This week’s project is a followup to my post last week about using 555 timer ICs. I was working through the examples in the Make: Electronics book – Experiment 17, in particular, which introduces the “astable” mode of the 555, where it basically functions as an oscillator. That reminded me of an idea I saw online for a “Light Theramin” – a sort of musical instrument controlled by light. The frequency of sound from the light Theramin increases with the amount of light falling on the detectors, giving it that quavering, continuously changing sound that’s reminiscent of a real Theramin. After a half hour of improvising with different components, I ended up with the circuit in the video:
The concept here is pretty simple. The 555 generates the basic tone, controlled by the resistors and capacitor connected to its Discharge and Threshold pins. I added a couple photoresistors in series with the resistance load. I’ve used these before – the resistance varies with the amount of light hitting the surface. Changing the resistance changes how fast the capacitor charges, with changes the frequency of the 555’s output.
I also kept the right-hand portion of the circuit from Experiment 17, which sets up a second 555 timer hooked up to the Control pin of the first one. The second 555 is a slower oscillator, with a button to turn it on and off. The result is to create a siren-line “weee-oooh-wee-oooh” sound. I added a red LED on the second 555’s output to highlight the effect.
The fun part of this circuit, for me, was the improvisation and trial and error of getting a good result. Yes, if you wanted to you could calculate out all the values to get the frequency range you wanted, but going into the project I didn’t know specifically what I wanted beyond a cool noisemaker. So instead I experimented with different values. First, I wanted a lower base frequency, so I increased the resistance so the capacitor would charge more slowly. Then I was concerned that the constant resistance would minimize the impact of the resistance change from the photoresistors, so I went with a different-sized capacitor instead.
At first, I only used one photoresistor, but that didn’t seem to give me very much range. In the end, I put 3 photoresistors in series, which I figured would have an additive effect, and it seemed to do the trick. Taking a close up of my breadboard, though, you can really see that this circuit was evolved rather than designed. That mess of resistors and photoresistors is a lot simpler than it looks! Then again, it also got built on top of and overlapping the Control input coming from the second 555 timer, and even the lines I used to connect my power and ground rails. If I were still in school, I’d expect to get docked points for that kind of shoddy construction. But for hobby work, it does the job.
There’s one other thing about this circuit, which is still puzzling me. When I built the original version of the circuit in the book, I put the speaker in series with a 100Ω resistor to limit the current – but when I tried it out, I didn’t get any sound out of the speaker except for a quiet click when I powered up the circuit. I’m not the only one to experience that, but I still haven’t found an explanation that completely satisfies me. The solutions I’ve found online put a capacitor in series with the speaker, and that seems to do the trick (100 μF worked well for me). I also found that swapping out my 100Ω resistor for 180Ω works, but I can’t explain exactly why. I suspect that would become clear if I had an oscilloscope, but those are pretty expensive as toys go.