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Need some help

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New Member
i am making a bike speedo using a motor attached to the wheel, but the problem i have is that i do not know how to work out the voltage for different speeds. such as 5mph 10mph etc etc. i need to know this so that i can change values of resistor so that at different speeds more LED's light up.

any help would be of help

thank you



New Member
Any help? Sounds like you want to generate electricity from your bike wheel and use this to light sequential LEDs. If that's correct, you have too many variables to do the calculations from scratch (generator output per rpm, pulley/gear ratio, etc). Eliminate the variables. The first thing you must do is mount the generator to the bike, go 1 mph (clock your speed with a car or a manual spedometer), measure the voltage output with a standard volt meter, and write the voltage value down. Increase your speed to 2 mph and write that value down. Continue until you have a chart of values. Use this chart to design your LED circuit.


Active Member
Hi Mark,

I'm gonna guess you have an old meter mounted on a
bicycle driven by a motor on the bicycle wheel.

About the fastest you could go on a standard bike
would be around 25 mph.

fiddle with the series resistor to get just under
full scale deflection, then mark FSD at 30 mph.

You could further refine it with the help of
a car driver.

Best of luck with it,
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