engifineer

Ok, I was just explaining this to someone on another car forum recently, then saw a few projects that this may help around the scion forums, so I figured I would post up a quick thread on it. Many times it is nice (or necessary) to have a circuit controlled by a momentary pushbutton or momentary power source. This may be necessary if you need to start something with a momentary switch, but need it to keep running until another momentary change in the circuit (or another momentary switch) is activated. I have included a quick diagram I made below for this to make it easier to understand. I will explain it here.

First, the big box is a typical relay. The coil (relay control) is on the left and I have shown two normally open contacts. You can get relays in a million configurations, but this is the minimum needed to do what we are doing here. The DEVICE is the load, whatever that may be in your case (lights, ign, etc). The two contacts shown above the box are two momentary pushbuttons. The one on the left is normally open (meaning no circuit until you press it) and the one on the right is normally closed (meaning pushing it interrupts the circuit). The 12V labels just indicate batter power, and the other symbol with 3 lines is ground.

Lets look at what happens when you push the NO (normally open) switch on the left. When pressed, you are applying 12V to the coil, which is already grounded on the other end. This is what you will recognize most from a standard relay based circuit. This activates the relay.

When the relay is activated, the two sets of normally open contacts close, which applies power to the same point the switch does. This effectively latches the circuit, so when your finger releases he pushbutton the relay stays on.

To turn it off, you press the NC (normally closed) pushbutton, which breaks the circuit powering the coil and disengages the relay.

So you have something that is now activated and deactivated by momentary changes in the circuit.