Electronics buff's

[Supa Dexta]

Hey guys, looking to get into a little project here. On my motorcycle the speedo is driven off a 5v pulse from the motor, not anything to do with the wheels. So if you change your final drive sprockets, you mess your speedo calibration up (not to mention they aren't acurate from the factory to begin with). This unit here fixes that:

http://www.speedohealer.com/eng/intro.htm

They want a 100usd or so for it..

I'm curious to see whats inside this thing, but no one has a schematic, or can open one, as its sealed in epoxy (likely because the circuit is quite simple and they don't want people to know)

So since the speed sensor outputs a 5v pulse I believe, if I were to get a frequency generator, and hook it up and find the range 0mph - say 160mph or thereabouts) that would be a start. I'm thinking it may involve a 555 timer circuit, perhaps an opto coupler, but I don't know just yet, just getting into things.. Got a lot of reading and figuring to do at this point.. and recomendations?

[Unknown303]

If thats the case its probably is running off of some kind of amplifier circuit into a counter.

[tulit]

Or not even...

A simple micro (AVR) could do it.

Feed the signal from the motor to an ISR line (possibly buffering it first) and simply have it count the pulses in a certain time frame (couple it with a TIMER). Hook the output onto one of the PWM lines and simply adjust the frequency of the output on the fly based on the interrupt counts (change one register). You'll have the ability to continouslly calibrate the system across the whole range this way.

Using just a counter won't help unless you are trying to divide the frequency by exactly an integer value. Using a 555 would be a possibility too but I can't think of a really direct way right now to make it into a voltage controlled frequency generator (their timing is based on the time constant it takes to charge/discharge an external cap). And in any case, it will end up being a more complex solution than just using a small micro.

$5 worth of parts and a bit of coding and you're good to go.

[Supa Dexta]

Thanks guys, not sure what to do just yet. No time at this point to round everything up to get working on this.. we'll see

[Unknown303]

Whoa never even thought of doing it that way.

[Supa Dexta]

anyone have a freq. generator i can borrow? :) the more I think about this, the more I get turned around.. Somewhere in this circuit, it knows what gear it is in as well... we'll see..the avr seems like it may be the way to go..

[Supa Dexta]

Well holy hell, I found a pic of a circuit board for a competing brand, and its not terrible quality either.. one more step foward... This internet has everything..