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Need help...PWM circuit for LEDs...

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tmod

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From https://www.bit-tech.net/article/51/ ...
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I would like to adapt this circuit as an LED dimmer (C1 will be 33nF so the PWM frequency is higher...), but instead of the thermsistor, I'd like to use a phototransistor (less light = brighter LEDs). However, I'm not exactly sure on how to do it since a phototransistor lets more current through under a greater amount of light. What would be the best way to make the effect the inverse of this?
 

I think you could get the desired effect if you exchange the positions of r1 and r2. You should mount the phototransistor at position r2 and use a fixed resistor as r1.

hope this works
 

what is it ?
 

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