MyMotionSensor - a low-power sensor for motion and ambient brightness

[canyouhearmenow]

I needed a couple of motions sensors to automate lighting in hallways and other rooms. Since the lights should only turn on when it is dark, the system also needs to know the ambient light level in the location where the motion was detected -- so I added a simple brightness sensor to each node.

This is based on an Arduino Pro Mini clone, it is low power (runs for >18 months one a pair of AA batteries) and low cost (cost of materials <EUR 10 ).

Source code and schematics are available in Github, and a more detailed description here.

I chose the smaller HC-SR505 PIR sensor rather than the larger HC-SR501 sensor used in the MySensors motion sensor example. The PIR sensor appears to be sensitive to power supply fluctuations. In the first prototype, it would often re-trigger immediately after its output returned to low (inactive), presumably because sending a message via the NRF24L01+ module would draw current from the battery, and cause a drop in supply voltage. Therefore, the software ignores transitions of the PIR sensor output for a few seconds after going to sleep.