The Medway Handyman submitted this idea :
Now that I've found an adaptor to connect my Wickes/Earlex vac to my circ
saw, I was wondering...

You can get vacs with a socket outlet on, so you can plug a power tool into
it. When you start the power tool the vac switches on, stopping the power
tool switches off the vac.

I assume some kinda electronic gizmo senses the load and does the switching.

Can you buy said gizmo to fit to an existing vac?

Don't know about the suitability (check the ratings), but you can get a
power strip for a computer, which turns on/off several sockets operated
by the current drawn as you turn the PC on and off. Turn the PC on and
the other sockets become live, turn it off they all turn off.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk