> Could someone suggest me a simple transistor based schmitt trigger circuit
> that uses 3 volts as Vcc and has big difference in turn ON and turn OFF
> voltages at output, i.e. big hysteresis window? I want to use it to
recover
> a slow changing but noisy binary signal (signal that is made up by only
two
> voltage levels between 3 volts).

Jinx's circuit which I've just seen posted is typical of the 2 transistor
basic design and is probably excellent for many applications. Try it for
yours and see if it handles your rise times. If not, consider an IC design
as below.



        Russell McMahon

________________

You will find traditional 2 transistor Schmitt circuits on the web. However,
my experience is that getting a simple design to work well on a really slow
slewing signal is not trivial. I put some effort into this and found that
while I could make something that LOOKS like it works like a good Schmitt it
still didn't work for very low slew clocks (many ms). I could have worked on
it but decided that other solutions were superior in my case.

Unless there is a good reason not to, consider using an existing IC
Schmitt - even if you only use one date. The eg 74c14 / CD40106 is quite
cheap and very effective. The designers had the advantage of using as many
transistors as they felt they needed to do a good job. If you choose
appropriate family you can run from about 2V to about 15V (not necessarily
with the same IC in all cases.)

If you use several gates with Schmitt triggers you can adjust hysteresis
levels to whatever you want with a few resistors to feedback to the input.

Also consider using an opamp. An LM358 (2 amps) is very cheap and am LM324
(4 of the same amps) is often cheaper.
These will operate on from around 5v to around 30v supplies and only draw 1
mA per amplifier. Low power draws less but is a poorer amp. Other opamps
work down to 2v supplies or less but cost more.
Op amp allows total flexibility in design.

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