Mystery component in a 1 Euro device - Page 1
Forum section: electronics / beginners

author: sn60pb40
author_rank: Newbie
timestamp: January 26, 2014, 11:19:39 pm
content: 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.

author: wilheldp
author_rank: Regular Contributor
timestamp: January 26, 2014, 11:23:47 pm
content: It's a COB (Chip on Board). Probably a very small microprocessor die, without a package, bonded directly onto the board. It's nearly impossible to determine what it's doing since you would need to be a chip designer to decipher what chip it is, then have a JTAG interface to get the program off of it.

author: sleemanj
author_rank: Super Contributor
timestamp: January 27, 2014, 01:09:14 am
content: My assumption is that there is going to be a single NPN transistor die (or probably N channel mosfet) and an on board resistor under that blob.Example showing what the circuit probably looks like (except I've used an NPN but it's probably a FET and a much higher value pullup (R2), in order that the off current is low)As for WHY they'd do it like this with the inverted logic... I can only assume it was the cheapest way.

author: rexxar
author_rank: Frequent Contributor
timestamp: January 27, 2014, 05:30:58 am
content: Why would you need any sort of silicon in something like this? Surely the reed switch can handle the current for two LEDs.

author: Codemonkey
author_rank: Regular Contributor
timestamp: January 27, 2014, 07:29:27 am
content: Are the 2 wires on the component side of the sensor bit joined together ? If so, this is probably forming a small loop inductor. The chip under the blob will be measuring the change in inductance when the "magnet", if it is one, maybe just a bit of metal, gets close to the loop.

author: amyk
author_rank: Super Contributor
timestamp: January 27, 2014, 12:41:34 pm
content: I think it's a COB MOSFET. The production quantity was probably high enough to justify the (very slightly) decreased cost over using e.g. an SMD part.Also, it seems you didn't overpay that much...http://www.aliexpress.com/item/7764-YL-358-Battery-powered-sensor-light-LED-automatic-closet-magnetic-light/831828154.html

author: sn60pb40
author_rank: Newbie
timestamp: January 27, 2014, 07:42:04 pm
content: Quote from: rexxar on January 27, 2014, 05:30:58 amThis is exactly what I was thinking, too.

author: sn60pb40
author_rank: Newbie
timestamp: January 27, 2014, 07:47:01 pm
content: Quote from: sleemanj on January 27, 2014, 01:09:14 amThanks for the schematic - I'll definitely try this.

author: DmitryL
author_rank: Regular Contributor
timestamp: January 27, 2014, 07:54:10 pm
content: Quote from: Codemonkey on January 27, 2014, 07:29:27 amIt looks like a piss-poor reed switch, but without a glass case. "A samurai without a sword is like a samurai with one, but only without one."

author: Rufus
author_rank: Super Contributor
timestamp: January 27, 2014, 08:05:53 pm
content: Quote from: sn60pb40 on January 27, 2014, 07:42:04 pmBecause the light needs to be on when the magnet is removed and normally open reed switches are much easier to make.