I have a hobby project that's been on the backburner for about 3 years now. It's very simple – just a solar-powered oscillator hooked up to a speaker, that changes frequency in response to the level of sunlight.
This is the circuit I'm using right now. It's mostly designed by Wilf Rigter from the BEAM circuits mailing list:
The thing is I'd like to invert the frequency/sunlight response. At the moment in dark light it's a quiet tone with low frequency, and in bright light it's a loud tone with high frequency. Instead, I'd like to have a high-frequency tone in low light and a low-frequency tone/pulse train in bright light.
I'm really up against my knowledge limit in understanding this circuit as it is. What I do understand is that the 10nF capacitor charges through the 22k resistor until its voltage reaches the Vbe threshold for the NPN transistor on the bottom left, at which point the transistor opens and the capacitor discharges through the speaker, making a click. Then something happens to reset the system and it starts again. With more light the capacitor charges faster and so the tone has a higher pitch.
Inverting the frequency/light level response, though, is beyond me. I think I'd need to somehow slow down the charging of the timing capacitor here as the light increases by diverting the current somehow, and I suspect that I'd want to be storing this diverted current in another storage capacitor to be discharged through the speaker, instead of discharging the timing capacitor through the speaker. Actually building this into a schematic, though, is too difficult for me right now.
Any help much appreciated!
Best Answer
I was able to simulate this circuit and play around with it a bit. The relationship between the voltage of the solar cell, the 330k resistor and the 10nF cap sets the operating frequency of the circuit. The difficulty in inverting this relationship is that the condition in which you want highest frequency (low light) also corresponds to the condition where VCC is lowest. Unless you can introduce an additional voltage supply independent of the solar cell, I don't see how you can achieve what you want. Sorry...