Three years ago I build an automatic switch that I use to turn a shop vac on when I use my miter saw. And although it has been working perfectly ever since, I wanted to make an updated version that is less complex and easier to make. This new version uses just three components to make it work: a current sense transformer, a small bridge rectifier and a solid state relay.

First, a word of warning: this project deals with mains voltage, and if you have any doubts or concerns about working with it, this project may not be for you. There is a serious risk of shock, fire and possibly death unless all safety precautions are followed. Do not attempt construction of this project unless you are completely confident in your ability to work with mains voltage safely.

Here are the parts laid out that I used to build mine:

Missing are some wire nuts to join the wires inside the box. I used a standard 5″ metal electrical box to house the switch and put together the lead wires from plug ends that I had. Another way is to use short extension cords with the plugs already attached, but you need to make sure the lead in wire is heavy enough to carry the combined current from running both machines. My wire is 14 gauge and I kept it fairly short.

The circuit diagram:

The transformer is a Triad CSE187L that is rated for 30 amps continuous load. The bridge rectifier is a small 2 amp device used for the sake of convenience and the relay is the typical solid state type that switches on from as low as 3VDC:

I’m using a solid state relay for this version because the current sense transformer doesn’t put out enough voltage and current to drive a mechanical relay. Like the previous version of this automatic switch, there also isn’t enough current to charge a capacitor to extend the run time of the vacuum that will be controlled by this switch. I have found in using the original for the past three years that there is no need to have the vac run longer – it works just fine as it is.
The current rating for the relay (in amps) should be comfortably more than the machine you are switching with it. Mine is rated at 25 amps and will likely control a shop vac that runs on 10 amps.

It’s worth mentioning before we go deeper into this that this switch will generally only work for tools that draw more than 10 amps (1200 watts). Again, this is a limit of the current sense transformer and the current it produces. The higher the loads, the more current the transformer will put out. For example, it will easily work for a miter saw that is rated at 12 – 15 amps (1400 – 1800 watts). It will not work with the typical random orbit sander or other small tool that uses less power. I may revise the circuit at some time in the future to make it work with smaller tools, but kept it as simple as possible for this version.

I used a small piece of perf board to mount the transformer and rectifier. Note the 14 gauge copper wire used to connect the primary side of the transformer:

I’m using quick connect terminals that are soldered directly to the copper wire, which makes it easy to put together.

To mount the circuit board in the box, I drilled holes for two #10 machine screws:

Since the circuit board has mains voltage, it must be fastened securely so that it will not short out against the box. As I said in the build video below, the box MUST be grounded for safe operation.

After the board was installed, I used hot melt glue to lock the nuts on the bolts to keep them from coming loose:

The relay is screwed onto the metal cover for the box and the box itself will act as a heat sink for the relay. If only used intermittently for a miter saw, the relay won’t even get warm, but it depends upon the size of the shop vac that is used.

I made a video for this project going through the build: