@S Rice posted:... a reliable tachometer for measuring small motor RPMs.
It's the "small motor" that makes it challenging. The reasonably priced (say, $20) contactless HOBBY tachs I've seen (and used) detect optical reflections as GRJ points out. It's amazing that they include a built-in laser to provide the optical source. Can you find a surface that can be marked that is spinning/moving at the motor speed?
My question is are you measuring the RPM on a test-stand (rollers) so that you aren't trying to target the tach laser on a moving train? Or are you dis-engaging the motor from the transmission/gears so as to measure the no-load RPM characteristics as might be found on a motor datasheet?
Depending on what you're trying to measure, there are alternatives. For example if you determine the gear ratio and wheel diameter, you can measure track speed and back-out the engine RPM. Track speed can be measured with a stopwatch and ruler...or with model train speedometers...or with a digital movie camera counting frames.
On the esoteric side, if you have access to electrical instrumentation like current probes and oscilloscopes you can measure electrical characteristics such as commutation discontinuities to determine motor RPM. Yes, I've done this and while tricky it is exact if you can extract the signal-from-the-noise so to speak.
