To be honest with you, they are so inexpensive why spend the time and money getting parts to build one.

http://www3.towerhobbies.com/cgi-bin...&I=LXPT32&P=ML

This one is $24.99 at Tower.

But, if you are willing and able, it certainly could be done. I am sure someone with some electronics imagination could make one. It would have to be some sort of opitical pickup and a counter with a divide circuit, then some sort of display circuitry. I am an Electronics Engineer but would just buy one pre-made in a small, easy to handle package

Obviously somone can do it because they are sold.. but again, why bother!!

I found this site using Google Search:

http://www.floridaame.org/Plans/Kamr...achometer.html


This one uses four segment display for up to a count of 9999. You would want to go above that, so a five segment counter display would be necessary. In place of the Hall Effect sensor, you could use some sort of optical pickup and then count the shadow of the prop as it passes by. Divide by two for a two bladed prop, by three for a three bladed prop, and so on.

CGr.