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I'm using a HC-SR501 PIR sensor.
The sensor is working fine, it outputs 3.3 V on motion detection.
But when I connect the PIR sensor to pin 2 (interrupt) of an Arduino pro mini,
the voltage output is only 0.289 V on motion detection.
The sensor is detecting motion, it's just not giving enough voltage.
How is this happening?

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If the sensor is putting out 0 to 3.3 V when open circuit, and 0 to 290 mV when connected to a microcontroller, then the micro is obviously loading the signal.

A normal CMOS input wouldn't load a signal like that, so obviously that input of the micro is not acting like a normal CMOS input. Possible reasons include:

  1. The pin is not configured as a input in firmware. Check your code.

  2. The pin is dedicated to some other function. Check the datasheet.

  3. Something else is loading the pin. Check the schematic and your wiring.

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