Dynamo Speed Sensor

[Solocle]

Currently in the early phases of this project. But behold!

It seems that 220 mph is about the limit with ANT+ and the Roam...

Currently I'm just fiddling with one of the ANT+ sample projects on an nRF51 discovery board. However, the nRF51422 seems to be a fairly capable chip.

The idea is to run the rear dynamo wire through a current sense resistor (that way the speed sensor also doubles as "your light is still going!"). This will be connected to a current sense amplifier (basically an op-amp), with over-gain parameters. I don't really care about the magnitude of the current, just that there is current, so setting the gain high like this should give a nice digitalish signal to run into the microcontroller, which should just need to increment a count and reset a time count!

 

[C R]

Interesting project. So, are you aiming to map current to speed?

 

[Solocle]

Interesting project. So, are you aiming to map current to speed?

Current would be hard to map to speed, especially as it ideally produces 500 ma. No, the output is AC, and the frequency correlates directly to speed. I already have an op-amp arrangement that can detect current reversal, and have tested the cycle counter functionality. All that remains is to wire it all up, and then handle the fact that with a 28 pole dynamo you get 28 cycles per rotation! I'm not sure whether I can handle that by just setting my wheel circumference to 2155/28 = 77 mm (- 500 ppm), or whether I'll have to stick an extra counter in the interrupt handler.

 

[C R]

Current would be hard to map to speed, especially as it ideally produces 500 ma. No, the output is AC, and the frequency correlates directly to speed. I already have an op-amp arrangement that can detect current reversal, and have tested the cycle counter functionality. All that remains is to wire it all up, and then handle the fact that with a 28 pole dynamo you get 28 cycles per rotation! I'm not sure whether I can handle that by just setting my wheel circumference to 2155/28 = 77 mm (- 500 ppm), or whether I'll have to stick an extra counter in the interrupt handler.

Ah, that makes more sense. Cool project.

 

[Ming the Merciless]

The Dynamo output is AC but the current in wires from front light to rear is DC I believe. It’s post rectifier.

 

[C R]

The Dynamo output is AC but the current in wires from front light to rear is DC I believe. It’s post rectifier.

But the current will be whatever the lights draw, which, within certain limits, will be independent of speed, that's what confused me at first.

 

[Solocle]

Aand it works!

 

[Ming the Merciless]

But the current will be whatever the lights draw, which, within certain limits, will be independent of speed, that's what confused me at first.

What I’m getting at, is that with DC there won’t be a frequency from which to derive speed.

 

[Solocle]

What I’m getting at, is that with DC there won’t be a frequency from which to derive speed.

Almost certainly not the case on my setup. B&M topline brake plus. I think that depends on an AC supply to the rear.