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I need a simple device to tell me the speed I am traveling on a bike through two 7-segment displays. Is there a simple way to do this?<p>Thanks, Mike

Well initially I'd say "go get a speedometer from a bike shop" and save some time. But that's not the tinkerer's way to do it.<p>Commercial speedometers for bikes generally use a magnet mounted on a spoke, and a reed switch mounted on the fork. You're probably best off using a microcontroller. I would personally recommend a PIC, but obviously Basic Stamps are supposed to be easy. Convert the length of time between wheel rotations to a speed based on the circumfrence of the wheel. I suspect that for your precision level, a lookup table of some kind would be easiest to convert the rotation time to speed.

I never worked with microcontrolers, stamps and stuff like that, and don't want to put the money in for one project. Is there a way to do this using the same reed switch/magnet combo, but without the microcontroller?

LM2907<p>ICL7137<p>
You will need a frequency to voltage converter and an analog to digital converter with display driver.<p>[ July 24, 2003: Message edited by: ljbeng ]<p>[ July 24, 2003: Message edited by: ljbeng ]</p>

Mike,
The difficulty with timing one wheel revolution by the magnet/reed switch is that the speed is inversely proportional to the measured time so that whatever you use you have to take the inverse. What you might do is to fit a photo-electric pair to the front forks and arrange them so that the spoke connectors - or the wheel spokes interrupt the beam. If you then set up a two digit counter to count the beam interrupts then you could use a 555 to set the timing interval over which you count spokes. In order to make the counter direct reading in MPH the interval set by the 555 would be of the order of one tenth of a second. Hope this helps.

If your willing to spend $20 for a cheap digital voltmeter or surplus panel meter, then all you need is a way to generate an average voltage proportional to the rotation speed. For example, a magnet, coil, diode, and capacitor. The faster the wheel rotates, the higher the voltage and more frequent will be the pulses. The diode rectifies and the capacitor averages it out. Or use the previous photoelectric idea and average that instead. You would use a pot set up as a voltage divider to calibrate it.

Isn't a PIC (Stamp or whatever) required to run a LCD display? It seems that no matter how you arrive at the speed value you are going to have to run the analog signal through a PIC to be able to display the value on a LCD.

I do have a small 0-1 ma meter. How would I go about connecting it?<p>Thanks, Mike

This is one case where you would never be able to beat the price of a commercial product since I have seen full featured bike speedo's for under 20 bucks. Nevermind the circuit just the case mounting wireing and sensors will cost you more than that.<p>I have done some priliminary work with a bs1 just to see if it would be fast enough to work as a speedo and it would. <p>Another idea is to measure both the speed of the wheel and the chain wheel with a stamp. Calculate the ratio and convert it with a lookup table to read out as a gear indicator.
That is a feature I've never seen at any price.

One quick and dirty method would be to use a small permanent magnet motor as a tachometer. This could have a rubber wheel attached to the motor shaft and bearing either on the tire or the rim. The motor will generate a DC voltage proportional to speed. This voltage can be read with your 1 ma meter by using a series resistor to convert voltage to current. I would expect a couple of volts out of the motor, so the resistor would probably be about 2K to 10K ohms. Three parts, no batteries. Calibration may be a problem.

ljbeng, you said two parts I would need. I ordered samples from maxim for the one and am going to order samples from national of the other parts. How would I use these parts in my application?