How To Build A Passive Magnetic Pulse Extension Cord

[Sparky49]

Bicycle speed and cadence is measured using magnets. (https://buy.garmin.com/shop/shop.do?pID=1266&ra=true) However, some cycles (http://tridenttrikes.com/stowaway.htm) are configured so that the wheel is too far from the pedals for the Garmin sensor to measure the pulses and thus rotations on both. How can a pulse from a moving magnet be transmitted to the sensor on the Garmin device?

 

Could a coil be placed to receive the pulse of one of the magnet and simply wire it to another coil next to the sensor? If so, what shape and size should the coils be? What would be the best orientation of the coils relative to the magnets? What gauge wires would work best for the coils? What number of turns of wire in the coils would work best? Should the same number of turns be used on both coils? What would be the best gauge wire of the connecting pair? Should shielded twisted pair cable between the coils or unshielded be used?

 

(I'm feeling old and short of time so I'm asking a lot of questions. I hope it is not rude.) I hesitate to ask about possible donor coils like earbuds.... I do want to avoid using a powered circuit as there are already batteries in the sensor, watch, and chest transmitter.

[CraigD]

Welcome to hypography, Sparky! :)

 

On the subject of biking, check out this gallery photo of Hypography’s founding member and owner (he who worries about paying for hosting and making things work when they break) Tormod:

It’s been years since I regularly rode (a collar-bone breaking close encounter with a car on my daily commute and a violently concerned wife was the main cause of my stopping), so I’m way out-of-date on all things bike related, so I had fun reading and doing a little research on your question.

 

Bicycle speed and cadence is measured using magnets. (https://buy.garmin.com/shop/shop.do?pID=1266&ra=true) However, some cycles (http://tridenttrikes.com/stowaway.htm) are configured so that the wheel is too far from the pedals for the Garmin sensor to measure the pulses and thus rotations on both. How can a pulse from a moving magnet be transmitted to the sensor on the Garmin device?

 

Could a coil be placed to receive the pulse of one of the magnet and simply wire it to another coil next to the sensor? If so, what shape and size should the coils be? …

My guess is that about any pair of little coils would work. I’d skip the questions, and just take an unneeded or cheap set of earbuds, snip and solder the wires from the 2-wire cords together – polarity shouldn’t matter, position one near your wheel magnet and the other right up against your wireless sensor units wheel magnet sensor, and give things a spin. I can’t think of any reason this shouldn’t work

 

If there are problems, my next step would be to perform a bit of earbud surgery and remove the magnets from withing the coils, on the through that they’re moving when the wheel magnet moves past, and messing up the induced current, or reducing the induced magnetic field at the sensor end. As this may mess up the waterproofing of the earbud, some post-op silicon seal might be called for once things test OK., and may be needed anyway, as you’ll need a neat way to make the earbuds, which aren’t shaped right to be tied to a bike frame, tie to it securely.

 

Once you’ve gotten things working in a test setup, you may need more wire length than comes with the earbuds, I’d just scavenge another set, or shop an electronics part store for some two-wire cord that looks about the same. Since the currents involved here are tiny, I don’t think picking a specific wire gauge or shielding is important.

 

If you can’t get anything to work, or don’t want to hassle with all the crafting and experimenting (though I’m guessing you’re the sort who enjoys this sort of hassle :)), here are a couple of backup options:

• Get a “tethered, independent speed and cadence sensors for optimal placement of each” sensor like this Wahoo Fitness one. Though pre-wired with a very short connecting cord, according to Wahoo, it can easily be extended to work on recumbent bikes. I suspect your Garmin receiver is ANT+ compatible – if it isn’t, shame on Garmin!
  
• Give up on wireless and replace everything with a wired unit like this CatEye. Most of the wired units appear to have separate sensors for wheel and crank magnet, though they may not come pre-wired with enough cord, so would need a little out-of-the-box cutting, soldering, and sealing.
  

I’m curious to know what you work out, and hope you’ll post pictures when you’re done. Good luck, and hope you stick around our humble website for some science fun!

[Sparky49]

Welcome to hypography, Sparky! :)

 

On the subject of biking, check out this gallery photo of Hypography’s founding member and owner (he who worries about paying for hosting and making things work when they break) Tormod:

It’s been years since I regularly rode (a collar-bone breaking close encounter with a car on my daily commute and a violently concerned wife was the main cause of my stopping), so I’m way out-of-date on all things bike related, so I had fun reading and doing a little research on your question.

 

 

My guess is that about any pair of little coils would work. I’d skip the questions, and just take an unneeded or cheap set of earbuds, snip and solder the wires from the 2-wire cords together – polarity shouldn’t matter, position one near your wheel magnet and the other right up against your wireless sensor units wheel magnet sensor, and give things a spin. I can’t think of any reason this shouldn’t work

 

If there are problems, my next step would be to perform a bit of earbud surgery and remove the magnets from withing the coils, on the through that they’re moving when the wheel magnet moves past, and messing up the induced current, or reducing the induced magnetic field at the sensor end. As this may mess up the waterproofing of the earbud, some post-op silicon seal might be called for once things test OK., and may be needed anyway, as you’ll need a neat way to make the earbuds, which aren’t shaped right to be tied to a bike frame, tie to it securely.

 

Once you’ve gotten things working in a test setup, you may need more wire length than comes with the earbuds, I’d just scavenge another set, or shop an electronics part store for some two-wire cord that looks about the same. Since the currents involved here are tiny, I don’t think picking a specific wire gauge or shielding is important.

