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Mains current detection

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Hi, All

Does anybody have a mains current detection circuit as follows:

Detect mains current flowing through a wire (powering a saw), and switching on another 220 v device (vacumn cleaner) startup delay of a a few se! cs would be nice, and switching off once saw is off (after a couple of secs would be nice too).

Anything out there already?

Many thanks.


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If it is for your saw (noisey device), why not use a sound activated switch trimmed to the sound difference of when the saw is turned on? When turned off, as the blade winds down, the switch would still be activated until the blade winds down to the quiet threshold you have set.

Just another concept for the same result, except you do not have to mess with the mains.


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I like the Moderator's idea, but would this device accidentally get triggered by other noisy power tools? (obviosly the power to YOUR device would have to be ON).

Just something to keep in mind.

Here's a poor man's solution: Do you have to pull a trigger to operate the saw? If so, can you somehow use the same trigger for your vac? 8)

Good luck.


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Darrin, good idea, the sound trigger could be connected after the switch. Of course, there would have to be another way to keep the vac on after the switch is turned off. Perhaps RC time constant.

Another method around this would be to make the sound switch only trigger right at the high threshold of the saw noise, which would always be louder than other equipment.

Still, another way might be to wrap some wire around an iron core and place it close to the power cord of the saw motor to make a pseudo-transformer. Then use this circuit as the trigger to turn on the vac. As I recall, Ante has done something similar to get an extra power source from a transformer. Ante? Do you have an idea for this?


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  • 2 weeks later...

I went looking for such a device at my local hardware store. I sure had trouble explaining the function and purpose. I finally found a vacuum cleaner with one built in, but didn't feel like buying that day. Just the same, a slave outlet would be handy. MP and Ante, I would and will use a method similar to what you suggest- I have on hand a few current transfomrers (1000:1 ratio). When I get mine done I'll let you know how well (or poorly) it went.

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