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Externally powered light/dark switch

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Newbie level 2
Sep 23, 2009
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Light/dark switch

Hello everyone,

I have a question that I (hope) is pretty basic. I'm working on a project where a small hydrogen fuel cell powers a small car motor. Everything is peachy, but I want to include a sensor to cut the circuit when light is no longer incident upon it.

What I tried before (unsucessfully) was having a photoresistor in series with the fuel cell and the motor. The fuel cell gives about 6-8 volts (on a good day). The problem was, even with as much light as I could get on the photoresistor, it added too much resistance (around 12ohms with light, 200 ohms without) to the circuit and was no longer able to move the motor.

So, what I was wondering is if there was a way to make a photoelectric switch thats externally powered. I'm not allowed to power the motor with anything but the fuel cell, but I can power any other component on this car with batteries if need be.

I feel like this is a really elementary question, but I am pretty uninitiated. If anyone could help me out, I'd appreciate it! I just want to know if this is possible, and if so, where to get the parts to make it happen.


Re: Light/dark switch

I think what you need is a comparator driving a relay so the cell switches off at a certain, predetermined light level.

Use the photoresistor and a fixed resistor to make a potential divider and feed it to one side of a comparator. Use a potentiometer on the other comparator input so set the light level at which it will change state. The using a transistor as a current booster, turn the relay on or off. The relay contacts go in series with with the motor.
Power all this off your battery, you should be able to draw very little current until the relay coil is energized.

If you use the schematic suggested by Treitmey add a fixed resistor in series with the variable resistor otherwise setting it to minimum resistance will cook the photo resistor and probably the control as well!


Thanks guys. I'll try your suggestions. They've been a big help.

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