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The Cyclotron mini is a battery-less cyclocomputer that uses a hub dynamo for both power supply and speed measurement.

cylotron mini

Motivation and key ideas

As I frequently ride my bike after sunset, I needed a cyclocomputer that's readable in the dark, i.e. has an illuminated display. This rules out devices powered from a non-rechargeable coin cell as a backlight would lead to unsatisfactory battery life. I also didn't want yet another gadget that needs to be charged.

Fortunately, proper bikes these days come equipped with a hub dynamo that provides an always-on power supply at low mechanical resistance. Apart from power, a hub dynamo also provides accurate speed and distance measurements since its output frequency is directly linked to the tire's rotation.

Features and facts

  • All of the usual cyclocomputer features
    • Speed
    • Distance
    • Max. speed
    • Average speed
    • Time in motion
    • etc.
  • Stores the last 15 trips
  • Trip-independent distance accumulator
  • Backup power provided by 1.5F supercapacitor
  • Barometer for altitude measurement
  • Backlit 8 character LCD that's also readable in sunlight
  • 3 buttons (Ok, Down, Up)
  • Ambient light sensor for automatic backlight control
  • MSP430FR4133 MCU
  • Closed-case firmware updates

Hardware

As usual, the PCB layout was done in Horizon EDA and boards were ordered from JLCPCB. The components got soldered on a cheap electric hotplate after applying solder paste with a stencil.

cylotron mini

PDF schematics

MCU

The Cyclotron mini is based on the MSP430FR4133 MCU. It has way more pins than actually needed, but I had one spare and it's still available amidst the great chip shortage.

LCD

The LCD is a DOGM081-A 1×8 character LCD with amber backlight. I went with a character display rather than a graphical one as these require MCUs with large program memory and RAM to store fonts and framebuffer respectively. Apart from that, a character LCD effectively limits firmware feature creep such as graphs.

The backlight LEDs are rated to run at 80mA, anyhow they provide sufficient illumination at around 2mA. Since the the LCD is a transreflective one, the backlight is only required when it's dark and thus doesn't need to be all that bright.

Unfortunately, this particular display doesn't support any meaningful low-power (<1µA) standby modes, so it's powered from an IO pin.

Power supply

power supply

The AC voltage from the dynamo is clamped to about ±7V by two antiserial zener diodes D8 and D9. The 170V TVS diode D9 provides some additional transient protection. It's probably not really necessary, I only included it as other people were using similar protection on their dynamo powered circuits.

The LDO U6 regulates the rectified and smoothed voltage to 5.2V to charge the 1.5F backup capacitor via a 50 Ohm (actually 2 100 Ohm in parallel) series resistor. I did some simulations with various current source circuits to charge the capacitor faster, but couldn't come up with one that worked significantly better than a simple current-limiting resistor. D16 prevents the capacitor from discharging into the unpowered LDO.

When there's power from the dynamo, but the supercapactior voltage still is too low, D15 and D17 provide a bypass to power the device while the supercapacitor is charging.

Finally, the LDO U7 regulates the bus voltage to 3.3V as it's required by the rest of the circuitry. The BOM lists an ADP162AUJZ-3.3 from Analog Devices, but an an S-1313C33-M5T1U3 by ABLIC with lower quiescent current is a pin-compatible drop in replacement that I ended up using.

To measure the capacitor and bus voltage without loading them too much, they can be momentarily connected to a voltage divider by the P-channel MOSFETs Q2 or Q3. The RC circuit provides the level shifting to turn on the transistors. I got that idea from Mike Harrison's 2017 Supercon badge.

It turned out that standby time (with the LCD off) is mostly determined by the leakage of the schottky diodes surrounding the supercapacitor, leading to a current consumption of about 8µA which will deplete the supercapacitor in about 4 days. It might be worth experimenting with non-schottky diodes or parts with lower leakage to extend that time.

Without backlight, the idle current consumption is about 150µA.

Cycle counting

The hardware has provision for three ways to count the cycles of the AC signal generated by the dynamo as it's needed for speed and distance measurement.

Digital inputs

Measurements confirmed that the dynamo produces its nominal voltage well below 10 km/h, so the simplest way to count cycles is directly connecting the AC signal to digital inputs of MCU with series resistors and clamping diodes. As this method provided good results, it's the one I ended up using.

ADC measurement

In case the above approach didn't work for some reason, the selected MCU pins can also be multiplexed to its internal ADC.

Comparator

If all of the above methods would have failed, there's provision for a low-power comparator to turn the AC signal from the dynamo into a logic-level signal to be processed by the MCU.

Barometer

The go-to barometric pressure sensor these days appears to be the BMP280 by Bosch, at least judging by what's available as modules for makers. Unfortunately, it's apparently a victim of the 2021 chip shortage and impossible to get hold of.

Instead, I went for the MS563702BA03 by TE connectivity. It's connected to the MSP430's I²C peripheral. Since it consumes less than 100nA when not doing measurements, it can be directly connected to the always-on 3.3V power supply.

It's I²C interface is a bit odd as it doesn't use repeated starts for reads requiring an address. It also doesn't provide any means to poll if a conversion is still in progress, instead one has to wait for the expected conversion time and hope that that's been long enough since reading the result too early corrupts the measurement. At least the result will be 0 to indicate that it's not valid.

Since there still was sufficient space on the board, there's provision for using the BMP280 instead.

Ambient light sensor

Influenced by part availability I picked the ambient light sensor LTR-329ALS-01 made by LiteOn. Same as the LCD, it doesn't have any useful sleep modes, so it's also powered from an IO pin. This meant that it couldn't share the I²C bus with the barometer as its clamping diodes would have adversely affected the bus. As there's only one I²C peripheral in the MCU, its I²C bus is bit banged.

