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[on:]

Hi all,

I have have circular saw used for woodworking and I want to be able to turn a number of other AC outlets on when the saw is in use (eg, fan, vacuum for sawdust, etc). Does anyone know of an existing implementation of this idea?

If not, it looks like a CT (Current Transformer) is the most obvious way to detect that the master outlet (circular saw) is operating. Any other less obvious, and possible less expensive, ideas as a sensor for doing this?

Once the current has been detected turning on a relay to power the other outlets is easy ...

[Reply #1 on:]

Yes, you could use a current transformer such as http://au.element14.com/triad-magnetics/cst-1005/current-sense-transformer/dp/1615369. Another possibility is a Hall current sensor, although that will involve a little more work making the connections safe.

Where do you switch the saw on? Depending on the answer, it may be sufficient to just detect that there is mains voltage in the cable to the saw. That's easy, you just need to wrap a wire around the cable and connect that to an arduino input with a high value pulldown resistor. Adjust the value of the pulldown resistor to get the sensitivity you need. If you are not using an Arduino, you can use a 2N7000 mosfet and a couple of protection diodes to pick up the signal instead.

Why not just power the other devices from the switched supply to the saw?

[Reply #2 on:]

The idea is that the saw is plugged in and activated with a remote switch on the saw handle. The other devices are separately plugged in and turned 'on' but not operating because there is no power at their outlet. Once the saw is turned on, this is detected and power allowed to flow through to the other devices.

Your wire idea may work ok. Does this need to be around one of the AC wires (ie just the Active line)or will it work ok wrapped around the entire cable, which in my case will be earth, neutral and active?

[Reply #3 on:]

Your wire idea may work ok. Does this need to be around one of the AC wires (ie just the Active line)or will it work ok wrapped around the entire cable, which in my case will be earth, neutral and active?

All that's needed is that there is some capacitance between the live (active) wire and the pickup wire, so either should work.

I remember making a device that used this technique around 40 years ago. One application was to find out which lamp in a string of series-connected Christmas tree lights had failed, without having to take all the lamps out.