Lite-On LTH-1550 Reflective Photointerruptor

This part combines an infrared LED side-by-side with a NPN phototransistor. When no object is present in front of the sensor, the infrared light dissipates and the transistor stays off. When an object is close (but not touching), it can reflect light into the phototransistor so it can turn on.

Lite-On LTH-1550-01


Top, side, and bottom views of the sensor. The LED current flows from anode to cathode, the transistor current from collector to emitter. Note that a subtle package corner notch marks the LED anode. Dimensions are in millimeters (inches).


Hand-drawn sketch of the part. Note that the clear lens is the LED, and the opaque lens is the phototransistor. The pins are A,K,C,E for anode, cathode, collector, emitter.

In typical use, the part needs two resistors, one to limit the LED current and the other to form a voltage divider with the phototransistor. Typical values for 5V operation are 220 Ohms for the LED ballast and 22K for the phototransistor bias.


Hand-drawn reference circuit. The sensor is drawn in a top-view for ease, although that means the LED cathode is awkwardly at the top. The pins are physically too close for convenient insertion in our breadboards, but male-female jumpers can be used to extend the leads for testing.