There is no Application jou can just build out of
a 555
Actually I recall a 70's hobbyist homebrew optical paper tape reader
(in BYTE?) where the Schmitt Trigger for each photodiode was nothing but
a 555, NOT BEING USED AS A TIMER, just as a Schmitt Trigger. Unlike
using real "schmitt trigger" parts the 555 had a built in resistor
network that set trip points to 1/3 and 2/3 Vcc with no external parts.
There's a hand-pulled commerical tape reader like that shown in the
'S100 Handbook'. A scehamtic is give, but the ICs are given a hose number
only (all are the same). They are clearly 555s from the pinout, I assume
they were selected to have the right thresholds.
-tony