Saturday, March 13, 2010

Capacitive On-Off Switch

I dont really like the idea of cutting a hole in the perspex case for mounting a switch. And besides, most computer momentary switches that you find are pretty plain.

So - for the box I will be using a capacitive proximity sensor to turn it on/off.
One of the easiest to find here in China is one of those industrial devices - see below for the one I am using.

Not the smallest thing in the world - however there is not too much of an issue with space around the area where I will be mounting it.

This capacitive switch cost around 35 RMB, which is around 5 USD ??
It has a sensing distance of around 8mm, and works OK through the perspex. Not sure how well it would work through other plastic types, but Perspex is OK.

This is a 12 V device, so it needs to be run directly from the 12V power supply. As it is a 12 V thing, cannot just directly connect it to the motherboard and expect not to have some problems. The switched connection floats up to 12V when not active, so connecting this to the 5V-related on/off pins on the motherboard is probably not a good idea.

So I also grabbed a 12V Reed Relay - to give some isolation. The relay is in a SIP package - so I wired it into the lead of the sensor - can see the lump in the cable in the above diagram.

I mounted the switch so it was at the side of the case towards the front. Reason for this is that others kept suggesting it was a bad idea to capacitive sense - what happens if a cat walks past too close ??
So i figured that if it was on the side, this would not be exposed to places that cats would walk.

So when in place, the case basically just slides down over the top, with the capacitive sensor roughly flush against the inside of the case.
Now when you want to start the PC, you basically rest your hand on the side of the box for a brief moment, and it fires up. Can also turn the PC off.

There are various other capacitive sensing products out there - would be nice if I had the spare time to investigate them... later maybe.

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