Hi,
I have made an impulse purchase off the 1 Euro shelf of our local household appliances shop recently. I got an automatic closet LED light, pretty much identical to this one:
http://www.amazon.com/Automatic-Magnetic-Opening-Drawers-Cupboards/dp/B007TMBIXA. I did not expect it to work very well given the price and the Amazon reviewers seem to agree with me that it is a bit crap. I don't mind, because I just wanted to take it apart to see what is inside and how it works anyway. I don't fully understand how it works though.
The thing uses a magnet and a magnetic sensor that looks to me like a sort of cheap reed switch with the "normally-open" bare contacts soldered directly to the PCB. If the magnet is close enough to the device, then the switch is closed and the LEDs are off. The thing draws around 2 microamps in this idle state, i.e. when the closet door is shut and the LEDs are off. If you move the magnet away, the switch opens and the LEDs turn on.
I am pretty sure the switch is "normally open". I shortened the switch with a cable instead of using the magnet to make sure. Now I have two questions:
1. Any idea why the designer used this "reverse logic"? I would have expected the switch to work the other way around, i.e. let the switch be in the normally-closed position and if the magnet is close enough, open it to turn the LEDs off.
2. What is this mysterious component under the black dot?
Thank you.