Introduction
Many modern microcontrollers like ESP32, Raspberry Pi Pico, and STM32 use 3.3V logic. Traditional PIR sensors like HC-SR501 are designed for 5V. Connecting them directly can cause problems.
The Problems
1. Power Supply Voltage
HC-SR501 requires at least 4.5V to operate reliably. At 3.3V, it may not power up at all, or may behave erratically.
2. Output Voltage Level
Even if powered at 5V, the HC-SR501 output is typically 3.3V, which is safe for 3.3V inputs. But some clones output 5V, which can damage 3.3V inputs.
3. Logic Level Mismatch
If the sensor outputs 5V and you connect directly to a 3.3V input pin, you risk damaging the microcontroller.
Symptoms
- Sensor doesn’t power up when connected to 3.3V.
- Erratic readings, random triggers.
- Microcontroller eventually fails (if 5V applied to input).
Solutions
1. Use 3.3V-Compatible Sensors
The best solution: choose sensors designed for 3.3V operation:
- AM312: Works from 2.7V to 12V, 3.3V output.
- Panasonic EKMB series: 3V versions available.
- Excelitas PYD 2597: 1.4-3.6V operation.
2. Power HC-SR501 from 5V, Use Level Shifter
If you must use HC-SR501, power it from a 5V source (USB, separate regulator) and use a level shifter or voltage divider on the output.
PIR OUT ──── 10k resistor ────┬──── GPIO (3.3V)
└──── 20k resistor ──── GND
3. Check Your HC-SR501 Output
Measure the output voltage with a multimeter when triggered. If it’s 5V, you must use level shifting. If it’s 3.3V, you can connect directly (but still need 5V power).
4. Use an Optocoupler
For complete isolation, use an optocoupler between the sensor output and the microcontroller.
Case Study: ESP32 with HC-SR501
A user connected HC-SR501 directly to ESP32, powering the sensor from ESP32’s 5V pin (when USB powered). The sensor worked, but after a few weeks, the ESP32’s GPIO pin stopped working. The sensor output was actually 5V, slowly damaging the pin. Adding a voltage divider solved the problem permanently.
Conclusion
When using 3.3V microcontrollers, either choose 3.3V-compatible sensors or implement proper level shifting. Don’t assume HC-SR501 is safe – test first.