Using an aircraft’s altimeter to replay mountain bike journeys

Maker Glen Akins wanted to relive his best mountain bike ascents and descents, so he hacked a vintage aircraft altitude indicator (also called an ‘altimeter’) using Raspberry Pi Pico and a little digital data conversion wizardry.

You may have already guessed that an altitude indicator, however old, is not designed to record the modest speeds reached by mountain bikers, which means Glen’s rides are displayed at 60 times his real speed.

From the rider’s wrist to Raspberry Pi

Glen’s fancy watch captures his latitude, longitude, and elevation when he’s mountain biking. After finishing a ride, he exports the data as GPX files onto a Raspberry Pi 400, where a Python script reads the elevation data at 60 times speed and passes them to a Raspberry Pi Pico over TTL Serial. The Pico converts the elevation data points into synchro angles and passes them to the digital-to-synchro conversion board. The final step is to turn those synchro angles into single-control voltage waveforms that the vintage altimeter can understand.

The top left graph shows the Python script’s output. The oscilloscope display shows the single-control voltage waveforms sent to the altitude indicator

Digital data conversion magic

Glen, being an actual wizard, had already built his own digital-to-synchro converter. It did require some updating for use in this project, however.

The Raspberry Pi Pico sits on an adapter board (pictured below) that connects to the converter board using a right-angle 7-pin header socket. Glen felt this offered a more secure connection and was easier to use instead of the alternative: lots of jumper wires running between the Pico and the converter.

Glen’s description of the demonstration video explains how the nifty digital-to-synchro (D2S) converter works:

“The updated D2S converter fits on a single board and uses three Microchip MCP4802 DACs and three TI OPA548 power operational amplifiers to produce high-power 400 Hz AC waveforms to power and control the servo loop in the altitude indicator.”

The updated D2S board next to the Pico on the adaptor board feeding it data

There is tonnes more detail on how each step of the conversion process works in the original project post on

An analogue throwback

This is a nice and simple — and slightly steampunky — piece of retro tech, perfectly sized to be a discrete desktop display.

When installed in an aircraft’s instrument panel, the two cylindrical pieces sticking out from the bump at the top of the altitude indicator house lights to help pilots read it in the dark.

Simple functionality

The hand on the large dial makes one full circle for every 1,000 feet of altitude. Each large number on the dial marks 100 feet and each white line between the large numbers marks ten feet. The inset counter shows the aircraft’s altitude in hundreds and thousands of feet.

Nice work, Biker Glen. This projects ticks all of our over-engineered and retro-tech boxes. We’ll leave the high-altitude cycling to you though.

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