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. I was really thrown for a loop when I saw the schematic. Nine 555's, no R's or C's. I was sure it was a typo or an April Fool's joke, but no, it was for real. Similarly folks cleverly use PIC's (not sure Arduino can do the same but probably) choosing Schmitt trigger inputs and/or internal pull-ups. I'm sure someone could make a 1/3 and 2/3 Schmitt trigger using PIC's and some clever cross-wiring of analog and digital ports. Tim.

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.