# Help regarding comparator

#### narsil

Joined Aug 12, 2006
2
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. http://www.swarthmore.edu/NatSci/echeeve1/Class/e72/E72L2/Lab2(OpAmp).html
( 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.

#### thingmaker3

Joined May 16, 2005
5,083
How are you hooking up the touch switch contacts to the op amp?

#### narsil

Joined Aug 12, 2006
2

This is how I connect my touch pad to the op amp.

#### thingmaker3

Joined May 16, 2005
5,083
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.

#### nomurphy

Joined Aug 8, 2005
567
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.

#### Rail Ranger

Joined Aug 22, 2006
15

This is how I connect my touch pad to the op amp.
correct me if i'm wrong, but...is this a form of a schmitt trigger? and if so, why is there a third resistor, and it's purpose?

#### pebe

Joined Oct 11, 2004
626
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.