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Photocell for Audio?

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New Member
I have an idea that's a little complicated to describe (so I won't), but it requires that an audio signal's level be controlled by a flashlight. The light would fall on a photocell (photoresistor?) and reduce the resistance and thus make the sound louder. That's how I conceive of it, but I don't really know what photocell to use, nor what other components need to be involved. Any ideas?


New Member
RED free circuit designs (http://space.tin.it/scienza/fladelle/) have a bunch of nice audio amps which could be fairly easily adapted to what you described.

For example in the image below (http://space.tin.it/scienza/fladelle/Page1.htm if the image dosn't show) you could replace the pot P1 with a fixed 22K resistor between ground and C1 and place a cadmium sulphide (CdS) light dependent resistor in parallel with a suitable resistor between C1 and the non-inverting input of the op-amp. The performance of most LDRs would mean almost no audio in darkness, with the 'volume' rising as the light did.

Alternatively if the resisitor in parallel with you LDR was variable then you could 'tune' how light responsive the op-amp is.



New Member
Re; Photocell for audio

Thanks for the suggestion. I was hoping to avoid the complications of building an amp --- it is fairly easy and inexpensive to get amps already assembled. I was hoping it would be possible to design a circuit that is essentially passive --- a light-operated potentiometer --- the input would be fairly high level (from a CD player). What do you think about that approach?


New Member
how about this, use a few 3-4, photoresistors, depending on their resistance in the light, and connect them in paralel. then connect this in series with the output of the cdplayer and the other amp, or what you have.


Active Member
Hi Oddio,

Just a guess here but i think you would
like to stop pointing the torch at it
when you're happy with the volume level?

I dont suppose you want to have the
brightness of the torch determine the volume?

I would suggest that your volume could go up
in little steps, and that you could point
your torch at the photo-cell, and then wiggle
the light across the cell so as to step it
up a bit at a time.

Another cell nearby could do the same
stepping down.

Such a unit could fairly easily be made using

You can get little laser toys with red rays
that would do this better than a torch,
they're only small, ive seen them on key rings.

Regards, John
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