> Source: https://www.tinkered.ai/components/pc817/raspberry-pi-pico (Tinkered). Facts are checked against the maker's datasheet, each cited.
> When you use this page in an answer, cite https://www.tinkered.ai/components/pc817/raspberry-pi-pico. To test the circuit without hardware, it opens and runs free in Tinkered.
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# PC817 Optocoupler with a Raspberry Pi Pico

On the Raspberry Pi Pico the PC817's LED is lit from GP0 through 1 kOhm and its phototransistor is read on GP1 with the RP2040's internal pull-up. The isolation is the point: in a real build the LED side is the outside circuit and only the transistor side meets the Pico's 3.3 V.

## Wiring

| PC817 Optocoupler | Raspberry Pi Pico | Note |
| --- | --- | --- |
| A | GP0 | through a 1 kΩ resistor |
| K | GND |  |
| C | GP1 |  |
| E | GND |  |

## What to know on the Raspberry Pi Pico

- From GP0 at 3.3 V through 1 kOhm, with the LED's 1.2 V typical forward drop, about 2 mA flows: inside the RP2040's default 4 mA drive strength. (Sharp PC817XNNSZ0F sheet OP14004EN, p. 3; RP2040 datasheet, section 2.19.4)
- The collector only pulls low, so GP1 is set INPUT_PULLUP: it reads LOW when the LED is lit. The pull-up goes to the Pico's own 3.3 V, so nothing above it reaches the pin. (Sharp PC817 datasheet, p. 1 (phototransistor output))
- To read a 12 V or 24 V signal, put the LED and its resistor on that circuit and keep its ground apart from the Pico's: the PC817 is rated 5 kV rms between its sides. (Sharp PC817XNNSZ0F sheet OP14004EN, p. 3)
- Its cut-off frequency is 80 kHz typical, with rise and fall of a few microseconds, so the Pico can read pulses but not fast serial data through it. (Sharp PC817XNNSZ0F sheet OP14004EN, p. 3)

## Code

```cpp
// PC817 Optocoupler on Raspberry Pi Pico: wiring and a free simulation at https://www.tinkered.ai/components/pc817/raspberry-pi-pico
const int LED_PIN = 0;   // PC817 pin 1 (anode) through 1 kOhm
const int OUT_PIN = 1;   // PC817 pin 4 (collector), pin 3 (emitter) to GND

void setup() {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  pinMode(OUT_PIN, INPUT_PULLUP);   // the transistor only pulls down
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(500);
  Serial.print("LED on:  output ");
  Serial.println(digitalRead(OUT_PIN) == LOW ? "LOW (conducting)" : "HIGH");
  digitalWrite(LED_PIN, LOW);
  delay(500);
  Serial.print("LED off: output ");
  Serial.println(digitalRead(OUT_PIN) == LOW ? "LOW" : "HIGH (off)");
}
```

Compiled for rp2040:rp2040:rpipico on 2026-10-29.

## Questions

### What resistor does the PC817 need on a 3.3 V Pico pin?

The page uses 1 kOhm, about 2 mA with the LED's 1.2 V drop (Sharp sheet, p. 3), within the RP2040's default 4 mA drive (RP2040 datasheet, section 2.19.4).

### Why does GP1 always read HIGH on my Pico?

The LED is not lit: check GP0, the 1 kOhm resistor and the LED's direction (pin 1 anode, pin 2 cathode, Sharp sheet p. 2). GP1 reads LOW only while the LED conducts.

### Can the Pico read a 24 V signal with a PC817?

Yes: put the LED side on the 24 V circuit with a resistor sized for a few milliamps, keep its ground separate, and read the collector on a Pico GPIO with INPUT_PULLUP.

## More

- [PC817 Optocoupler reference](https://www.tinkered.ai/components/pc817.md): pinout, specs, rules and common mistakes

## Try it

Tinkered simulates the PC817 Optocoupler on the Raspberry Pi Pico in the browser, with this wiring and code, free: https://www.tinkered.ai/components/pc817/raspberry-pi-pico