The author had a problem: the neighbours had exactly the same type of doorbell as he did (actually a 50 Hz buzzer), so it wasn’t always clear who needed to answer the door. To avoid confusion, the author augmented the existing doorbell with a wireless model a reasonably inexpensive option at current prices. All that was necessary for this was to arrange for the existing button and wiring to also actuate the wireless doorbell.
Circuit diagram :
Tandem Doorbell Circuit Diagram
The author opened up the button enclosure of the wireless doorbell and used a multi-meter to find out which set of contacts were closed when the button was pressed. This is where the relay output should be connected (see the schematic diagram). The circuit is virtually self-explanatory: when the existing doorbell button is pressed to actuate the buzzer, the voltage is rectified by the bridge rectifier and regulated at 5 V by the 7805. This voltage drives the relay directly, causing the switch in the wireless doorbell button to be shorted. As a result, along with the buzzer a sizeable Big Ben chime indicates that some-one is at the door. Now the author just hopes that his neighbour doesn’t copy his idea.
http://www.ecircuitslab.com/2012/05/tandem-doorbell.html