 

If you can’t get anything to work, or don’t want to hassle with all the crafting and experimenting (though I’m guessing you’re the sort who enjoys this sort of hassle :)), here are a couple of backup options:

• Get a “tethered, independent speed and cadence sensors for optimal placement of each” sensor like this Wahoo Fitness one. Though pre-wired with a very short connecting cord, according to Wahoo, it can easily be extended to work on recumbent bikes. I suspect your Garmin receiver is ANT+ compatible – if it isn’t, shame on Garmin!
  
• Give up on wireless and replace everything with a wired unit like this CatEye. Most of the wired units appear to have separate sensors for wheel and crank magnet, though they may not come pre-wired with enough cord, so would need a little out-of-the-box cutting, soldering, and sealing.
  

I’m curious to know what you work out, and hope you’ll post pictures when you’re done. Good luck, and hope you stick around our humble website for some science fun!

 

Hi Craig,

 

Thanks for the kind welcome, your time and efforts too.

 

I'm not good at trying different things to make it work. I get distracted and things never get finished. It is better for me to have a plan and do it in one sitting (or standing).

 

As far as your design is concerned, it sounds a lot like my thoughts. I guess I'll just have to try different approaches like you said.

 

I was hoping someone here would have an understanding of how magnetic sensors work and the coil placement, shape etc.... This is such an exciting and interesting forum. I'm so glad I found it. Thanks are due to Tormod!

(Something I found years ago that still is a puzzle is that when I make a voltage transformer, the number of turns around the armature is

proportional

to the voltage out, but when making a current transformer, the number is

inversely

proportional to the current out. That is to say, my understanding of such things is limited. Further, I have found that asking questions sometimes reveals answers and other times one must wait for others to answer.) Hmm, perhaps the reason for the difference between the transformers is that one coil is the primary (current transformers) and the other the secondary?

 

As far as Ant+ compatability is concerned:

from the REI site:

"The Garmin GSC 10 Speed and Cadence sensor measures your pedaling strokes per minute, providing valuable cycling training feedback.

 

Performance cyclists know it's more efficient energy usage to maintain a set cadence than chase a set speed over varying terrain; GSC 10 makes it easy

Self-calibrating, wireless unit works with Colorado 300, Colorado 400c, Colorado 400i, Colorado 400t, Dakota 20, Edge 305, Edge 500, Edge 705 and Edge 800

Also works with Forerunner 305, Forerunner 310XT, Forerunner 405, Forerunner 405CX, Forerunner 410, Forerunner 50, Forerunner 610, Foretrex 401 and FR60

Also works with GPSMAP 62, GPSMAP 62s, GPSMAP 62st, Oregon 300, Oregon 400c, Oregon 400i, Oregon 400t, Oregon 450, Oregon 450t, Oregon 550, Oregon 550t and Dakota 20

Garmin GSC 10 Speed and Cadence sensor weighs 1.3 oz. and measures 2.75 x 1 in.

Compatible GPS Receivers: Colorado 300, Colorado 400c, Colorado 400i, Colorado 400t, Dakota 20, Edge 305, Edge 500, Edge 705, Edge 800, Forerunner 305, Forerunner 310XT, Forerunner 405, Forerunner 405CX, Forerunner 410, Forerunner 50, Forerunner 610, Foretrex 401, FR60, GPSMAP 62, GPSMAP 62s, GPSMAP 62st, Oregon 300, Oregon 400c, Oregon 400i, Oregon 400t, Oregon 450, Oregon 450t, Oregon 550, Oregon 550t, Dakota 20"

 

Nothing there about it being Ant+ compatible, but I cannot argue from a negative can I?

 

Sparky49

[CraigD]

As far as Ant+ compatability is concerned:

from the REI site:

"The Garmin GSC 10 Speed and Cadence sensor measures your pedaling strokes per minute, providing valuable cycling training feedback.

...

"

 

Nothing there about it being Ant+ compatible, but I cannot argue from a negative can I?

This product list makes me think that all the Garmin bike computers that sense speed and cadence are ANT+ compliant, so I don't think you'd have a problem swapping your one-piece GSC 10 sensor for something like the cord-connected 2-piece Wahoo Fitness sensor.

 

An emailing to Wahoo (or some other maker of a similar sensor unit) asking them to if this would work seems a good idea. Since they don’t make a receiver/computer unit (The usual receiver/computer for their system seems to be an Apple iPhone 3 or 4, stuck in protective case with a little ANT+ receiver that plugs into the iPhone), I expect they’re pretty good at answering compatibility questions like this.

[Sparky49]

This product list makes me think that all the Garmin bike computers that sense speed and cadence are ANT+ compliant, so I don't think you'd have a problem swapping your one-piece GSC 10 sensor for something like the cord-connected 2-piece Wahoo Fitness sensor.

 

An emailing to Wahoo (or some other maker of a similar sensor unit) asking them to if this would work seems a good idea. Since they don’t make a receiver/computer unit (The usual receiver/computer for their system seems to be an Apple iPhone 3 or 4, stuck in protective case with a little ANT+ receiver that plugs into the iPhone), I expect they’re pretty good at answering compatibility questions like this.

 

Thanks Craig!

 

<Update with answer from Wahoo Fitness>

 

 

"Our speed and cadence sensor might work for you, but you would need to

modify it to lengthn the wire connecting the two sensors. A better

solution might be to buy separate speed and cadence sensors such as

those branded by bontrager and others. As long as they are ANT+, I

believe that they should work with the Garmin but can not be sure as

the Garmin might only allow readings from combo speed and cadence

sensors. If that was the case you would either need to live without

cadence or speed it buy a sensor like ours and modify it. Modification

can be a fairly easy task if your handy with a soldering iron. You

would just cut the wire connecting the two, solder in a new extension

and wrap it with some sort of electrical shielding."

 

This might answer the question about using shielded twisted pair...