Hacking the gap - (opto) Photo Interrupter
Channel: Mike Seyfang
Date: 16/11/2013

The altronics z1670 (BPI-3c1-13) Photo Interrupter ships with a 3mm gap between IR LED and Detector. I cut one in half and moved em apart till it stopped working. Inside on a cool day I could (just) measure a voltage difference between gap open and closed at 20mm (as analog input to Arduino). 13 mm was the sweetspot for a clean enough voltage drop for digital input.