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Photocell Problems Answered

I'm trying to add a photocell light sensor to a device that sits under a liquor bottle that lights it up Only when it's dark. However the photocell (or at least I'm pretty sure it's a photocell) only completes the circuit when there is light shining on it, rather than when there is not. I removed the photocell from a broken night light that's been gathering dust in my junk cabinet. The night light logically lit up at night, but the sensor seems to be doing the opposite. Does anyone know what component I would require to reverse the effect of the cell? The first picture below is the circuit wired simply on a mini breadboard with a LED to test for continuity. The photocell is the only other component on the board, and is causing the LED to shine dimly. The second picture has a light shining on the photocell. It is hard to see, but the LED is lit much more brightly. I want to reverse this effect. All suggestions are welcome!

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user
westfw (author)2008-02-14

Yeah, the sort of photocell you have there gets MORE conductive as it gets brighter (resistance goes down), so you need a sort of logical inverter to make it into a "dark detector." A tech-king says, a single transistor will do fine.

See also the Evil Mad Scientist take on the same problem

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user
tech-king (author)2008-02-14

is the led powered off the photocell?

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user
Firebert010 (author)tech-king2008-02-14

Nope, two AA batteries and a resistor.

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user
tech-king (author)Firebert0102008-02-14

this is killer jack a lot's circuit. it should fill your needs.

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user
matseng (author)2008-02-14

This instructable shows what you need to get your led to light automatically at dark. Blue-Bawls-automatic-LED-light

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