# Schmitt trigger circuit design suggestions

There is a simple switching sensor which will be wired to a circuit by a BNC cable as shown in the above illustration.

As you see, when the switch is ON the potential at the point "A" will be high; and when the switch is OFF the potential will be zero. Pulse freq will vary from 1Hz to 40Hz.

For some reasons, I need this ON-OFF pulse output to be sharp and clean. So I want to take the point "A" as an input to a circuitry(Schmitt trigger) and obtain a better output.

I need advice when designing a Schmitt trigger circuit which satisfies the following expectations for the output pulse:

It shouldn't bounce, it should be immune to noise and it should be as sharp as possible(very short rising falling edges). And it should work for 5V to 12V Vcc range.

I would appreciate any generic circuit or idea.

• Nothing is ever immune to noise. Also, just do some classic debouncing and whack a schmitt trigger IC after it, not sure what the big deal here is. – PlasmaHH Sep 6 '16 at 10:07
• a known design would help – user16307 Sep 6 '16 at 10:09

Something like this?

simulate this circuit – Schematic created using CircuitLab

R1/R2 and R3/R4 are the voltage dividers to set trigger offset and hysteresis, R5/C1 is the input low-pass for debouncing ($f_c = \frac{1}{2 \pi R C}$). Output is inverted, obviously.

I used V3 to model bouncing and to test the low-pass, you might want to use different resistor/cap values.

• Thanks. Does V2 correspond to the input "A" in my illustration? Is it ok for 5 to 12Vcc supply range as well with the same values? – user16307 Sep 6 '16 at 11:45
• and can I use LM2903 instead? Is TL081 better? – user16307 Sep 6 '16 at 11:47
• Im having 1.8V offset at the output low in LTspice simulation. How make low around 0V? – user16307 Sep 6 '16 at 12:13
• V2+V3 correspond to your input A. Please consider that your "pull-up" must be a magnitude bigger than 330 Ohms, or your switch will not pull A down enough. I didn't choose an OpAmp, that's up to you. If you need 0V low, you need an OpAmp that can handle rail-to-rail voltage, the LM2903 should probably do (~400mV). – michi7x7 Sep 6 '16 at 12:17
• thanks I opened a new question hope u might have an input: electronics.stackexchange.com/questions/256554/… – user16307 Sep 7 '16 at 12:17