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A personal account of building a homemade PBX (Private Branch Exchange) telephone system in 1992-93 using salvaged electronic components and an 8031 microcontroller. The system supported multiple extensions with features like call hold, call forwarding, call parking, speed dial, ring cadence selection, DTMF/pulse dial conversion, and conference calling. Technical details cover relay-based voice path switching, DTMF tone detection and generation using a CS8870 and DAC-08, ring voltage generation with zero-crossing synchronization, and firmware debugging via ROM monitor. The project was built purely for personal satisfaction and entertainment.

My Homemade PBX