how to control 120VAC circuit with 12V switch - Control Panel DIY project

Thread Starter


Joined Jan 15, 2018
How would one go about selecting an appropriate relay to run 120VAC circuit with a 12v automotive switch? I will be running shop lights, Audio equipment, TV, PC, venting fan, etc. Basically, I want to build a DIY control panel similar to what you might find in a Sci-Fi spaceship and have it control everything in my project room/hobby room. Attached is a pic of my inspiration. That control panel incorporates 4 Arduino's, but I am happy just using good old fashioned switched. DIY Control panel.jpg


Joined Jul 18, 2013
Calculate the total load being switched and rate the switching device, relay contacts etc, accordingly.
If using low level (12v) control, then the relay/contactor coil will be rated at 12vdc.


Joined Apr 11, 2010
I’d like to add that you need an appropriate 12VDC supply. Take the peak current draw of a relay and multiply it by the number of relays, if you use identical relays for each circuit. If you use different relays, total their individual current to operate* Then add 10 or 20%.

Your 12VDC supply needs to be able to supply this much current continuously.

* Relays have two current draws. One to operate it initially and a second to maintain its position.


Joined Sep 24, 2015
Only AC versions, does not apply to the DC coil.
[edit] Misread - or misunderstood your comment.

As stated above, you need a relay capable of handling the load. And depending on the loads being switched on and off, you need to factor in the startup current of each device. For instance, a lightbulb (as an example) has a much higher current draw when you first flip the switch but once the light is fully lit its current draw drops significantly. So a 50 watt bulb at 10 volts will run at 5 amps. But at startup it may draw much more amperage. My guess is that it can easily draw 20 amps or more to initially light up. Factor in all the devices you want to start simultaneously and you can be looking at a significant load your relay will have to switch.

In stead of using car relays I'd recommend looking into contactors.

SSR - Solid State Relay ...
I like the idea, but they have to be big enough to handle the startup current of the devices under control.


Joined Jul 18, 2013
[edit] Misread - or misunderstood your comment.
Replying to the following: 'Relays have two current draws. One to operate it initially and a second to maintain its position."
That is does not apply to DC versions of relay coils. Current limited by the resistance of the coil.