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how do you get a photo sensor to turn on in the dark instead of in the light? Answered

is it possible, I am trying to do this so I can make a laser trip wire, but it turns off in the dark instead of in the light.

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Wesley666 (author)2009-08-22

You put a transistor in (PNP) that is is connected collector to negative, emitter to resistor (1k ohm), resistor (You could also put another LED in with the resistor in series) connected to positive and the Photo Cell from negative to the base of the transistor. The negative is connected to a resistor and LED in series then to positive. I tried this, or something similar (you may want to use a breadboard first) and the transistor shorted the circuit enough to turn the LED off without killing the battery and when it gets dark the transistor turns off and power flows through the LED.

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miiwii3 (author)2009-07-08

plug it into a transitor that should reverse it. but i can't remember if it is npn or pnp get back to u on that.

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XOIIO (author)2009-07-07

nice! I just finished a laser tripwire! Super easy way is to go to a dollar store and buy a garden led light that turns on an night. Then just take out the circuit and mid it for a buzzer/ more LEDs anything that you want to hook up. Works perfect for me. Hope I helped!

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kelseymh (author)2009-07-07

If the sensor has separate pins labelled "NC" (normally closed) and "NO" (normally open), just swap your lead from the one to the other. Otherwise, take the output leads and use a relay to do the inversion. You'll need to supply power to the relay. Or use a logical NOT gate.

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