Wednesday, January 2, 2013
Wiring A Wireless Doorbell
My father-in-law installed a wireless doorbell. The ring-button outside the door contains a battery and a radio transmitter, the indoor unit contains a radio receiver and a speaker. It turned out that the battery in the ring-button outside does not like the cold Finnish winter, so it went flat. It seems this doorbell is best suited for indoors use...
We planned to replace the transmitter with an old-fashioned switch with wires, and move the transmitter indoors. Opening up both the receiver and the transmitter first revealed a rather sloppy soldering job, then something about the design of the transmitter and receiver.
The transmitter contains an encoder chip LP801B, while the receiver contains a LP8029 decoder. The radio circuitry appears to be built with discrete components in both units. The transmitter circuit board has two push-buttons: one for ringing and one for changing the melody. Only the ring-button is accessible when the cover is closed.
The receiver contains a melody generator chip, under a black plastic blob. Both units have a bank of four dip switches. Pressing the transmitter button causes a noise inside only if the switches in the receiver and transmitter are in identical positions, this is to avoid hearing the doorbells of all neighbors.
The initial plan was to move the transmitter inside, and connect the ring-button outside in parallel with the ring button on the transmitter. After a look at the Japanese LP8029 data sheet, I found a point in the receiver circuit that starts the ringing noise when momentarily connected to the positive supply. I wired the outdoors button to this point. The transmitter module is now needed only for selecting the melody, the receiver on its own works as an old-fashioned wired doorbell.
It was hard to find any information in English about the LP801B and LP8029 chips. An Austrian hacker found the same encoder chip in the transmitter for radio controlled mains switches. He figured out how to connected the transmitter to a microcontroller, to switch things on and off from a program.