The CNY17-3 is an optically coupled pair consisting of a gallium arsenide infrared emitting diode optically coupled to a silicon NPN Phototransistor. Signal information, including a DC level, can be transmitted by the device while maintaining a high degree of electrical isolation between input and output. The CNY17 can be used to replace relays and transformers in many digital interface applications, as well as analog applications such as CRT modulation.