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:
- AC mains detection
- Reed relay driving
- Switch-mode power supply feedback
- Telephone ring detection
- Logic ground isolation
- Logic coupling with high-frequency noise rejection