Build Your Own Shop Vac Automatic Switch





Introduction: Build Your Own Shop Vac Automatic Switch

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.

Step 5: Finished Product



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    7 Discussions


    1 year ago

    Two things. First, where is your schematic diagram? A circuit such as this seriously needs a schematic diagram so others wanting to duplicate your efforts have a known good schematic to go by. Second, they actually have these for shops and they also have a few seconds delayed off function for the vacuum cleaner to make sure all the debris is cleared from the vacuum hose before shutting off. I actually bought two of them from Sear. They are Craftsman brand. Nice project, but you really need to post a schematic for safety reasons. JMHO

    4 replies

    Hi JMHO,

    Please open the PDF Circuit Diagram under step 1.

    I provided a two second delay to clear debris after the machine is shut off. You can see it demonstrated in the video I included.

    I know there are others on the market. This is mine.

    Okay I just took a look at your schematic. I didn't see it the first time around. Sorry for that. It looks good. You could have eliminated the 6 volt power setup and used a self biased 2N3904 NPN transistor circuit to accomplish the same results. Therefor making the parts count even less. It looks like the only reason for the 6 volt setup was to allow a manual "on" capability. So I guess that would be the individuals discretion to do. Nice circuit anyway.

    Oh, I think you have the Zener diode in backwards in the schematic. Check to make sure. The cathode end should be towards the top and not ground. With that setup you will get about ~.6XX volts output on the anode end. And it should be a 1N5234 or a 1N753. Either would give you a ~6.2 volt regulation or there about. Just wanting to make sure others can make your circuit and it work. Not trying to be picky by any stretch.

    Just fixed the zener. Right on both counts. Thanks


    1 year ago

    Top picture ground connection looks pretty funky. Also the clamp fitting for the feeder cable is wrong way out and not correct for the feeder cable (nor should SO
    cable be used to power fixed equipment. Maybe there should be a fuse somewhere? Nice circuit and design though.

    1 reply

    Hi bfarm,

    Thanks for your comment. Right on about the clamp. Thanks.

    I should have explained about the funky ground wire. The feeder cable is 3 conductor and grounded to the chassis from the plug. The external ground (funky) is a wire I pass through the center of my vacuum system to reduce static build up. see