Z80 CPU Bluetooth Rotary Phone

This is V2 of my rotary phone system. The V1 system was built in 2017 and used custom interface circuits, an Arduino, and Fona 2G GSM modem from Adafruit. It worked well enough at the time. You can easily find old videos of it on my channel.

Due to T-Mobile gradually reducing 2G service, V1 of the rotary phone lost reception and was unusable from about 2021 until a couple months ago when I completed these upgrades.

I redesigned the whole system from the ground up. The Arduino and Adafruit Fona are gone. It now uses a custom hand built Z80 computer as a control system.

Why a Z80? First, because I like it. This is my 2nd Z80 computer and I wanted to apply the lessons I learned building the first one to make an improved system. Secondly, I wanted a homebrew Z80 system that does something REAL other than just make blinky lights. Finally... the way I see it, the Z80 CPU is period correct for a rotary phone system since the two technologies coexisted during the 70's and 80's. Combining a rotary phone and Z80 makes this a TRUE retro system that actually works today.

All the supporting circuits have been redesigned as well, and uses Zilog chips throughout the whole system (Z80 PIO, SIO and CTC). The Z80 SIO communicates with a Waveshare SIM868 Bluetooth module via serial, and connects to a 4G LTE flip phone as a Bluetooth headset to get cell service. The serial port controls calling functions via Bluetooth AT commands, and the audio connection from the rotary phone to the Bluetooth module is all analog.

The rotary phone LED animation is a new feature. Any self-respecting Z80 computer needs blinky lights. This system is no exception.

All the code is written in Z80 assembly and is programmed into a 32k EPROM. It runs at a blistering 1.789 MHz!

There are many advantages to using Bluetooth. It allows the system to route calls through any Android smartphone (or 4G LTE dumbphone) using Hands Free Profile (HFP). It inherits the functionality of the connected phone, such as Voice over WiFi, caller ID, blocking spam, etc. Upgrading to 5G will be as simple as Bluetooth pairing a 5G smartphone.

It took over 2 years to design, build, and debug this system. I find it a bit poetic and bittersweet that I finished building this just as Zilog is ending production of the Z80. I'm very pleased with the result and plan to use this phone for a VERY long time.