Light switch sensing

[George]

I've built a device that you plug your bedside lamp into, and after a preset time, it slowly dims down to a preset value (upto as far as not being illuminated) the trick is... I have to be able to sense the light switch at any phase of the operation - if switched off - then timer etc resets. I've done this by winding a current transformer and putting output to an opamp. However this has some drawbacks, namely if the amp is set too sensitive it gets triggered during the dimming phase soley by the capacitance in the lead and on the other hand not sensitive enough and it doesn't pick up lightbulb when either fully on or pretty much off - you can tune it so it works with a bulb of a certain wattage and a lead length of a certain amount - but not ideal!!! Any other ways anyone can think of for sensing when the light is turned on/off? I was thinking about trying to send a 5v pulse down the wire at every zero crossing to pick up if switch is on or off - but am a little unsure if it will work or how exactly to do it. Any ideas would be really appreciated.

Cheers

[BobK]

Hi George,

www.Smarthome.com has 2-way X-10 devices that can tell you the status of the device you are controlling. But if you want to continue on with building your project yourself, Micromint sells a chip called the PLIX. It handles al of the X-10 coding so you don't have to. The chip is available both in parallel and serial configurations. Check out www.Micromint.com for more details. They have examples of using a PIC with the serial device and the parallel device is for use with your PC.

BobK

[George]

Thanks, tho I'd like to build myself, just for the experience if nothing else! I'm thinking of setting up an oscillator which will send a tone down the line at every zero crossing, however I'm not sure on how to receive the tone on the other end due to mains potential. So basically I have a triac powering - just before I fire the triac I'll be listening for the tone on the output of the lamp, if the switch on the lamp is turned off, no tone should come back.

Does anyone have any ideas how I can isolate that nasty mains potential either side of my the lamp circuit (bear in mind also that there will be rapid voltage transitions when in the middle of dimming.

Thanks

[ardhuru]

Hi Bob,

Why not look for the 50/60 hz pulse itself, taken from the live side, and thru' an opto?

Or am I missing what your reuirement?

Regards,

Anand

[George]

Because if the triac has the light on, you won't see it as that side will be grounded, you could do it if you missed a pulse now and then - but you would also see that by the light flickering.

Thanks

[Charles Linquis]

Have you thought about using a Hall-effect current sensor. The output is directly proportional to the current, and can be heavily filtered eliminating any noise from the switching.