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How to illuminate a wall-clock with leds and a light sensor ? Answered

I want to illuminate a wall-clock using a light sensor and leds from a 1.5\3.0\9.0 volt batteries.What components i need for a long lasting but simple circuit ?

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Smokedasphalt (author)2009-09-28

what is the light sensor for? You want to turn on the LED's when it gets dark??

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arikkru (author)Smokedasphalt2009-09-28

hello smokedasphalt, That is correct.I want the face to be illuminated during the dark hours,and leds to go off during the day.

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Smokedasphalt (author)arikkru2009-09-28

Use an LDR. Check out Seandogue's answer, i was gonna tell you to do the same thing. But, instead of an op-amp, rather use a comparator. Like the LM339. It is a very common IC.

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seandogue (author)2009-09-28

One method is to use a CD cell for light sensing. Basically a light dependent resistor, also known as an LDR. They last for years. You can use it in a voltage divider that drives one of the inputs to an opamp with moderately high gain, the other input is set using a fixed reference. When the voltage exceeds the value, it flips the state of the output from high to low or low to high. This output is either used to drive a Fet, BJT or relay which in turn powers the LEDS. To adjust the circuit performance, one uses a trimmer on the fixed reference so that the circuit operates according to expectations wrt "light" and "dark" or conversely uses a trimmer on the voltage divider in the same basic manner. There are other methods for achieving the result. This is just one.

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Re-design (author)2009-09-28

Or this one. You'll have to modify it to drive more leds though.

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Re-design (author)2009-09-28

See if this link gives you enough info to figure out what to do.

If not google "light sensor schematic" and start from there.

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