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# How can i use a CDS cell to turn on a LED? Answered

I want the CDS cell to turn on the LED when there is no light, and turn it off when there is. I need a diagram and parts list, i am using a 9 volt battery. Whoever gives the best answer will get credit in an upcoming instructable.

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## Discussions

There's a circuit linked from THIS question which nearly does what you want.

Here's my comments on the circuit I sent to the original poster when I helped him off-line.

The problem is that this is really too simple a circuit to do the job. It would do very well as a light that comes on slowly when the room gets darker, but it's not really suited to coming on suddenly at a certain drop in light level. We would need an extra transistor and a couple of extra resistors to make it work /really/ well.

However, I've constructed this circuit and got it to work /reasonably/ well. What you need to do is to replace the 1M resistor with 1M variable resistor (potentiometer) in series with a 10K resistor.
(The variable resistor has 3 connections - you would use the middle and one of the outside ones.)
You would then have a resistance you could vary from 10000 ohms (10K) to 1010000 ohms (1M + 10K).
The 10K is there to stop the transistor burning out when the pot is right at the low end.

To set it up, cover the light dependent resistor (LDR) and turn the pot until the LEDs are just at full brightness.
Any light should now begin to dim the LEDs, and turning the room light on should turn them completely off.
Of course, none of the light from the LEDs should fall on the LDR as this would stop the whole thing working.

the cds cell is a resistor - it (someone correct me if I'm wrong) gets less resistance as you add light - so wire it to the base of an npn transistor - search how to use a transistor to switch an led, and you're golden. Dont forget the current limiting resistors!