Using a ground signal as trigger

Question

So here is the problem:

First, I have a door opener device that gives me one wire to control it. This wire should connect to ground so the device works. We do not want to make the door opener to work all the time, obviously.

Second, I have a device for remote control, when a button is pressed on the remote, a ground signal is output on the box for a couple of seconds. This is the only output of the controller and unfortunately It can't serve as a ground to the door opener. It has simply too much impedance to make the door opener work. It works with lighter loads such as leds. Unfortunately I cannot change this control I'm treating it as a "black box".

What I need help with is figuring out a way to use the controller wire as a "trigger" to activate the door opener.

Summing up:

• Both devices use a 12v power supply

• Door opener wire (should connect to ground)

• Controller wire (ground signal)

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

It seems that the control box can only power low current devices. So this is why using a relay can be helpful. To activate the relay, the pin 85 is connected to the "controller box wire" and pin 86 is connected to 12v power supply. So when the remote control is activated and in turn the control box wire is "activated" and the relay's coil is powered using a very small amount of current.

The door opener needs a direct ground connection and wer use relay pin 30 (common terminal) to connect directly to ground. Pin 87 is connected to the common terminal only when the relay's coil is powered. So we connect the relay's pin 87 (normally closed) to the door opener wire.

A diode was added across the relay's coil to prevent damage, thanks to the comment of @SamGibson

I hope this helps you.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Insert a optocoupler.
Connect the LED of the optocoupler (if necessary, with a resistor) between the 12V and "output wire of control box" and the transistor of the optocoupler to the "wire to control this door opener" and ground.

My phone does not support the built in schematic editor. Below an attempt to draw the implementation: