Devices that require 220 volt split-phase circuits, like larger pumps and electric water heaters in North America, can be difficult to find “smart” switches for—especially when you’re looking for a non-cloud solution that respects your privacy.
I wanted a Wi-Fi switch for my pool pump that I could control with my OpenHAB server. After a few unsatisfying online search sessions, I decided I’d make one myself using a Tasmota-flashed Sonoff S31 smart plug and a 220V contactor with a 110V control circuit.
A “contactor” is really just a type of relay used to switch higher-voltage, higher-current AC circuits. Contactors are unlike ordinary relays in that they’re virtually always normally open, and typically switch multiple conductors. They are made of heavy-duty materials, are often spring-loaded, and tend to have extra safety features.
For this job, I ordered from Amazon this Packard 40-amp-rated 220V contactor with a 110V control circuit. You can get contactors in a number of different control voltages. 24V is especially common because it’s what HVAC systems typically use. But I’m going to use 110V for mine, because that’s what comes straight out of the wall, and that’s what my smart plug is designed for.
As you can see in this picture, the contactor’s control circuit has tabs for spade terminals on each side of the unit. I crimped a couple spade terminals onto the line and neutral wires of an old power cord and attached them.
Plug it in, and the contactor makes a satisfying “thunk” noise as the electromagnet inside pulls its contacts together.
Before wiring it up to your pump circuit (or whatever you’re using it on), you can ensure your contactor is working by setting your meter to check for continuity, then touching your probes to each of the main terminals on opposite sides of the contactor. When your (110V) control circuit is energized, you should see continuity on each of the two legs of the main circuit. Unplug the control circuit, and continuity should go away. It’s just a big electric switch.
Now you’re ready to wire up your main circuit. Just connect your two “line” wires (they’re typically red and black) to the terminals on each side of the contactor. If you have spade terminals, you can use the tabs that are provided. If not, use the screw-down terminals. That’s what I did.
I mounted mine inside the existing control cabinet next to the old mechanical timer, and just bypassed the timer. I figured leaving it in place would be a good idea in case I ever wanted to sell the house. I can always just remove the contactor and wire the mechanical timer back up if needed.