Ok, Here's another one from the wild03 cookbook. I'm sorry to say that this project requires a Multimeter with frequency measurement capabilities. and that's why It doesn't qualify as a "CHEAP Homemade Tachcometer"
Anyways, I used a simple Infrared Emitter and Decoder circuit from:
www.boondog.com/\tutorials\irled\ir.html
as you can see the parts are pretty cheap! This circuit produces a clean square wave when the beam is blocked.
I used an old PC mouse to get one of the two decoder wheels inside the mouse I covered all the slots but two, I then removed the little tab between those two to make a nice square hole. I then attached the wheel to the ZZ motor, and the output of the circuit to the Multimeter (SunPro CP7680)
With every turn of the encoder wheel the output signal will complete one cycle, the meter will measure cycles per second and all is left to do is to multiply that by 60 to get RPM!
Here are some numbers: The First test involved running the engine on the car, no gears, no load. the second involved a used AA battery, no load.
MOTOR..RATING....hertz on CAR....hertz with AA
RED.......21500....328 = 19680....390 = 23400
GREEN..16000.....260 = 15600....300 = 18000
BLUE.....10000....185 = 11100....205 = 12300
Again I'll have details and pics on my site as soon as I can!