Hello everyone,

I would like to create a circuit that will compare two input voltages and turn a small 1.5V motor on/off accordingly. This sounds like the perfect job for a comparator!

The comparator I am using is a LM339 Quad Comparator from radioshack. All the specs can be found here: http://support.radioshack.com/suppor...oc31/31971.htm

I attached a basic drawing of my circuit. (my MS paint isn't working, sorry ) The circuit is currently on my breadboard and is not working. The power supply is a 9V battery. I have no idea why it isn't working, (incorrect circuit???)

If you know what I did wrong please tell me so I can get this circuit to work.

thanks,

George

Attached Files

circuit-help.doc (22.5 KB, 88 views)

 

You said you want to compare voltages, but your drawing shows photocells. You need to tell us exactly what you are trying to do.

 

Hi George,
1) A comparator compares different voltages at its inputs. Your circuit always has the same voltage on its inputs. The photocells must be part of a voltage divider so that the inputs change their voltages when the light changes on the photocells, and the input voltages must remain within the input common-mode range of the comparator.
2) The quad LM339 is the same as an 8-pin dual LM393. Their minimum output current is only 6mA which probably isn't enough to drive your motor.

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Uncle $crooge

 

How far are your photocells from each other?
are they ON one at a time or both can be ON at the same time (By ON I mean expeosed to light)
Put pull down (connected to GND) resistors at both the inputs of the comparator (better to put same value resistors) and then
Just remove the motor from the output and try measuring the output with a multimeter.

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