Park Frazer's 1970s King DME Aircraft Instrument Lives a New Life as a Power-Hungry Desk Clock

Electrical engineering student Park Frazer has given a piece of a 1970s aircraft radio navigation system a new lease on life — by creating a very literal "radio clock" from the Panaplex indicator.

"I found this King KI 266 DME Indicator, part of a 1970s DME [Distance Measuring Equipment] radio navigation system that would have been mounted in an airplane cockpit," Frazer explains of the hardware at the heart of the project. "I only have the display part, so unfortunately I won’t be able to use it to navigate, but it does have two awesome gas-discharge seven-segment Panaplex displays."

Built around the concept of cold-cathode neon gas discharge, Burroughs' Panaplex displays were launched as alternatives to Nixie tube and LEDs and found homes in everything from calculators to, as Frazer discovered, aircraft instrumentation. Unlike Nixie tubes, Panaplex panels used a more LED-like seven-segment layout — giving the device a modern appearance with a retro glow.

The Panaplex-based gadget Frazer picked up was in a reasonable condition, bar corroded connection cables internally — replaced with hookup wire to bring the display back to life. "I powered it up with my best guess from what little I could find online and reversing the PCBs, and it worked on the first try," Frazer says. "Eventually I found the full manual. My guess was close enough, apparently."

Originally, the King DME would have been used to indicate the distance between the aircraft and a ground station — flicking between showing the measured ground speed or time-to-station at the click of a switch. Given its 1970s vintage, though, there's a surprising amount of hardware in the casing — including the need for both +15V and -15V power supplies, plus 5V logic and a hefty 175V to the Panaplex anodes.

To this, Frazer added a "backpack" to convert the device from displaying distance to displaying time — built around a Microchip PIC16F1454 microcontroller and a MCP47CVB12 10-bit digital to analog converter (DAC) to generate the analog signals required by the King DME's display. "The tricky part was the power supply," Frazer notes.

"I initially considered trying to build a mains power supply, but, in the interest of not electrocuting myself, I ended up with the crazy idea of boosting ±15V from a USB charger. I found the [Texas Instruments] TPS65131, which is a chip with both positive and negative converters that meet the specs to power the indicator."

After a little debugging, the backpack proved capable of both keeping time and displaying it on the DME using any USB power supply. "Without a USB connection [to a PC]," Frazer says, "the King logo [on the backpack] is a button to set the time. And then it's a functional, if power hungry and somewhat difficult to read, clock! The power draw depends a lot on the automatic dimming on the DME indicator, varying between about 0.7A and 1.2A (3.5-6W), and significantly more when it is first turned on."

Frazer isn't the only maker turning to scrapped aircraft for parts: Glen Akins has spent the last year working with various aircraft gauges, primarily based around mating vintage synchro gauges with modern digital systems — and in doing so turning tachometers into CPU usage gauges and altimeters into devices for reviewing recorded mountain bike descents.

Frazer's full project write-up is available on his website; design files and source code have not been made public.