Help regarding comparator

Thread Starter


Joined Aug 12, 2006
Basically, what I need here is make use of this comparator to act as a switch. As long as the user touchs it, it gives a logic 1 and releases it, it gives a logic 0, I'm using LM311 comparator. I am somehow making use of this main property of this LM311, the output is digital, either logic 1 (high) or logic 0 (low), cause its output is always either at its maximum value, or its minimum value.

I actually build a circuit exactly like one 1 source I got online.
( Pls scroll to mid region )

Now, the question is after building out the circuit and testing the output on an ocilloscope, the output do not exhibit such property of IN+ (input 2) is greater than IN- (input 3), the output is high, IF IN- is greater than IN+ the output is low (ground, GND). Basically, the circuit is a very simple one, so there can't be much troubleshooting to be done, what left that makes me nto sure is whether two of the pin, pin 6 and 7, does it have any effects on the circuit? Pin 6 is balance strobe and pin 7 is balance, and I do not have much idea what is it for. Therefore I left it untouch.

Pls do not hesitate to share your comments or help, help is much appreciated. :)


Joined May 16, 2005
Have you checked the voltage at Vi when touching the pad? Is it greater than 2.5v? If not, you might need to adjust your voltage divider values for a different threshold.


Joined Aug 8, 2005
The (-) input is floating, it has no reference point when the touch-pad is not conducting. Try a 1 Meg (or maybe 10 Meg) to ground.


Joined Oct 11, 2004
Connect a 10Mohm resistor from the inverting input to the +v rail.
Connect the LH side of your touch switch to 0V.

When you are not touching the switch, inverting i/p will be at 5V and o/p will be low. When you touch the switch, the voltage on inv i/p will fall below noninv i/p, and o/p will go high.