# Help regarding comparator

Discussion in 'Homework Help' started by narsil, Aug 12, 2006.

1. ### narsil Thread Starter New Member

Aug 12, 2006
2
0
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.

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
How are you hooking up the touch switch contacts to the op amp?

3. ### narsil Thread Starter New Member

Aug 12, 2006
2
0

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

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
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.

5. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
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.

6. ### Rail Ranger Member

Aug 22, 2006
15
0
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?

7. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
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.