I am happy to report that the PhotoSynTheremin prototype is built, and I am starting with a small batch of qty 25 boards. The next batch of 25 (last one had a slightly small LED footprint – Unacceptable!) will arrive in a week, electronics arrive today, and plexiglass cover should be here in about a week. Once everything comes in I will make the kits available on Tindie.com.
The original prototype had some issues which required a second board-turn:
- As the gain was turned up, the frequency would oscillate and shift up an octave.
- The frequency range of the audio oscillator behaved differently than the breadboard. What a surprise.
- In an effort to mitigate ambient light & saturation of the photodiode, I opted for a different pair of photodiode and IR LED; This needed testing.
In the above picture you can probably see a bunch of MLCC capacitors strung together in a rats-nest. This was an effort to find the right capacitor for the integrator circuit and eliminate the unpredictable oscillator action. I ended up increasing the capacitance from the breadboarded 0.001uF to 0.01uF on the PCB. I suspect that parasitics in the breadboard caused skewed results (which is usually the case) when adapting to the “real” PCB, but in doing so we trade off stability with frequency range. That’s ok, because it’s ear-piercing enough!
The gain of the LM386 was reduced by adding some resistance (or attenuation) at the amplifier input which helped with the gain & range issues. Lastly, the new photodiode and IR LED combination worked a treat.
Now, to just finish up drafting the instructions, start the new YouTube channel, get some boxes and other ancillary items and it’ll be ready to annoy The World!