Simple yet effective way to reliably control the DC Motor speed?

[KedarMhaswade]

TL;DR Summary

Is there a reliable way to control the speed of the DC motor used in this stroboscope model? Should different resistors be used?

As a part of our physics high-school self-study, we are making a stroboscope. We have a small 5-V DC motor that powers the strobe disk.

It works as expected, but, clearly, the motor makes the disk spin at a constant speed. Is there an easy, but effective and reliable way to control its speed? I don't mind joining some resistors in series if resistors can do the job (no variable resistor needed). If resistors is indeed the way to go (I don't think so), should I empirically decide the values of the resistors (or, should I employ Ohm's law)?

 

[berkeman]

Don't use series resistors to drop the drive voltage. Instead, learn about how to use Pulse Width Modulation (PWM) to control the DC motor speed...

 

[Baluncore]

The speed of an unloaded DC motor is determined by the supply voltage. The torque of the motor is determined by the current. If you use a series resistor, you will be limiting the current and therefore torque, so it will tend to wander in speed.

As berkeman pointed out, PWM should be used to regulate the speed by changing the duty cycle. A cheap and easy way to do that is to use a simple "buck regulator", which will be PWM on the inside. Take a look on the web for a cheap "DC-DC adjustable voltage buck regulator". That should cost you less than about $5.
Adjust the speed of the motor by adjusting the regulator output voltage to the motor.
https://www.ebay.com.au/itm/394143426291

 

[tech99]

A stroboscope requires a fine adjustment of motor speed. A good method is to use a potentiometer wired across the supply. Schools sometimes have a big wire wound rheostat, maybe 10 Ohms, which can be used. This tends to hold the speed steady and allows adjustment down to zero. https://electronicguidebook.com/can-i-use-a-potentiometer-to-control-motor-speed/