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You can use a strobe light.
Make a simple led circuit that can be driven by a frequency generator.
Put a small mark on the motor shaft, then adjust the frequency of the strobe untill the mark is stationary.
This project kit from "www.united77.com/projects.htm" may help you. It comes with PCB, components, programmed microcontroller and the project details.
Microcontroller based Tacho meter is an easy-to-make photoelectric based tachometer project for measuring the RPM (revolution per minute) of rotating shaft, shop-floor tools and many household machines without any mechanical or electrical interface. The circuit employs AT89C2051 microcontroller as the main controller. Some of the major components used in the circuit are CA3140 operational amplifier, ULN2003 current buffer and KLQ564 4-digit, 7-segment display
I believe you have a Brush DC Motor (only 2 wires!)
The best choice is to use optical rotary encoders or other stuff. But you can also do it without additional hardware, you only need a small microcontroller with an internal ADC.
This method is called: "Back EMF Motor Speed Sensing".
The motor RPM can be measured by understanding that a motor is a generator. As long as the motor is spinning, it will produce a voltage that is proportional to the motors RPM. This is called back EMF.
You can control the motor speed with a PWM signal applied to a MOSFET. If the PWM signal to the motor is turned off and the voltage across the windings is measured, the back EMF voltage can be sensed from there and the RPM's can be known.
Hi
There is not a lot to it when using a strobe light.
You make a circuit that is driven by an audio frequency generator that switches a high brightness led on and off at the driven frequency. You then make a small mark on the shaft of the motor.
You run the motor and hold the led close to the shaft. You adjust the frequency generator until the mark on the shaft appears stationary. The motor is now running at the set frequency revolutions per second.
Multiply by 60 to get RPM.
Be careful you dont get a harmonic as the mark will appear stationary at different multiples of the frequency.
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