Jeff Lau's Open Source Bluetooth Adapter Brings a Mitsubishi Carphone Classic Back From the Dead

Software engineer Jeff Lau has brought a vintage Mitsubishi carphone back to life by building a Bluetooth adapter to bridge it to a modern cellphone — and has released the design for anyone else with classic telephonic hardware gathering dust to try for themselves.

"I have an old car phone in my early '90s car. It's impossible to activate service for these old phones," Lau explains of the project's origins. "The DiamondTel Model 92 Portable Cellar Telephone is a hybrid transportable/mobile (mobile, as in, 'car phone') manufactured by Mitsubishi in the early 1990s. I made [it] actually work like a real phone."

While previous projects focused on bringing back long-obsolete vintage cellular telephones have focused on using software-defined radios (SDRs) to provide a compatible radio station with which it can communicate, Lau took a different approach: a black box, driven by a Microchip PIC18F27Q43 microcontroller, which sits between the wired handset and its transceiver base, intercepting their communication in order to swap out the vintage AMPS system for modern Bluetooth.

"The Bluetooth adapter fully implements all behavior that is experienced by interacting with the handset. Much of it is a replica of (or inspired by) the DiamondTel Model 92's original behavior," Lau explains. "The Bluetooth adapter pairs with a modern cell phone using the Bluetooth Hands-Free Profile (HFP). This Bluetooth connection is used to: monitor/display cell service status and signal strength, etc.; handle incoming calls; send outgoing calls."

It's a clever hack, and one which means the original hardware can remain unmodified and there's no need to worry about radio licensing restrictions as with an SDR-based solution. It did, however, take some work, including careful reverse engineering of how the handset communicates with the transceiver based. "[It] requires a lot of custom software to replicate [the] original DiamondTel Model 92 functionality as faithfully as possible," Lau admits. "But that's OK with me: I'm a software engineer, and this is fun!"

Lau has released the project's hardware schematics and firmware source code on GitHub under a custom no-commercial-use license; you'll need a DiamondTel Model 92 or a Mitsubishi Model 1500 to use it, though, with no other models known to work with the adapter. More details on the reverse engineering and hardware build are available on the Electro-Tech-Online.com forum.