Sears sells a switch for this purpose called an autoswitch.
http://www.sears.com/sr/javasr/produ...=00 924031000

It's only for 110V though and works well with my shop vac and saw. I have
not tried it on my HF DC yet as
I have not run the duct yet so do not know if the load will be too much for
both on the same circuit.
Chuck

"Dave Mundt" wrote in message
...
Greetings and Salutations....

On Mon, 26 Sep 2005 22:46:17 +0100, "BillR" wrote:

My neighbour had a tradesman come round to fit some doors. He had a hand
planer and a vacuum cleaner. The planer plugged into a socket on the
vacuum
. When he switched the planer on the vacuum came on also - when the planer
was switched off the vacuum ran for 10 secs and switched itself off.

This sound like a good idea to adapt to my dust extractor and lathe. I
can't
figure out how to do it and don't know what the device was called, so that
I
can 'google' it.

Does anyone have a circuit diagram or know how such a device works?

Before anyone suggests putting my vac and lathe on the same switch, which
is
the obvious solution, my lathe is blessed with a NVR switch (i.e. if the
volts to the lathe disappear - the switch goes to the OFF position.)

Here in the USA, there is a readily available relay controller
that senses current and can do this with ease. It SHOULD be around
for y'all too, although I suspect it would be an item from a "real"
electrical supply house - not one that caters to the typical home
owner.
Fine Woodworking magazine (I think)-
http://www.taunton.com/finewoodworking/index.asp
had an article on wiring up this circuit within the
past couple of years. It is pretty simple to do, actually,
but, as with all things electrical requires a certain amount
of care and attention to detail. As I recall the sensor
is not all that expensive - $70 or so.
In short - you run the hot leg of the circuit for
your tool through a donut coil on the sensor block. You
attach a relay of appropriate capacity to the control
outlets of the sensor block. You then hook the dust
collector up to the power, through the relay.
When you turn on the tool, the sensor sees the
current flow, and automatically switches on the dust
collector. I believe that some models will also allow
you to specify a short delay for on/off, to ensure
that both the electrical load is lowered, and, the
last of the chips will get pulled away when the working
machine is turned off.
I have considered this for my shop, but,
at the moment it is sitting in the "projects it
would be NICE to do" bin. A lack of time and money
and the fact that I have wired the switch for the
dust collector in a very handy spot has made it less
urgent.
Actually, when woodworking in the shop,
I tend to simply turn the collector on and let it
run the entire time I am making dust. I find that
not only does it do a great job of picking up the
immediately created sawdust, but, it keeps the
air cleaner in the process - and that makes finishing
the project a happier and easier process.
Regards
Dave Mundt