10002 Motion sensors?

Motion sensor madness

Picking the right one for my smart home project is not trivial. From left to right – Osram Nightlux, Ikea Molgan, Ebay sensors with sensitivity and timing adjusment (BISS001), smaller ebay sensor with no adjustment, another ebay sensor. As you can see I have looked at 2 different options: use a ready made sensor and fit it with a radio, or build my own.

Ikea Molgan

I hacked one a while ago – was my biggest preference. It looks the best out of all of them and comes with a nice holder (beware – the glue is strong enough to come off with the wall finishing!). It burns 70 µA in sleep, so a 1 year life is quite on a tight budged from AAA batteries. But I discovered it has a problem: every time the radio was transmitting something else, the motion sensor would trigger, which is not acceptable. Even with separate regulator for the radio and micro, the problem stays. The radio probably induces enough interference to trigger the sensor.


I made a small test where I put one in a box along with 2 other nodes and confirmed that it is not affected by the radio of nearby nodes, but the power glitches caused by the radio. Even though it has its own separate regulator, without very big decoupling on the battery (towards at least 1000uF) I could not get it to not trigger accidentally.


Osram Nightlux

Was less sensitive and it since it was waterproof, it is quite difficult to get open and remove the LED part, where I hoped to plant the radio, so it is out. Wall attachment is also quite bad.

Ebay “big” and “small” PIR sensors

The one in the middle of the picture works off 3 AA batteries – no problems. The one on the right most is the sensor powered from 2 AA batteries with a step up using TLV61225, and surprise surprise – it gets very very insensitive. The same is valid for the smaller version. I have verified multiple combinations and also separately from a MCP1640, with the same results – a high frequency, high ripple power supply is a no go. If compactness is most important, than probably enough filters could make for a clean supply. But, as mentioned in Lessons Learned – there are more issues with step up regulators on this high power radio. These sensors are usually based on a BISS0001 IC and burn about 50 µA.

AS312 (AM312)

These are tiny integrated and fully digital sensors which consume only 15 µA. Sweeeeeet. They have no control of sensitivity, however they have the highest sensitivity I have seen. Range is not so much due to the small lens, but they are ideal for what I want: place them on the ceiling to understand where the motion is.  They will easily false trigger from things like wind and moving curtains. Because of the low consumption, they are the ones I chose. You can check out some initial progress using these with a charge pump.

A little teaser for what is to come, this is a completed motion sensor:

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  1. Pingback: 10002 Short assembly run – Electro Bob

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