Of course this project is for personal and educational use only. Please Decathlon don't sue me!

This exercise bike, like most around this price point is quite old (had mine for 7 years), and has very basic (dumb) functionality. An LCD display that shows cadence, and resistance levels. No "smart" features or integrations.

It utilizes a spoked cast-iron flywheel; slowed by a permanent magnet eddy-current brake. The gap between the magnet and the flywheel's rim (controlled by a positional potentiometer) determines the strength of the resistance effect generated in the wheel and hence felt at the pedals by the rider. Therefore the bike's resistance levels 1-15 are a direct measurement of this gap.

Makes it perfect for our project because a cheap microcontroller like a Pico can easily be integrated in a non-invasive way to add some smart tricks comparable to state of the line trainers, at a fraction of the cost.

Since every exercise bike has different characteristics, the project is carried out in 2 phases:

Phase 1: Calibration 

The powermeter's design is based on the law of conservation of energy. When the flywheel is maintained at a constant rotational speed, most mechanical power input by the user is dissipated as heat through the braking system.

By characterizing the power dissipation of the magnetic brake, we can directly approximate user power output.

The power it dissipates follows a predictable physical law: P = k · ω². Hence the calibration step is to determine this k value for the bike at each of the 15 resistance levels. 

Note: The manufacturer already has these values but they're not publicly available so we have to get them experimentally.

Phase 2: Operation

We load the bike's power characteristics into a look up table and modeled with Micropython onto the Pico. And wire it up to the bike's DC power and cadence sensor.

The powermeter will use the standard BLE service specifications for Cycling to broadcast the power, speed and cadence as a bluetooth sensor would. It should be detectable on the popular cycling fitness apps. Tested using Wahoo.

Source code and documentation. Have fun y'all: https://github.com/EugeneBad/pico-powermeter