Sensors & isolation
Optocoupler
An LED and a light sensor sealed in one package, facing each other across a clear gap. The signal crosses as light, so the two circuits share no wires and can be thousands of volts apart.
Think of it as: Signalling with a torch across a river instead of building a bridge.
Pins
- A / K (pins 1, 2)
- : The LED input. Needs a series resistor, like any LED.
- C / E (pins 4, 3)
- : The phototransistor output. Use it like a switch with a pull-up resistor.
The PC817 in a 4-pin DIP is the classic; there are SMD versions and faster ones with logic outputs.
What to look for inside
- The input switches on and off.
- When it is on, the LED shines across the clear gap inside the package.
- The light turns on the phototransistor, so the output circuit conducts. No electrical connection crosses the gap.
The numbers that matter
| Spec | Typical |
|---|---|
CTR (current transfer ratio) Output current ÷ LED current. It falls as the part ages. | 50 % to 600 % |
Isolation voltage The safety rating between the two sides. | 3.75 kV to 5 kV |
LED forward current Sets how hard the output turns on. | 5 mA to 20 mA |
Switching speed Slow parts round off fast signals. | 5 µs to 20 µs (PC817) |
Where you’ll use it
- Reading mains-powered signals safely with a microcontroller.
- Feedback in isolated power supplies.
- Breaking ground loops between two devices.
Common mistakes
- No resistor on the LED side.
- Expecting the output to switch fast or carry big current. It is a signal device.
Go deeper
Output current is the LED current times the transfer ratio.