Bike and programming interface

To provide a reliable electrical connection to the mount on the handelbars, the Cyclotron mini uses a 6-pin spring loaded header, the The remaining 4 pins are used for programming and one spare pin. The header is kind of expensive at around 5€, but works really well. As the pins are exposed, they're protected by ESD protection diodes.

Bugs

As always, there were some minor goof-ups:

  • The footprint for the MIC5295 voltage regulator appears to have slightly incorrect pin spacing
  • The display overlaps the top pad of the buttons by a little bit

Firmware

The overall architecture is very similar to the Pluto digital watch, with some simplifications here and there. The MCU spends most of its time in LPM3 sleep mode and is woken up about every millisecond (1/1024Hz) by a timer interrupt. The interrupt handler reads the button state and handles debouncing. If a button was pressed, the low power mode is left and the main loop resumes. It also counts the pulses generated by the dynamo. To periodically refresh the display, the interrupt handler also leaves low power mode every second.

The main loop then figures out why it got woken up, populates an event mask with the according event and calls the top-level event handler. This one then dispatches the event to the current view as well as other subsystems.

Same as pluto, the firmware is built using the msp430-elf GNU toolchain and a simple makefile. Enabling link-time optimisation helped to eliminate getter/setter functions in the resulting binary and reduced code size by about 1000 bytes.

LCD

The firmware maintains an 8-character "framebuffer" that is flushed to the display once the current view is done drawing. This ensured flicker-free display updates. The LCD's hardware cursor comes in handy to indicate the current digit when adjusting values.

Symbols such as °C and km are displayed using custom characters to save space on the screen.

Tachometer

The 1024Hz timer interrupt handler calls a function that reads the state of the two pins that sense the AC signal generated by the dynamo. For increased robustness, a pulse is only counted if a rising edge is detected on the other pin than the prior one. The accumulated pulse counter is then used by other code for calculating speed and distance. Overflows of this counter don't matter as other code only considers its difference between two points in time.

The current speed is calculated every second based on the pulse count difference, counts per revolution and tire circumference.

Distance is accumulated by another pulse counter that counts modulo the counts per revolution. When it overflows, the tire circumference is added to the millimeter accumulator. That one in turn counts modulo 1000mm, and adds one to the total distance in meters on overflow.

Barometer and altimeter

Calculating air pressure from the uncalibrated 24-bit pressure and temperature ADC readings provided by the sensor unfortunately involves 64bit math that consumes around 1000 bytes of program memory.

To avoid waiting for the conversion to finish, the ADC conversion result is only read in the next one-second tick, also eliminating the possibility of not waiting long enough and corrupting the measurement.

The formula to calculate altitude from air pressure looks quite intimidating to implement on an an MCU without floating point unit. However, a reasonable approximation can be made if the relative altitude change is small since the slope can be considered constant in that case.

As it turns out the derivative of the altitude only consists of elementary arithmetic after plugging in the reference pressure calculated from known pressure altitude and simplifying the equation.

Ambient light sensor

The datasheet of the ambient light sensor doesn't provide a formula for calculating the illuminance from the ADC readings, but we only need a threshold for turning on the backlight, this doesn't matter that much. Even though the sensor contains two channels tuned to different parts of the spectrum, using channel 0 proved to be sufficient.

Power management

To conserve power when the device is off, LCD and ambient light sensor are powered down and the MCU only only wakes up at rate of 4Hz to check if any button got pressed or the dynamo generated any pulses.

Construction

All parts have been designed in FreeCAD and were printed on a FlashForge creator pro 3D printer using PLA filament and TPU for semi-flexible parts. Finally some good use for the dual extruder!

CAD files

Due to licensing restrictions, the CAD files available in the mech directory don't contain the 3D models of certain components.

Even though each file contains all parts, only use the ones that are visible.

Case

The overall concept of the case is similar to the Hubble SFP multitool. The main circuit board is located by the two halves of the case, eliminating the need for screws or the like that'd consume space on the circuit board. Both halves are held together by 4 M2×8 (×6 works as well) torx screws threaded into the inner shell. As they don't have so support any axial forces, there's no need for metal inserts or nuts.

The ambient light sensor gets to see the outside world through a light pipe pressed into the rear shell.

cylotron mini

Same as with Hubble, the key caps are integral to the case, eliminating any chance of rattling. Thinning the front wall provides the required flexibility. The low pivot point of the mechanism makes it possible to to press the keys from the front rather than just straight on.

A case that's waterproof around the keys is currently work in progress.

Mount

Central part of the mount is a dovetail-shaped piece that's affixed to the rear shell with two self-tapping screws (see above). It slides into the part that's mounted to the handlebars and locks into an integral retaining clip with a satisfying click.

cylotron mini

The mount itself is attached to the handlebars by means of 4 u-shaped clamps that are held together using threaded rod as I couldn't find screws long enough. To provide better grip, the inside surface of the clamps (shown in red) is printed using semi-flexible TPU filament.

cylotron mini

Electrical interface

The spring loaded pins from the main board mate with targets pressed into the mount. The targets are installed by first fiddling the cable through the hole in the mount, soldering it to the target and then pressing it in using pliers.

Development mount

cylotron mini

For easy firmware updates and debugging, there's a variant of the dovetail receptacle with all targets populated. They're connected to the 5V and Spy-Bi-Wire pins of an MSP430 evaluation board.

Issues and Pull Requests

Unlike software projects, hardware projects like this one reach a state of completion once I'm happy with what it's doing. After that, I usually don't have much interest in continued development and move on to new projects. So don't expect me to review and merge pull requests. Happy forking!

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