Photointerrupters

Photointerrupters are a smart solution for detection of
rotation or linear movement of an object. The basic principle
behind photo interrupters is well known to everybody who ever
used an elevator: interruption of a light beam is used to detect
whether there is an object between the light source and the
detector. Both are placed face to face at the left and right hand
side of the elevator door.
On a much smaller scale, the same principle is used for photointerrupters.
In most photointerrupters the light source is an IR-LED
and the IR-detecting device is a phototransistor. This is the
most convenient combination because the output signal of the
phototransistor can be used without additional amplifier in the
evaluation circuit.
Some barrier is moving within the gap between the IRED and the
phototransistor. Depending on the position and the shape of the
barrier the light beam is interrupted or not. For most mechanical
applications the speed of the phototransistor is sufficient to detect
the movement of the barrier within the gap of the interrupter.
The basic characteristic of an optical interrupter is the width d
and depth h of the gap as well as the dimensions a and b of the
aperture in front of the phototransistor. The gap dimensions are
important parameters for the mechanical construction of the
system and mechanical tolerances of the whole system (e.g.
position of the interrupter and the barrier moving within the gap)
should be considered very carefully. The size of the aperture in
front of the phototransistors determine the position resolution of
the barrier moving in the gap of the interrupter.
Few interrupters have two phototransistors on the detecting
side. This allows to detect the direction of movement of the barrier
within the gap.