You're looking for a single-bit stream of information and very likely you can find it as an electrical signal inside your doorbell already. I wanted to replace

You're looking for a single-bit stream of information and very likely you can find it as an electrical signal inside your doorbell already. I wanted to replace the sound my wireless doorbell made so I took the basestation apart and it was a very simple thing, with three chips: a radio (NRF51), a microcontroller (PIC) and a blob of epoxy on a separate board that was connected to the speaker. It took maybe half an hour of beeping and scoping to understand how the PIC and the sound maker communicated - in this case five pins to select one of 32 sounds, and one pin to trigger playback. I simply took the playback trigger pin and connected it to a small MP3 player module and moved the speaker from the internal sound maker to that. If you can just attach wires to the button directly, it's even simpler. Of course, if the object is to use a pi zero to do some DSP, this is missing the point. But there's a good chance it's the long way round if you want to solve the problem of knowing when somebody is at your door.