My Bike Speedometer is eight times more accurate than yours

A consumer bike speed detector knows when a wheel has made a complete rotation and it will use Bluetooth to send that information. If you stop (or start) suddenly, the controller for the sensors can be sure you have stopped about three seconds later.² My notes (and complaints) on working with a consumer sensor are here.

I built my own speedometer that detects every time the wheel moves one eighth of a rotation. It can tell you an accurate cadence reading in 1.8 seconds and the controller knows a complete stop (or start) has happened within 0.1 seconds.³

(this is for a stationary exercise bike by the way, I’d build something different for general riding)

Here is the rig:

It’s an IR sensor hooked up to an Arduino Uno. It works because the bike wheel itself is divided into eight zones that are detectable by colour. You can see the setup in this photo of an early version:

It sends data (currently cadence, speed, and distance, even if two of these are arbitrary on a spin bike) to this display that we have on the desk:

I had to learn (and re-learn) an awful lot for this project. Before I started I didn’t know anything about 3D printing (the sensor cover and the display cover are both 3D printed designs), soldering (all the wires on the display), or even what crimping was (it’s getting wires the right size and using odd tools to lock them into holders). I didn’t know anything about an Arduino other than I could make it blink an LED at me and all my electronics was roughly remembered teenage taking-things-apart. I did at least know the maths and the C++ one needs to program an Arduino, but everything else has been a serious upskilling.

History

I considered two designs:

• Lots of magnets around the rim and a hall effect sensor that pings every time a magnet passes.⁰
• Measuring the voltage of a dynamo attached to the wheel.

However, some guy online suggested I use an optical sensor because my flywheel has eight coloured segments.

I did a proof of concept test with a photoresistor. It worked well enough that I was willing to buy an some of these IR sensors, because I understood they would be a bit more accurate and effective.

I made a simple stand out of scrap wood in the garage¹ and built this:

…and tested it on my bike setup

My test pilot got it up to 7.5 rotations per second so was functioning. It registers the speed accurately at very low speeds, which a consumer system doesn’t do and detects acceleration and deceleration much earlier.

Version 2

I took it apart and spray painted the stand.

I refitted the IR sensor, screwed it on properly rather than using an elastic band, and 3D printed a simple cover for it. I bought myself a crimping kit and tidied up all the wires.

The previous version required the laptop there all the time. That was fine for me but my partner is training for a triathlon. I 3D printed a housing for an LCD display and worked out how to display things with it.

I obviously needed a much longer cable. Nova and I found an old Ethernet cable (which had four wires internal wires, which is what we needed) and I used that. I’m finally getting some use out of my soldering iron…

I’m really impressed with how professional it ended up looking:

Next version

The next version needs some serious rewriting of the code. Not only does it need to be properly under version control under Github but it needs all manner of tests and diagnostics. This includes: * Making a histogram of light levels - I don’t believe we are getting false readings at the moment but it would be nice to have just in case it’s relevant * Experiment with how often to update the display and how often to read the sensor: that will give me information I need about if it is worth adding a second sensor to improve accuracy.
* Gentle improvements to the display - right now the numbers aren’t even left padded and it would be good to cycle though various metrics.

Version after that.

Adding a simple speaker that makes a noise if you aren’t peddling fast enough (I am regretting only having four wires in the Ethernet cable, because I could have mounted it in the display)

⁰ I’ve actually got a set of suitable magnets so I might do a test another day.

¹ I am ridiculously proud of this and I also know it looks very much of a bodge.

³ This is the result for the first design, I’m reasonably sure I can get it down to 0.03 seconds with some changes to the code, but it needs some testing.