Front End Detector

First created: Mon Jan 10 2000
Last updated: Sun Apr 8 02:28:29 EDT 2001

Pototransistor Circuit

phototransistor circuit

phototransistor.sch Cadsoft EAGLE schema file.

The float drives a paper flag that interrupts the optical connection between a phototransistor and a bright diode. As the float rises, the flag moves up and opens the passage of IR light from the diode D1 to the transistor T1. The phototransistor conducts and V1 drops.

As the float drops up, the flag moves down and closes the passage of light. The phototransistor stops conducting and V1 rises to 12 V.

In normal use, the signal V1 will be centered around 6V.


The infrared diode has a flat end on its cathode (most negative) side whereas the phototransistor has a flat end on its collector side (most positive).

For over a year I had the phototransistor upside down, confusing the collector with the emitter. Remarquably, this configuration, although much less sensitive, still works! I had to set R2 at 100K to better use the limited current and R1 at 0.5K to drive more light from the diode.