This is IC 4N35 an Optocoupler with Transistor Output, 1 Channel, DIP, 6 Pins, 50 mA, 5 kV. Each optocoupler consists of gallium arsenide infrared LED and a silicon NPN phototransistor

Applications:

1. AC mains detection
2. Reed relay driving
3. Switch-mode power supply feedback
4. Telephone ring detection
5. Logic ground isolation
6. Logic coupling with high-frequency noise rejection