electronics” as closed black boxes into which they pour data and code and somehow get a response.

Many years ago, I judged a high-school science fair where the projects were quite sophisticated; in fact, many were too sophisticated for my taste. For example, some students had access to advanced biomedical-lab equipment and ran various DNA tests. But when I asked them how they verified that the results these machines provided made sense, their response was “there’s no way to do that—we just assume it’s correct.” Their inability to do a sanity check or cross check on the results and just accept them is somewhat worrisome. I actually preferred the less-advanced projects where the student was able to get into the nasty details and could observe and explain what was going on.

That’s why I think that doing basic projects with simple, all-transistor analog and even digital circuits would be educational and also develop a sense of the realities of what circuit elements you must deal with at a most-basic level. Nothing fancy is needed: some leaded transistors and passive components, various types of switches, batteries or power supplies, a DVM or two, and some LEDs as indicators. An oscilloscope might be nice but is optional in the relatively static analog and digital worlds. Even the classic solderless prototyping board of the 1960s—and still widely available—would be fine (Figure 3).

Figure 3 The classic solderless prototyping board for leaded components is still widely available, and remains a viable circuit-experimenting tool, within limits.

What’s your take on the need for or wisdom of offering basic transistor-based STEM platform and resources? Are they useful and worthwhile or, as they say, that ship sailed long ago?

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2 comments on “Simple analog-centric circuits expand STEM perspectives

  1. wb9kzy
    June 7, 2022

    Interesting column. One thing: that lightning detector was used by the blogger without attribution, a little searching shows that it’s from the Techlib web site, it’s the original version of his lightning detector:

  2. DaveR1234
    June 20, 2022

    I don’t much care for either of these circuits to get started with STEM. One is too complex (and require a storm!) and the other requires Morse code.
    Start with the simple LED driver and a pushbutton then move to a 555 to flash at variable frequencies and PWM followed by driving a speaker with variable frequency and modulation to sound like a fire engine or ambulance. Connect a scope to compare sound with the waveform.

    Then work with dc motors that rotate in either direction, and can operate as a DC generator to drive an LED or a second motor. I built a “car” with two motors, one for each wheel. With the motors connected together, turning one wheel drove the other wheel (why) holding the 2nd wheel from turning made it tough to turn the 1st wheel (why?). Connecting two colored LEDs to the motor wires turned one LED on when turning the wheel, but turning the other direction turned the other LED on (why).

    Don’t forget the simple coil of wire wound around a nail.

    Fairly simple equipment can do a lot of teaching.

    Moving on to Arduino is endless in possibilities.

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