Based on the conversation I had with the students at the beginning of class yesterday, I came up with an idea for a lab that uses both the phototransistor and an FET that should be fairly fun and easier than many of the labs we’ve done. The students were saying they would enjoy a musical application—the student who mentioned it was thinking of a light harp, where the “strings” are light beams that you break with your hands to trigger synthesized notes. It would be pretty easy for them to make a circuit that detects the presence or absence of light, but all the fun parts of such a lab are in the programming, which is not part of this class (and which most of the students have no training in).
I decided to try making a sound that the students could control in some way with a phototransistor, but not necessarily a particularly musical sound. The obvious application of an FET is to put it in series with the loudspeaker and a power supply and turn the FET on and off. The relaxation oscillator that they did for the hysteresis lab this week could be used to drive the FET (at least, if we lower the frequency down into a comfortable part of the audio range). The students don’t have another 74HC14N chip, but they don’t need one, since all four nodes of the hysteresis oscillator (+5v, out, in, GND) are available at the screw terminals of the board they soldered.
Since we are interested in increasing RC, we can do it by increasing C—adding another capacitor in parallel with the one on the board. All they need to do is connect wires from the board to a breadboard, add capacitance to lower the frequency, and connect the output to the gate of the FET.
So we’ve got sound (pretty loud sound if they use a high pitch) out, all we need is control. The simplest thing I could think of was to put the phototransistor across the capacitor. In bright light, it conducts a large current, discharging the capacitor and keeping the input low (thus keeping the output high). In darkness, it conducts essentially no current and the oscillator oscillates as it did before we added the phototransistor. At intermediate light levels, it conducts some current, which means that it takes the oscillator much longer to charge the capacitor up to the high threshold, so the output-high time is stretched. If the current through the resistor is slightly larger than the current through the phototransistor, the oscillator oscillates at a low frequency. If it is much higher, the oscillator output is stuck high, and if it is much lower, the oscillator oscillates at its maximum frequency. So by shadowing the phototransistor, one can modulate the frequency of the output.
One problem with the design as a toy is that the resistor used determines the amount of ambient light needed to shut the thing off. I had to add resistors in parallel with the one already in the oscillator to make the current large enough to oscillate even in dim light, and I needed to increase the current a lot more to have any control in sunlight. When I had the current that high, though, the oscillator would not shut off with just room lights at night.
I suppose I can make a virtue out of this problem, though, by having the students measure the range of currents that they get from the phototransistor (with a bias voltage around the VIH threshold) for the lighting in the room and the shadowing they can do with their hand, then pick the resistance they would need to use in parallel with the existing resistor. After that, they could compute the capacitance they would need to add to get a reasonable high frequency in darkness.
This is a simple design exercise, gets another use out of the hysteresis oscillator board, and is sort of fun to play with (though the pulse-train buzzes do get annoying to listen to after a short while—I’m sure I’ll have a monster headache by the end of the lab session. I’ll try to write up this lab as soon as I get Monday’s quiz written.
I’m now thinking that next year I’ll rearrange the first 6 labs into several shorter labs:
- buying parts kit and familiarization with it. Marking bags of capacitors, learning to use multimeter to measure resistors.
- thermistor lab using multimeter (and make sure we have water hotter than 70°C and colder than 10°C)
- thermistor lab using voltage divider and Arduino
- microphone lab getting DC characteristics with Arduino
- microphone lab setting DC bias and learning to use the oscilloscope
- hysteresis oscillator on breadboard
- soldering hysteresis oscillator design
- phototransistor and FET lab
- characteristics of stainless steel electrodes
- characteristics of Ag/AgCl electrodes
- audio amplifier with op amp, dual power supply
- audio amplifier with op amp, single power supply
That order would give us more time to develop the notion of impedance before the electrode lab, as well as more time to learn to use gnuplot, but after 6 weeks will be in about the same place I hope to be at the end of next week after 6 weeks. It might be necessary to spend two of the shorter labs on the dual-power-supply op amp, but we’ll see.