I was wondering if anyone can figure out how the cadence sensor works on a Sigma Sport computer... I was thinking of making a shift light (haha yea like on a car's tachometer) so that as I approach 100-ish rpm cadence a red led would blink telling me it's time to shift... I'm thinking maybe somehow connecting a resistor / and a led to the two contact points between the cadence wire and the computer's bracket?

Yes I'm going out of control with this bike thing... I actually train on my mountain bike riding on the road cuz I can't afford a decent road bike yet... :D

im sure they would buy this one up

I think I'd rather be looking at the trail then waiting for an LED to light up!!

Most bike computers use a "reed switch" and a magnet attached to your spokes. When the magnet goes past the sensor, the reed switch closes, sending a pulse to the computer. Taking into account the wheel circumference that you've set the computer to, it works out how many revoloutions per minute you're doing, and converts it to a km/hr or miles per hour reading.

The same principle works for you cadence. There is no way that you could simple hook an LED up to the sensor or bike computer. At best, you might be able to get the LED to flash at each crank revoloution, but thats about it. The majority of bike computers these days have very few components, and use just a single custom made microcontroller. There is no real way to simply interface to it.

Cheers, Dave

I think I'd rather be looking at the trail then waiting for an LED to light up!!

Most bike computers use a "reed switch" and a magnet attached to your spokes. When the magnet goes past the sensor, the reed switch closes, sending a pulse to the computer. Taking into account the wheel circumference that you've set the computer to, it works out how many revoloutions per minute you're doing, and converts it to a km/hr or miles per hour reading.

The same principle works for you cadence. There is no way that you could simple hook an LED up to the sensor or bike computer. At best, you might be able to get the LED to flash at each crank revoloution, but thats about it. The majority of bike computers these days have very few components, and use just a single custom made microcontroller. There is no real way to simply interface to it.

Cheers, Dave


you could make your own with a programmable microcontroller. www.basicstamp.com

it's totally doable, just not as a hack on a cyclocomputer.

But it is just like any other thing that may become too busy. It is kind of like the study they did with fighter pilots. They could put everything on the HUDs if they wanted to. What do the pilots want? Target acquisition, target track, weapons selector, the three big A's (attitude, altitude, airspeed), %thrust and G's. The pilots don't want turbine speeds or any of that jazz on there. They just want the fighty stuff on the HUD.

People are like that. A shift indicator might be a bit much for most people. I know that when I am running too high a cadence, I want to shift for a lower cadence. Besides, there is also the consideration of why they are at that cadence? Having a machine tell you that you should shift to a harder gear while your bouncing through a rock garden may not be the best plan, especially if you pay attention, shift down, stall out and fall...

Besides, colour me clueless, but shouldn't you be spending most of your time looking 20-25 feet ahead when you're riding?

Lets think this through. I am not sure how the reed switch works, but I would guess that there is some sort of a charge associated with the pulse of the magnets. If you were to hookup a capacitor to the wires and then measure what kind of potential is collected at 100rpm. You would then need to pick a diode that lights up at that potential, or do a voltage/current divider that would split the current/voltage to the one required by the diode to light up. I think that you can completely bypass the cyclo computer here. Someone please correct me. Better yet, on your voltage divider, you could make one of the resistors a pot so that it can be adjusted which in turn would make your diode light up at whatever rpm you would want.

_MK

LOL - no such luck. :D

A reed switch is just a tiny glass housing with a contact and a thin bent wire inside that when near a magnet is pulled closed to create a circuit. They do not generate electricity, and can't handle large amounts of power going through them as they are so fine inside.

Agreed, it would be very easy to create such a "shift-light" but IMO it really is quite pointless!!

- Dave.