Controlling an electromagnetic bike with Arduino

I have received an eletromagnetic stationary bike without the actual controller. What is left is a board that connected the controller to the bike circuits (an electromagnet and two Hall sensors). While I have some experience with Arduino and have no problem with getting the sensor inputs, I have no idea how to get around to controlling that electromagnet (which adjusts the effort required to ride the bike). I see that the board has an MCP60021, which is an operational amplifier, but what I have read about it made it no clearer for me. Is this the chip that is actually used for the control? I also see that the connector to the controller supplies 24V, so it needs to be stepped down to power the Arduino, if I understand this correctly. Will the signal from Arduino to the amplifier need to be also converted? Any pointers would be useful...

can you show the other side of this PCB?

RV mineirin

Unfortunately, the other side is rather tricky to capture as the crank gets in the way, I hope this will do (the board is reversed, so power input is at the top):

What is the model of this bike?

RV mineirin

It is Capital Sports Arcadion. Unfortunately, there is very little information on that model.

I am not sure, but it seems the traces are either barely visible, or there is another layer inside. I cannot trace out the sensors, for example. But I can always check the connections with a multimeter, if needed.

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Does this cable that says "controller" have the other end connected to some other PCB?

RV mineirin

It should be connected to another PCB (the actual controller that should maintain the communication with the Bluetooth app, among other things), but this part is missing in my bike (which is why I got it for the tenth of the listing price).

The pins of the missing controller are marked G, +24, D, R, P. I assume G is ground, +24 is power supply for the controller, D and R connect to the inputs from the RPM sensors (they are marked 'Hall', but actually are reed switches). This leaves the P pin as the one that should control the electromagnet. It seems to lead to the op amp, but right now I cannot check the actual pin (I need to disassemble the crank further).

Considering that the board is analog only and that control must be done with a single pin, I have theorized that all is required is analog signal on that pin. I have tested that theory starting with very small voltages (ESP32 has an integrated DAC) and it turned out to be true. All I had to do was to crank up the pot on the board to get a decent resistance range.

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