Dark Activated Transistor Switch

Oct 24, 2007 4 Replies

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duff333 18 years ago

Setup: Voltage divider using a potentiometer and a cds photoresistor controlling the base current to a MPS2222 NPN transistor. 12v supply. Common emitter. Load is a cold cathode light driver circuit.

Problem: When the photoresistor is put into darkness, the cathode tube lights up only half-way. I can adjust the pot to get the cathode tube to light up all the way but when the photoresistor is brought back into brightness, the cathode tube only turns off half-way.

I'm guessing the transistor is not fully saturated. Is there a way to increase the range of the photoresistor (without getting a new one) or some other way to turn this fully on when the lights go out?

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Ecnerwal 18 years ago

Dumb question before you get too far into the driver electronics - is the light sensor adequately shielded from the light it turns on? What happens if you put your hand, or something else opaque, over the sensor?

Cats, coffee, chocolate...vices to live by

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John Larkin 18 years ago

Use a mosfet instead of the 2222. You'll have a lot more margin for tweaking.

A schmitt trigger might be nice, too, so the fet snaps on or off.

John

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Jan Panteltje 18 years ago

On a sunny day (Wed, 24 Oct 2007 12:58:17 -0700) it happened snipped-for-privacy@gmail.com wrote in :

Try using 2 transistors (darlington).

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Jamie 18 years ago

cascade another transistor behind that one. look up darlington transistors. What you need is more gain. I'm assuming you're trying to keep the part count down and advancement to a minimum ? Other wise, I'd use voltage comparator IC.. or an OP-AMP with a little schmitt trigger on it..

"I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5

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