Running Relay in parallel to switched neutral rocker switch

Question

Complete beginner here, so apologize if this is obvious - but I'm trying to hook up a fog machine to a relay module for my Arduino. The Fog machine has a plug for the wall, then a second drop cord plugs into the back of the fog machine that is connected to a little unit that has a light on it, and a rocker switch that, when depressed, causes the fog machine to spit out smoke.

The rocker switch is spring loaded, so letting go flips immediately back to the off position. The setup is exactly like this thread: https://forum.sparkfun.com/viewtopic.php?t=29518 There is a black wire that comes in, which is soldered to a post for a small light, then a blue wire is soldered on the other post. A small jumper goes from the same post as this blue wire, to the outside post of the rocker switch. Then the middle post of the rocker switch is connected to the white wire. I believe this is called a 'switched neutral' Hooking up a multimeter shows a 120v difference between both the black and blue wires, and the black and white wires.

On that thread, the solution was to wire the relay 'in parallel' to the switch, but it doesn't explain the circuit past that. How do I hook up a relay module before the switch so that I can trigger it from my arduino? The relay is SPDT relay with COMM, NO and NC posts. Hope I explained that clearly!

Answer 1

In this case you're looking at a relatively simple problem. You want to switch a load with a simple existing control mechanism. A switch controlling a load is a very simple circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Because you are trying to achieve function identical to the function of this switch, a relay in parallel to it with the right contact setting will bypass the switch and provide the same function.

^{simulate this circuit}

With regards your question about a short, the purpose of a switch is to connect(short) and disconnect its contacts in a controlled manner. Looking at the second diagram, you can see that with this arrangement, if either the rocker switch or the relay are actuated, the load will be connected to power. Once the switch has connected, the voltage on both sides of the other switch will match, and actuating the other switch will have little consequence.

When choosing your relay, make sure it's controlled contacts are rated to do the same thing as the existing switch. Match the current rating, voltage rating, AC or DC and any other relevant ratings and you should be fine.