[Cialug] Raspberry Pi 3

[Zachary Kotlarek]

On 29 Feb 2016, at 14:53, L. V. Lammert wrote:

> On Mon, 29 Feb 2016, Justin Richeson wrote:
>
>> 2 wires run to the "button" in the garage, but somehow it controls a
>> lock function, the light, and the door.
>>
> It is a digital signal, .. if you hooked an oscilloscope to it, you would
> see a series of pulses for each function.
>
> Pretty hard to replace with a button, *BUT* most openers will respond to a
> simple switch closure across the wires as that is an option if you can't
> use the fancy control module.


I’ve completed a similar project on a door opener with a 2-wire, line-powered open/lock/light button.

At least on mine the signaling was all analog. The door toggles with a simple circuit closure so you can use a doorbell button or a relay or anything else that will close the circuit to trigger it. This is pretty common; it’s the way all older openers and thermostats and doorbells and other residential signaling systems work.

The other signals use the same line and a couple of analog components to induce a slightly different signal. The light was a circuit closure through a resistor, so the return voltage is lower when you press that button than when you press the door button. I forget what the lock button was; it may have just been a different resistor but I cannot remember. I haven’t done general research to know how common that control system is but I would be surprised if any of them are much more complicated and still a wired controller.

You can draw small amounts of power from the input line but there’s a limit to how much you can sink into the ground/signal line because of its dual purpose — if you push too much current into the return line you’ll trigger the opener (or some other function).

If you’re confident you garage door will never put out something harmful on the signaling line you can interface it using a fairly simple transistor-based circuit. But you shouldn’t. Those 3.3v electronics are sensitive and a relay that will protect you is cheap. You can get solid-state relays that will trigger directly on logical-level outputs from the Pi and get both simple circuit design/construction and lots of isolation.

	Zach
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