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# Utterly Hipster Bicycle Speed & Cadence Gauges

Two analog voltage meters, ATtiny85s and some parts from a modern bike computer turned into speed and cadence gauges for a hipster's bicycle

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Here at Insight Machines Laboratory we like analogue gauges so when we found out how easy it is to convert an old analogue voltage meter into an MCU-controled whatever meter, we decided to give it a try. The result is a set of two large analogue gauges that show bicycle speed and cadence.

Trying out the speed and cadence gauges (video shows version 1.0, with slightly different dials and without LED illumination):

The main source of inspiration for this project was a desk clock built with analogue voltage meters and ATmega328 (article in Polish). For our project, we decided to use the smaller an less power hungry ATtiny85 running at 1MHz. The power comes from a CR2032 coin battery which should last for around eight months. We decided to use magnets and reed switches from a modern Sigma bicycle computer - mainly due to their low cost and reliable mounting. There's also a switching voltage regulator to make sure that the signal that goes to the voltage meter is always in the same voltage range (0-3.3V) despite the falling voltage of the battery. The regulator is a contentious issue as it uses a lot of power but we have not found any sound alternative. We're using a regulator that  has a standby mode so when the gauge shows zero, the regulator is shut down.

The diagram below shows how all the components are connected together:

The components fit nicely into the voltage meter's enclosure:

The screws, visible on both sides of the coin battery, hold the board in place and at the same time act as terminals for the reed switch. Thus, one of them is connected to the regulated power source while the other is connected to the gate of a NPN transistor and pulls down an ATtiny85's PB3 pin whenever the reed switch closes. This in turn causes an interrupt in the software that records the time of the signal. The speed/cadence value is calculated from the time difference between signals.

The gauge has a single blue LED that illuminates the dial. It is very useful after dark and adds a nice, cool look to the gauges:

The LED can be switched on and off using a simple spring loaded button at the bottom of the gauge:

The gauges are mounted on the handlebar using a plastic band with a worm-like drive. Tha band was adapted from a Cateye headlamp:

It is attached to the gauge's "can" with a single screw:

At this point, we consider the project complete. The gauges obviously need some tweaking but all in all, the devices work as expected and are sufficiently reliable.

### gauge.svg

Gauge graphics

image/svg+xml - 101.04 kB - 09/14/2017 at 16:09

### BicycleAnalogueGauge.ino

Arduino source code for both speed and cadence gauges

- 3.77 kB - 08/20/2017 at 18:45

• 1 × ATtiny85 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
• 1 × Analogue Voltage Meter We've used 15V meters but other ranges will also work (as long as you rescale them properly)
• 2 × NPN transistor Used to detect signals from the magnetic sensor and to drive the voltage meter
• 2 × 10kΩ resistor Used to limit the current going to the reed switch and to control the voltage meter (via NPN)
• 1 × 4.0kΩ resistor Used to change the voltage range of the voltage meter
• ### Dial illumination test

Insight Machines Lab10/29/2017 at 09:57 0 comments

Took the bicycle out yesterday evening for a windy test drive to see how the dial illumination performs. The results are satisfactory. See for yourselves:

• ### Testing the night light

Insight Machines Lab09/28/2017 at 19:41 0 comments

Version 2.0 of the gauges set will have a blue LED to illuminate the dial. Today, we've made a simple test to see whether a small 3mm LED, mounted at the bottom of the gauge, will be sufficient to light up the whole dial. The results are promising. See for yourself:

• ### New dials

Insight Machines Lab09/27/2017 at 19:57 0 comments

We printed a new version of the dials on a glossy sticker paper. They should prove more durable than the ones printed on regular paper and, of course, stick to the metal plates below them so they don't wrinkle or bend. We've also printed some dials in inverted colour scheme (visible on the left on the photo below) but that was not a very good idea so we'll just drop it.

• ### New test drive video

Insight Machines Lab08/28/2017 at 21:38 0 comments

Shot a new test-drive video depicting the behaviour of both the speed and the cadence gauges. The video is available in the "Description" section as well as here:

• ### Changing the looks

Insight Machines Lab08/28/2017 at 21:34 0 comments

Tried to remove the plastic ring around the gauge today. Without a proper stand for a drill to cut a perfect circle with the diameter of 65mm, we opted for a more brute-force approach with the Proxxon Micromot 230 and a routing bit.

The result wasn't perfect...

... But it has to do.

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## Discussions

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Insight Machines Lab wrote 09/14/2017 at 16:31 point

Thanks for your comments. The gauges are in daily use and they still work. They even survived a modest rain. However, some kind of weather sealing will be definitely necessary. Same applies to shock protection, although those devices aren't meant to be used on mountain bikes and it seems fair to assume that they will never be very rugged.

When it comes to the needle driving mechanism, the initial plan was to use a small servo because, just like you've written, it seemed obvious that the needles will wobble constantly due to vibration. However, out of pure laziness, we decided to try the original voltage meter mechanism first and surprisingly it proved very stable. The reason for this is that  the gauges are mounted almost horizontally so vertical shocks don't matter too much and horizontal shocks don't happen all that often on an urban bicycle. The only issue with durability that we have already observed is that the screws that hold the wires from magnetic sensors tend to loosen from time to time. This is annoying but not very severe and should be relatively easy to fix (one way or the other).

Are you sure? yes | no

James Murphy wrote 09/13/2017 at 17:12 point

Being an old 31E20 US Army Field Radio Repairman I LOVE the analog meters!  What Can we do to help?  PS Grandsons are 13 and 12 and ride their bikes many many miles per day.  Possible Test setup?

Are you sure? yes | no

Insight Machines Lab wrote 09/13/2017 at 20:18 point

Hello James!

Any volunteers for testing are welcome. The two gauges from the demo video are still operational and are used almost on daily basis, but that does not mean that more tests would not be valuable. Hence, if you have a moment to spare and are willing to build and test another gauge, that would be great.

The biggest hurdle right now is to figure out a way to get rid of the voltage regulator as it consumes too much power and is actually the most expensive part of the whole device (!). It was added to make sure that the voltage on the mechanism that moves the needle is always in the same range (0 - 3V) regardless of the voltage on the battery. The regulator performs quite well, but the problem is that it uses few times more power than the micro controller in sleep mode and significantly shortens the battery life. It should be possible to use the ATtiny to measure the voltage on the battery and as the voltage slowly drops, compensate for that by slowly increasing the PWM duty cycle. But that's just our theory...

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Lolucoca wrote 08/20/2017 at 22:17 point

That's a really cool project! I personally really like analog meters and in fact I'm using one right now as a sound level meter. I might actually do this to my bike!

Are you sure? yes | no

Insight Machines Lab wrote 08/21/2017 at 08:02 point

Glad you liked it! We were afraid that the needle would wobble when riding the bike due to vibration etc but it turned out that as long as the gauge stays horizontal, the wobbling is negligible.

Are you sure? yes | no

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