Glen Akins' Latest Upcycling Project Turns a Vintage Aircraft Tachometer Into a CPU Gauge

Engineer Glen Akins is back with another piece of upcycled aircraft instrumentation, this time turning an electric tachometer into a USB device for monitoring a computer's CPU load.

"When I purchased the tachometer, I really had no idea what I was getting other than that it had an electrical interface rather than a mechanical flexible shaft interface and it looked cool," Akins explains of the Kollsman Electric Tachometer which forms the heart of his latest project.

"On the front of the unit are two dials. The larger dial indicates the tens digit of the percent of maximum RPM and the smaller inset dial indicates the ones digits. The two dials are added together to get the total percent. On the rear is a receptacle with seven pin contacts and a nameplate."

The nameplate offered clues as to what would redline the device, but not about how the connector on the rear was wired nor how the tachometer would receive information about the engine's rotary speed in whatever aircraft it had originally been installed. So, naturally, Akins took it apart — discovering a direct mechanical link between the two dials and a spring which resets them to zero when force is removed.

Further disassembly unveiled a three-phase synchronous motor, as well as wiring for illumination for better visibility in the dark. It also revealed a puzzle: a copper cup which fits into the gap between a spinning magnet and an iron shield, revealing the device to be a magnetic drag cup tachometer. "The rotating magnet induces Eddy currents in the copper cup which causes the copper cup to rotate and apply a force to the shaft that counteracts the force applied by the hairspring," Akins explains. "The shaft rotates and the pointer moves to the point where the balance of the two forces is equal."

To make it work, then, Akins needed a three-phase alternating current power supply with a frequency which could be varied from direct current (DC) up to 120 percent of 70Hz. Repurposing an earlier design, Akins came up with a three-phase board which can be controlled by an external Raspberry Pi Pico development board. Software running on the RP2040's first core takes a user-supplied percentage as an input and works to smoothly position the dials, while the second core carries out direct digital synthesis of the three-phase signal and controls the digital to analog converters (DACs) in the power supply to actually make the dials move.

This isn't the first time Akins has repurposed vintage aircraft gauges for other uses. Back in November last year he built a Microchip PIC16F-powered adapter board designed to allow a Selsyn-based multi-gauge indicator to be controlled over USB from a companion .NET app. A month later, he showed off another Selsyn-based indicator adapter — this time based on the Raspberry Pi RP2040 microcontroller.

This time, Akins had a plan for the tachometer: where it would originally have shown how hard an aircraft's engine is working, now it shows how hard a connected computer is working — taking the current CPU utilization as a percentage input and moving the dials accordingly.

Akins' full write-up is available on his blog.