This might seem strange, but I have found it hard to buy a decent door bell pushbutton. They all seem just wrong. And I don’t feel like spending $40 on a pushbutton that looks horrible either. Anyway, I was looking through the WES catalog to buy some supplies, and found some Video Door Intercoms. The camera unit was $40 for B&W, or $50 for colour. And they included a pushbutton. WOW. This one was a LOOK-C unit.
This did not include the base unit, but who cares. Debugging the four wire interface I worked out how they work. First, the 12V is only active when the unit is fully active. The video signal is therefore only active when 12V is supplied. And the ground is simple. The audio does the rest. When there is 12V or so on the audio line, the internal relay is activated causing the door strike (if attached) to open. And if the button is pressed, the voltage on the audio line goes to ground. The audio line normally sits at 5V.
Audio signals going both ways sit on the single audio line, meaning that there must be a hybrid in the camera unit allowing for hands free.
To interface to my alarm, to use the Alarm as a door bell, I purchased the KC5377 Voltage Switch Relay Kit from Jaycar. This was under $30. To simulate the 5V, I took the 8V from the regulator and connected this to the input of the switch via a 100 ohm resistor. Then I connected the input to ground via a 220 ohm resistor. This gave a bit over 5V at the input. The I then tuned the relay settings so that it would detect the pushbutton.
This seems to have worked well. I have yet to install it, but I am sure it will work