Design Challenge: AM radio

Once again I had a group challenge as part of the electronics final. This was something I first tried the last time electronics was offered (two years ago, see laser tripwire). I felt that it was just as much fun as last time, and the class certainly engaged with it and came through victorious.

The format for the final was a 1.5 hour written exam (individual) and a 1 hour group challenge. I provided a stack of parts, some basic test equipment, and an outline of the challenge:

You are stranded on a desert island after a plane crash. Fortunately the plane was full of physics majors and you all survived. The following parts and equipment were not incinerated after the fiery crash:

  • two 555 timers
  • an NPN transistor (2N3904)
  • various capacitors and resistors
  • numerous scrap wires
  • one breadboard
  • a battery-operated oscilloscope (don’t leave home without it!)

The oscilloscope is on loan from your local hackerspace so you cannot tear it open for parts! To survive you must build a transmitter capable of sending a morse code message over the AM band. You estimate that the search party will be nearby within an hour, your transmitter must be working by then. Good luck!

Since we had not discussed AM radio in any substantial way, I told students who finished the written exam early that they were allowed to search for information about AM radio and radio transmitters. They were allowed to take notes on what they find, but the internet would not be available during the challenge.

This turned out to be crucial… otherwise I would have had to get them started with some “outside information” and that wouldn’t be as fun. Several of them had enough time to jot down schematics but since they didn’t know what parts would be available, these weren’t as useful. The most useful notes had general information on the frequency range for AM radio, and the basic idea that a carrier is modulated by an audio signal.

During the semester, we had briefly covered the use of transistors as mixers and the class all remembered their most basic use as “one current controlling another”. These pieces were enough for them to recognize that one strategy was to design two oscillators with astable 555’s (one for the carrier and one for the signal) and then mix them on the transistor.

It turns out to have worked very well and I was easily able to tune in their “signal” on an old clock radio. The range wasn’t great with just a single wire antenna (no impedance matching or amplification) but they satisfied the spirit of the problem.

I learned that it is tricky to do these group challenge problems with 11 students. Last time around I had 7 in the class and that was just fine… two natural groups of 3 and 4 students. This time, there were always one or two people standing around. They broke out into four groups, with two groups working on the carrier and two on the signal. This let them speed up the prototyping & testing process. The adjustment I had to make this time was to provide extra parts to tinker with (duplicates from the parts list above). That way there were enough parts so everyone could be prototyping something. When someone got the oscillator to work they could say what component values they used and that circuit could be built into the final answer. If the course is this large again, I’ll have to be creative about finding a challenge that can be easily distributed.

The class learned about AM radio and I’m willing to bet a longitudinal study that they will remember how to use a transistor as a mixer. I’m using the testing effect here to teach some concepts that weren’t central to the course (i.e., they didn’t cram for them the night before) but they are relevant, useful, and fun to know.

Overall, it was another fun challenge, and the result is a circuit that’s fun to play with. Below is a schematic for the answer the class turned in. One nice feature of this problem is that there are actually many answers that work as long as the basic criteria are met. Due to the huge number of harmonics generated by the astable oscillators, you can pick up this signal all over the AM dial. In fact, the fundamental frequency of the carrier doesn’t even have to be in the AM band, even if a higher harmonic fall in the AM you can still hear the transmission. Low specificity is bad for radio stations, but good for rescue beacons! It is important to avoid challenge problems that have tight tolerances or that require very specific solutions.

An AM radio built from two 555 timers.


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