A longtime telecom hobbyist finally assembled a home‑made PBX in 1992‑93 after years of tinkering with intercoms, shortwave radios and garage‑sale phones. A stint at Bell‑Northern Research gave him access to junked components and microcontrollers, which he repurposed for a dial‑telephone system built from relays and analog circuitry. The project remained a personal experiment, not a commercial product, and shared schematics online.

The eight‑extension switch supports true telco voltages—48V on‑hook, 90VRMS 20 Hz ringing and roughly 25 mA loop current off‑hook. It can drive three rotary “500” phones in parallel without false trips and routes a single central‑office line with selectable DTMF or pulse dialing. Three voice buses allow up to three concurrent calls, while built‑in progress tones cover dial, busy and audible ringing.

The design favors readily available parts over integrated circuits; a relay matrix handles switching while oversized capacitors smooth ringing transients. Off‑hook detection relies on software timing of relay contacts, and a Crystal Semiconductor 8870 chip decodes DTMF tones for call control. Though inefficient, the build showcases how hobbyists can recreate core telephone functions without proprietary hardware for fun.