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Is there a way to lower a photocell's resistance to amps? Answered

I am making a device which uses a laser to lower a photocell's resistance to the point where enough current can flow to begin spinning a motor. Unfortunately, the motor (4.5 V) starts off by drawing about .75 amps and once running falls to .25 amps, but apparently the photocell can only draw a lot less- the only word I've been able to find is that it typically draws about 1 milliamp, which is nowhere near enough. I have two questions- first off, does anyone know how many milliamps I can run through a photocell, and is there any way that is pretty cheap so that I can lower the amps drawn or the resistance of the photocell without stopping the motors?

7 Replies

kelseymhBest Answer (author)2012-01-27

Use the photocell to trigger a relay.

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seraine (author)Re-design2012-01-27

That's what I was thinking, but I haven't really used relays and I have no clue as to what kind I'd use or how I'd do it. Any ideas?

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kelseymh (author)seraine2012-01-27

Well, you need a relay which can handle 1A at 5V (to allow for overages) on the output. For the photocell, you need the relay's input to trigger at a few mA, at whatever voltage you've got across the photocell. Search on Digikey, or Mouser, or Jameco, for a relay with those parameters.

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seraine (author)kelseymh2012-01-28

I went on jameco, and from what I know I think that one of these two would do the job: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_187240_-1 or http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_174450_-1 .

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lemonie (author)kelseymh2012-01-28

Or an appropriate transistor.


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iceng (author)2012-01-28

I have a TO-3 size ( cadmium sulfide ) 1.5Amp photo resistor
not available any more and that would be a sledgehammer way
to run your motor.
The photo resistors reasonably available to you are in the millamp range so you need a device that can control your motor with a limited
current sensitive photo resistor.

A sensitive relay is the best way to go to run your motor on / off.
Or an op-amp if you want speed control.
Or a micro processor if you want more sophistication.

You need to select a std battery voltage before any circuit designs
can be suggested.
( 3 AA cells deliver 4.5VDC ) not very good for relays or for op-amps.

There are low current coil relays that work on 5v that could run your
motor but you will need a higher current photo resistor
see the pictures
Otherwise you will need a transistor amplifier before the relay,
Anyway this problem is electronic engineering with what you have
and what you can get.

kelseymh gave you good advice but I sense that  you need more, like
some body to design this just for your application.


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