transistors

I’ve started teaching some basic electronics classes at the Ann Arbor District Library. They recently setup a makerspace they’re calling The Secret Lab and hired a guy named Steve Teeri to run it. I got hooked up with Steve after I inquired about the possibility of teaching some basic electronics classes. As it turned out, they had recently gotten to the point where they could start doing things like electronics classes in the Secret Lab so my inquiry had come at a fortuitous moment. I’ve since become the de-facto electronics/ham radio consultant to the library.

The first class we held was on how to use a digital multimeter. Five people showed up, and it turned into a class on circuits as much as a class on how to use a DMM. Overall, it went pretty well, and we followed that up with a class on how to use an oscilloscope.

This evening, we held a class on basic transistor circuits. This evening, we had nine people show up. There were a couple of  older guys, three younger guys, two women, one who brought her two kids. The kids were eight and ten.

Initially, I had planned to have the students breadboard two circuits – a simple switch circuit and a common-emitter amplifier circuit. Both of these circuits can be found on the sheet below.

I figured that we’d be able to blow through the switch circuit, then dig into the amplifier circuit. WRONG! It took us nearly the entire class for everyone to get the switch circuit to work.

Some of it was my fault. First of all my schematic was lacking. It was a schematic that I got off the Internet, and while it was correct, it wasn’t detailed enough. As you can see from the red ink above, there were several omissions and errors:

1. On the schematic, I didn’t explicitly show which pins on the transistor were the emitter (E), base (B), and collector (C).
2. I didn’t include a diagram showing which pins on the transistor package were the emitter (E), base (B), and the collector (C).
3. I didn’t explicitly show how to connect the battery.
4. I included a switch that we did not use in class.

An added complication was that the resistors that the library had purchased had blue bodies and very thin color bands. The result was that it was really difficult to really read the color code. At first, I thought it was just me and my failing eyesight, but I was relieved when one of the students had the same complaint. We actually had to dig out the multimeters and measure the resistors to make sure that we had the right ones.

Another reason that it took us so long is that I had to teach the students some really basic stuff, even before we got to the point where they could put the circuits together. This included the resistor color code and how to use the proto boards that we were using. This was certainly OK, but I hadn’t anticipated having to do that.

After about 45 minutes, all of the circuits were built, and the LEDs were lit. I asked them to disconnect the 2.2 k resistor to demonstrate how removing the base current turns off the transistor, and I think they all got that idea. I also explained how in practice that 2.2 k resistor wouldn’t be connected directly to a power supply but to perhaps an Arduino’s digital output. I also mentioned that instead of just turning an LED on and off, we might use the transistor to do some real work like switch a relay on and off. I think they got those ideas, too.

One guy asked how much current that the 2N2222 could switch. I had brought along with me a 2N2222 data sheet, and we looked up the maximum collector current for a 2N2222 (1.0 A). We then discussed how running the transistor at its maximum current rating might not be a good idea.

There was just enough time to go over the amplifier circuit quickly. Fortunately, I had the foresight to bring my own protoboard with the amplifier circuit already assembled on it. I quickly hooked up the scope probes, the signal generator, and the 9 V battery, and demonstrated how the circuit turned a 100 mV signal into a 2 V signal. For those who were interested, I was also able to talk a little bit about biasing.

So, the first thing that I take away from this experience is that I really need to gear down the level of the presentations. Second, I need to be a little more explicit with my instructions to students.

I’m hoping to do a lot more with the Ann Arbor District Library. I think that perhaps the next class will be an Arduino Basics class. At some point, too, I’ll want to reprise my DMM and oscilloscope class. I really love it that the library is giving me the opportunity to do this.