Automatic Door Release with Bell Pattern
Concept sketch — the doorbell button's press pattern travels the existing intercom wiring (SCS bus) as short/long electrical bursts, much like Morse code. A microcontroller taps that bus passively, recognizes one specific pattern, and pulses the door release only when it matches.
A DIY doorbell-triggered entrance release — listens for a specific ring pattern and automatically unlocks the door when it's recognized, skipping the need to buzz visitors in manually.
Major Challenges
- Undocumented protocol — the intercom's bus signal had no public spec to build against. An entire custom analog front-end was designed and simulated to tap it, then dropped in favor of piggybacking on an existing intercom module that already conditions the signal.
- Reverse-engineering the encoding — the bus turned out to use Return-to-Zero encoding, not plain UART: a "0" bit is a short pulse, a "1" bit is silence. Sampling at the usual mid-bit point misreads it, so the decoder had to look for a pulse anywhere in the bit's time window instead.
- Sampling resolution — captures stayed unreliable for a long time. The real cause wasn't the decoding logic at all — it was the microcontroller sampling too coarsely against a pulse barely wider than the sampling interval itself. Tightening the sample rate fixed it.
Draft placeholder — full write-up and details coming soon.
← Back to iAlmirPro.Home