Shawn Ellis wrote:
>
> Ok, stupid question time..  Could someone please define for me once and for
> all the meaning of the "Schmitt Trigger" and how it differs from TTL?  Also
> the Schottkey, but I think this is just a low power TTL...

Schmitt trigger circuits are designed to convert slowly varying analog
inputs into fast switching digital outputs that can be fed into standard
logic gates.  Digital gates want their inputs to switch instantly from
low to high and vice versa.  They tend to do strange and nasty things,
such as oscillate or burn up, when their input voltages rise to the
point where the output starts to switch and stay there.

Typically, the output of a schmitt trigger stays low as it's input rises
above zero until the input reaches a threshold voltage that is usually
somewhere around half of the power supply voltage.  At this point, the
output instantly (a few nanoseconds) changes from low to high.  The
output then stays high until the input voltage goes a bit lower than the
original threshold voltage.

The difference between the input voltage that switches the output high
and the lower voltage that switches it low again is called hysteresis.
It keeps the schmitt trigger's output from switching rapidly up and down
due to noise and ripple on the input.

Schmitt triggers with inverting outputs are also available, of course.

Dave, N9LTD