I’m working on a project to replace my doorbell. The existing system uses an AC transformer, about 13V, with a solenoid chime. I want to use the existing wiring, but replace with 5VDC and tie into a microcontroller or Raspberry PI so I can make it play MP3’s and hook it into the home automation system.
The switch shown was a traditional momentary doorbell switch with an incandescent lamp in it. I replaced the lamp with an LED and resistor. The way the old system works, there’s always small current flowing to keep the lamp illuminated. When the switch is pressed, the switch shorts past the lamp which allows enough current to flow to activate the solenoid, causing it to strike the chime. The new circuit works similarly with the exception that I use an op amp (comparator) to detect the voltage change when the switch is pressed. Currently, it just lights the LED, but the output is suitable to tie into the GPIO on the PI. I just have to get around to finishing it…
One cool thing about using LED’s, they are brighter, and use significantly less power.
I made a schematic somewhere. I will post if it anyone is interested.