Not specifically, anyway. But that’s ok! I figure for a start, I want to learn how the display on the bike gets its data. Because I put cadence and speed monitors on my road bike a few years ago, I had a general idea of the kinds of mechanisms it might use.
A lot of bike computers, cheaper ones in particular, use Hall effect sensors to tell when the wheel or the pedals have completed a rotation. Hall effect sensors show a change in voltage in the presence of a magnetic field. Mount the sensor, slap a magnet on your cranks and/or your wheel, and a microcontroller can keep track of each time a rotation is completed by watching the voltage. Easy!
Some other sensors are based on accelerometers which work by tracking the change in acceleration experienced by an object during rotation. Some fancy bike computers use GPS to measure speed.
I had no actual data on what was going on inside the Schwinn, but I had some suspicions. I felt confident ruling out GPS for a stationary bike. I also thought it was pretty unlikely that Schwinn would use accelerometers, since they’re usually more complicated and expensive than magnet-based sensors. Knowing that there’s spinning going on, I also thought it was possible they were doing something funky with electromagnetic induction, the same way a generator works.
Without looking up any videos or guides, I’ve decided to mess around and see what I could learn by taking apart the little display unit mounted to the front of the airbike.
After unscrewing the display unit from the bike, I found that it was connected to the bike by a 2 pin header. Two wires meant that it couldn’t use a Hall effect sensor or serial data connection, because they both require at least 3 wires for supply voltage, ground, and signal. This had me a little worried, since I thought a Hall effect sensor would be the easiest to deal with.
It was still possible that the bike was sending an analog voltage signal. Although I really didn’t need to take it apart to figure this out, I realized that the unit itself was powered by a bunch of AA batteries. This made me think it was unlikely that the bike used EM induction, because part of the benefit of that approach is that you could use the power from it to run the little display.
To definitively rule out EM induction, I had to check whether the bike was generating a voltage across the two pins. I ordered a bag of 2-pin headers from Amazon, hooked up my ancient multimeter in voltage mode, spun the pedals, and watched.
Aaaaand - drumroll please - absolutely nothing happened.
Which was good news. No voltage meant I could pretty confidently rule out an analog signal produced by EM induction.
Out of curiosity, I flipped my multimeter into continuity mode (which is just a fancy name for “beep if there’s nothing between these two wires”) and pedaled again.
Jackpot! As the pedals were spun, the multimeter beeped for a short interval which suggested that a switch was briefly completing a circuit.
Since I had already ruled out a Hall effect switch, I was curious about what mechanism the bike was using internally. I did a little bit of googling and I found a maintenance guide for the Schwinn AD6 that helpfully included part names and numbers. Apparently, I was dealing with a reed switch.
I’d never run into them before, but it turns out reed switches are like the simpler, low-tech cousins of Hall effect sensors. A reed switch is made up of two little pieces of metal in a sealed tube. When a magnet is brought close to the reed switch, the two pieces of metal touch and complete a circuit. Wikipedia says they’ve been around since 1922, so that’s pretty cool. I like when solid, simple devices stick around.
Based on how frequently the connection was made, I could tell that the switch would trip based on the rotation of the fan, not of the pedals. This meant that if I wanted to measure cadence, I needed to figure out the gear ratio of the bike, or in other words, how many times the fan would spin for each rotation of the pedals. I got my partner to hop on the bike and rotate the pedals 4 times while I tallied the number of times the switch tripped. We repeated this a few times just to make sure, and ended up with a ratio of 43 fan rotations for each 4 pedal rotations.
With just a screwdriver, a multimeter, some googling (and a little help from a patient partner) I was now armed with the information I needed to start building a replacement for the display.