I have an electric motor stand that I want to add an arduino tachometer to measure the rpm’s of the prop. I know about the ir sensors and sensors that are on either side that breaks the beam and counts the revolutions. I have several sensors that I tried to use by pointing at the propeller but it doesn’t work. Commercial tachometers can measure rpm’s by just pointing at the blades of the props. I took one apart and there is a simple photo resistor that counts the revolutions but the difference I see I is that there appears to be a lens in front of the photo resistor. Has anyone made a tach with only a photoresistor?
How many wires connect your photo resistor?
Hi,
I would think it is a photo-diode or photo-transistor, they rely on a light being reflected from the targets surface.
Usually a piece of reflective tape is stuck on the targets surface.
A photo-resistor, LDR, would be too slow in response to give a reliable output.
Have you Googled;
arduino propeller tacho
Tom.... 
