Sunday, August 16, 2015

Adventures in Low Power PIR Sensors



As part of a larger project to be discussed later, I have been working on coming up with a very low power wireless motion sensor. PIR (passive infrared) sensors are normally used for these kinds of applications. The problem is that they are very thirsty, usually consuming something in the neighborhood of 80-100 uA while sensing, and 200+ uA once motion has been detected.

Looking at most cheap PIR sensors available from China, we can see that there are usually one or two active items on the PIR module: a voltage regulator and/or op amp. I reasoned that these components would be responsible for a good chunk of the power consumption, and started looking for alternatives.

I stumbled on this small PIR sensor which appeared to have no active components like regulators or op amps, and bought a couple. I measured the current consumption at 3V and was impressed to see that it was indeed roughly 27-28 uA.

The image you see above is a Moteino with the ATmega328 put into power down, the RFM69 radio module is in power down, and the PIR sensor providing an interrupt to wake the ATmega328. The only trick was to swap out the 3.3V regulator for a 3V variant, because I found that the PIR sensor consumed significantly more power at 3.3V compared to 3V. As you can see, this achieves an overall power consumption of about 29uA.

I decided to pop the white plastic lens off the PIR sensor and see if I could find the manufacturer's part number - rather than just the "SimplyTronics ST-00082" part number.




It appears to be an AM322, manufactured by Nanyang Senba. They could probably be purchased for pennies in large quantities.

While 29uA for the entire wireless motion sensor was pretty good, I wasn't done yet. I wanted to see how low I could go. I poked around on Digikey and was intrigued to find a bunch of PIR sensors from Panasonic that claimed to consume a miserly 1uA of current while sensing. I picked up an EKMB1101111 from Digikey. Unfortunately I think the 1uA figure is bogus. It appears to hover much closer to 100uA, which is more realistic. Perhaps there were other factors influencing my measurement, but I doubt it. For now, the AM322 is the best part I have found for this device.

(Article originally published 11 February 2015, and appears to have been mistakenly deleted somehow.)

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