# Lasers and motion detectors



## Arilou (Oct 6, 2013)

A lot of new construction has motion detectors in the restrooms, which turn on the lights. If the lights go out on you, you can stand up and wave your arms, or point a laser at the motion sensor.

I've noticed that, at least for cheap laser pointers, red lasers seem to work better than green ones for this. This surprises me a little bit. PIR motion detectors are designed to react to >8000 nm, so I wouldn't have thought 530 vs 650 nm would make that much of a difference. My violet laser also triggers the motion detector easily, but this is probably due to the lack of an IR filter.

I'm assuming this effect is just due to the laser heating up the pyroelectric sensor, but maybe something else is going on?


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## PhotonWrangler (Oct 6, 2013)

I would not be surprised if the pyroelectric sensor had a small amount of photovoltaic sensitivity also. I'm guessing the pointer causes a little bit of heating along with a little bit of photovoltaic effect. The trigger circuit is looking for pulses, which is why the fresnel lens is divided up into lots of facets - as the object crosses each facet boundary it creates a small pulse in the sensor.


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## Arilou (Oct 7, 2013)

I'm guessing it's mainly dependent on the total power output rather than the wavelength. The green laser appears much brighter than the red laser, but that's just because the human eye is more sensitive to this wavelength.

Most cheap 532 nm lasers are frequency-doubled 1064 nm DPSS lasers, and they usually don't have infrared filters. I'd expect quite a lot of IR output from these lasers, so it surpises me a bit that it appears to be less effective than the direct 650 nm diode laser. Maybe I'm overestimating the amount of IR that actually comes out of a 532nm laser.


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## PhotonWrangler (Oct 8, 2013)

I found a pyroelectric sensor that I had sitting around so I hooked it up to a VOM set to read in the megohms range. I was able to get a significant reaction from the sensor when flashing a small keychain light on it, and the light generates almost no heat so I'm gathering there is a small photoelectric effect.


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