Comparator/Schmitt Trigger 'low' is too high!

Thread Starter

dannybeckett

Joined Dec 9, 2009
179
Hi guys, I've made the schmitt trigger circuit 2/3 the way down this page -

http://www.swarthmore.edu/NatSci/echeeve1/Class/e72/E72L2/Lab2(OpAmp).html

When I tested this circuit with an LED everything seemed fine. I then inserted a transistor to the output stage which made the LED light continuously. I measured the voltage at the op amps output - the circuit was still working, but the 'low' voltage was about 1.5V! High is about 15V.

I switched back the LED and I observed the same thing, but the LED responded as if the low was 0V (completely turned off). Maybe to do with the op amps output current limitations? Anyway that isn't the issue here, I'm using this circuit to drive a relay so I need that transistor in the output stage. How can I make the op amps 'low' voltage 0V? I'm using a single rail power supply by the way.

Cheers guys!
 

retched

Joined Dec 5, 2009
5,208
You could use a pair of diodes to get a 1.4 to 1.5v drop, or a resistor in series with the transistor.

Your opamp likely will not go to 0v without a split supply.

I would try the diode drop, or the resistor to get closer to 0. Depending on the transistor you are using, you will need to drop that voltage below ~.7v to shut it off.

If you have a negative supply available, use it. It will make your life that much easier.
 

Thread Starter

dannybeckett

Joined Dec 9, 2009
179
Unfortunately I dont have one available in this circuit. Ive put a 100K resistor inbetween the base and output of the op amp, it did help, the LED dims when the op amp is off and is bright when it is on. I think the string of diodes is the solution to this problem, albeit a lame one (that isnt a dig at you btw lol)
 

eblc1388

Joined Nov 28, 2008
1,542
You really need *TWO* resistors, forming a voltage divider to drive the transistor properly from the input voltage of range of 1.5V ~15V.

Try 22K from Schmitt Trigger output to transistor base, with 10K from transistor base to 0V.
 

Thread Starter

dannybeckett

Joined Dec 9, 2009
179
last update - the potential divider network was not allowing enough current through to switch on the transistor properly. It has been removed and replaced with 6 diodes which does work 100% =]
 

tom66

Joined May 9, 2009
2,595
Instead of 6 diodes you could try a 4.3V zener in reverse bias. It's close enough to the 4.2V approximately dropped by the diodes.
 
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