In the MMDigest 990202 John Kleinbauer described a concept for a simple
roll reader.  He wrote in part:

> The output of each wire could then go to a 10-Kohm pull-up resistor
> that would feed a gate in a 74LS14 Schmitt trigger chip.  The Schmitt
> trigger removes any mechanical bounce from the signal.

The circuit will actually have to be a bit more complicated to provide
the desired bounce removal.  With the circuit as designed, switch
bounce will cause the input to the 74LS14 to swing between ground and
+5V at a rate limited only by the rather small parasitic capacitance of
the wiring and chip input.  The signal swing will be far greater than
the hysteresis voltage of the Schmitt trigger input.

To remove switch bounce, add a capacitor from the gate input to ground.
Choose the capacitor value so that the RC time constant of the capaci-
tor and the pull-up resistor is large enough to keep the input to the
Schmitt trigger from reaching its trip point while the switch contacts
are bouncing.  A time constant in the neighborhood of 0.01 to 0.1
second is probably a good starting point (1 uf capacitor with 10K
pullup).

Paul West