I developed this project over time so this is a reverse engineered instructable.
I will show parts of another device I started to make as well as the completed one.
Although there may be many ways to do this, this is my way.
Don't attempt this unless you are competent working with electricity. Make sure connections are tight and insulated. Double check your work. Please be careful!
Step 1: Circuit Diagram
This is the circuit diagram of the device.
Step 2: Start With a Metal Box.
Start with a triplex utility electrical box, 2 duplex outlets with metal covers. I used a piece of aluminum in the center space on which to mount the manual switch.
Step 3: Terminal Block
The three terminal Bakelite connector provides a clean way to feed power and ground the box.
Step 4: The Circuit
The relay is DPDT 20 Amp @240 Volt. The Coil is DC 6 Volt. I measured 41 ohms across the coil with an ohm meter. That comes out to 150ma @6 volts. I installed jumpers on both circuits to provide double current carrying capability and I put a diode on the coil to prevent reverse voltage induction when the coil voltage collapses.
The third photo is my home-made transformer. The primary is 14-gauge magnetic wire on a 3 3/4" long bolt. The winding is 1 3/4" long, two layers, approximately 40 turns.
The secondary is 36-gauge magnetic wire approximately 1200 turns. Output is 12 volts. The primary is in series with the power tool in use and the secondary triggers the relay to power the shop-vac.
The transformer is mounted in an acrylic frame. Acrylic is easy to work and can be "welded" with acrylic solvent cement.
The 6 volt circuit if from a common wall wart which I took out of its case. This circuit is used for the manual on switch and to supply power for the transistor to energize the relay coil. It can produce 300ma @ 6 volts.
The transistor switch is a 2N3904 NPN bi-polar transistor good for 200ma @ 40 volts. I put in a 6 volt zener voltage clamp in the input because of the 12 volts from the home-made transformer.