PIR sensor firing every 20 seconds

[Jan Aerts]

Hi all,

For some reason my PIR sensor (HC-SR501) fires exactly every 20 seconds. These false triggers happen even if the sensor is put inside a closed cupboard (so definitely no movement or temperature changes), and turning the knobs on the sensor itself does not have an effect.

Does anyone have a clue what might cause this?

Thank you!

[mfalkvidd]

Hi @jan-aerts, welcome to the forum!

Does it trigger even if not connected to an Arduino? (power the PIR from battery or stable bench power supply and measure the output with a multimeter to eliminate the Arduino and the power supply as possible source of the problem)
esp8266 is known to cause false triggers.

Broken sensor och short circuit somewhere on the board could cause problems as well.

[bgunnarb]

In my experience PIR sensor are very sensitive to spikes in the power supply, especially if you run them on voltages close to the lower limit. I feed mine with a separate +8 volt supply. If you run it on the same 5 volt supply as the ESP8266 you are almost guaranteed to get false triggers. The 20 seconds is probably the time after which the PIR resets. This causes a spike on the supply which triggers the PIR again. This happened to me.