Salvaging an old stationary bike RPM sensor

Posted on 2011/03/20


I had to dispose of an old stationary bike that little by little fell apart. Before throwing it away I wanted to see if I could salvage something from its simple electronics. The bike has a sensor that somehow detects the rotation of the crankset and then is used to display the RPM on a small modal LCD. The “brain” of the bike is a microcontroller located below a plastic cap that is glued to the PCB so much that I couldn’t remove it, so I can’t identify the component. Everything is powered by two 1.5V batteries in series.

I took away the sensor itself and the PCB that connects it to the microcontroller that manages the display. The sensor has two terminations: one is connected to the negative pole of the batteries, the other is connected to the microcontroller and also to the positive pole of the batteries with a resistor whose colored bands indicate it’s 620kΩ. To understand what this sensor does, I hooked it up with my Arduino Uno.

RPM sensor connected to Arduino UNO

RPM sensor connected to Arduino UNO

The positive termination of the sensor is connected to the A0 analog input, and the sensor is powered from the 3.3V output of the Arduino through a resistance of around 620kΩ; the negative termination is connected to the GND pin. I also connected the 3.3V Arduino output with its own AREF pin to make the Analog-to-Digital Converter use the full voltage dynamic. I plan to use the Arduino as a (low-performance) oscilloscope to capture the sensor signal during various external stimuli.


Posted in: Hardware