This page is about the Doorbell. It may sound strange for an entire page on this subject, but it is remarkably complex. You see, The Crazy Engineer did not like any of the NORMAL doorbell buttons on the market. They all looked so ‘boring’ and ‘out of place’. We finally found a design that seemed to look OK. It was in proprtion with the rest of the house, and the brickwork, and seemed to say ‘Press Me’. But things were not perfect. Despite the price being right at under $50, it did come with a video camera.
A decent amout of reverse engineering was needed to work out how to attach this doorbell to any electronics so that it could make a sound. My first attempt was over-engineered, but worked well for over a year. It is only in writing this article that I decided to fix the engineering and bring things back to earth. I eventually worked out that the resistance between pins 2 and 4 of the video intercom changed when the button was pressed. This took longer than I thought it would take!
In order to do something with this resistance, I connected 5V to the orange wire (Pin 4) via a 100 Ohm resistor. Then I connected a 270 Ohm resistor between the orange and blue wires (Pins 4 and 2), and connected the blue wire (pin 2) to ground. This gave a voltage differential that I could look for. This was connected to a kit from Jaycar – a voltage dependant relay. The output was a clean contact that I connected into my alarm system, which would chime when someone was at the door. I will not bother with the full details of the relay since I ended up getting rid of it. Overengineered!
I replaced the Jaycar kit with a much simpler circuit. I connected the Orange wire (Pin 4) to 12V via a 100 ohm resistor, and then connected a 270 ohm resistor between the Orange and Blue wires (Pins 4 and 2 respectively), and connceted the blue wire to ground. Ignore the 5V regulator on the circuit board – I was using it, but changed my mind. It ended up not being used. The blue wire (pin 4) was then connected directly into my alarm system.
This circuit gave me 9.9V on the alarm panel normally, and 7.8V when the button was pressed. I then told the alarm system that this device was ‘Normally Open’ and to say ‘Front Door’ when things changed.