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Just a side-note, I am a complete beginner when it comes to circuitry.
I am attempting to control a garage door controller with a relay powered and controlled by a Raspberry Pi. The garage door already is controlled by a button with some simple 2-core wire.
How can I get both the button and the relay to trigger a signal to my garage door?
I guess what I am trying to create here is an OR gate.
Anyways, I hope this is enough information.
When the button is pressed, it allows electricity to flow through it, between its two connections.
You can wire the relay's N.O. and C. (not N.C.) connections in parallel with the button (i.e. wire one to each terminal of the button). It doesn't matter which one goes to which wire on the button. When the relay is activated, electricity can flow between N.O. and C. just like a switch.
So electricity can either flow through the button, or around the button and through the relay. Or both at once, if you happen to press the button and activate the relay at the same time.
Just to help your understanding of circuits a bit (you garage door won't actually be wired like this): if the button worked the other way - if it blocked the flow of electricity when you pressed it - how could you wire the relay?
In that case you could wire the N.C. and C. connections of the relay in series with the button, so the electricity has to flow through both. Then, pressing the button would block the flow of electricity, and activating the relay would also block the flow of electricity, since the relay only allows electricity to flow between C. and N.C. when it's not activated.
By the way, when I talk about "the flow of electricity", it's not the electricity going to the garage door motor - it's a separate, very small trickle of electricity, which the control circuit tries to put out in order to sense whether the button is pressed.
My youngest garage door opener dates back to the 80's, so things might be different now. But back in my day ...
Except for the motor, the control system in an opener runs on 24-28 Vac relays. The button closure connects the relay coil to the 28 V source, and one set of the relay contacts keeps the relay closed after you release the button. When the door hits the end of its travel, it trips a switch that opens the circuit and sets it up to run in the reverse direction the next time. There are variations on exactly how things work internally, but that is the basic concept.
Somewhere on the motor housing should be a 2-terminal strip where the button wires are attached. Measure across these terminals with a battery-powered meter in both AC and DC modes. If your system is like mine, you should read almost 0 Vdc and 24-28 Vac - ish.
The normally-open contacts of a relay driven by the pi can be connected at the button, the motor head, or you can run another wire pair from the head to just about anywhere you want. The current being switched in my systems is 0.7 A (700 mA), but newer systems probably have a lower coil current. Still, use a relay with contacts rated for 2 amps AC or more. Switching a highly inductive load is hard on relay contacts; overrating is good.
