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Sense motion and environmental changes with a passive infrared sensor (PIR) on the ATtiny1627 MCU family.

When I was in college, I was friends with one of the lab engineers that managed the electronics labs in the department. One of the problems that he worked on was to count students in a room in order to quantify lab usage over time. There are many approaches to this problem – visual cameras, radar sensors, door switches, etc… However, the solution would also have to be compliant with laws regarding privacy and be installable without damaging the lab space – this meant that some solutions, such as visual cameras, were almost immediately off the table.

While he has since retired, the problem is still an interesting exercise. One possible way to solve this problem would be to use a Passive Infra-Red (PIR) Sensor. PIR sensors passively absorb IR radiation in the field of view of the sensor. When the amount of IR in the environment shifts, the sensor’s output changes. This appears as a small AC signal on a large DC bias. By monitoring the differential voltage, which cancels out the DC offset, changes in the environment can be detected.”

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