A photocoupler is a passive optical component that transfers signals through infrared light between two isolated electronic circuits. The isolation between the input and output circuits is maintained to prevent the high voltage on the input side from damaging the signal receiving electronic circuit. There are various types of photocouplers depending on the source-sensor combinations used to make them. Some of the most common combinations include LED-phototransistor, LED-LASCR, and LED-photodiode pairs. Logic output photocouplers are based on LED-phototransistor source-sensor combination.

Digital logic mode is one of the main operating modes of transistor output photocouplers. Photocouplers that are optimized for operating in this mode are generally referred as logic output photocouplers. Unlike LED-phototransistor combinations in the linear mode that output a magnified copy of the input, logic output photocouplers output logic high or low only. The logic high is obtained when the voltage is approximately equal to Vcc while a logic low is obtained when the voltage is approximately equal to ground level voltage.

The phototransistor outputs the logic high when it is in saturation. In this saturation condition, the power consumption is usually very low. Photocouplers have a wide selection of applications in computing and communications fields. They are also widely employed in photocopiers, automation systems, and light measurement instruments.