DC motor RPM display

I have a simple system of some 12v DC motors attached to some PWM speed controllers that use a knob to control the speed of my DC motors, but it has no numbers on the dial knobs and really no way to know what exact position I am at, and I want to know at what speed my motors are running. is there a way to use an LED/LCD screen to display what RPM my motor is currently running at? My initial project is a spirograph that uses two DC motors and the motor speed to control the drawing arms for designs, so knowing the motor speeds will allow me to tweak and repeat designs.

You can put encoders on the shafts and count RPM or you can put arbitrary numbers 0 - 10 on the dial. Good Luck & Have Fun! Gil

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If that "knob" is an encoder, numbers won't make sense. If it's a pot, it may make a bit of sense.

Note that also you're not controlling the rpm of your motor, you're controlling the power supply to the motor through your PWM setting. The rpm the motor is running at is only partly related to this; if you want to set the rpm of the motor you have to first measure it, and secondly change the PWM setting accordingly.

Without rpm measurement the best you can do is display the PWM value, as 0-255 or as 0-100% value for motor power.

I think using the power will work, a 0-255 marker. basically I just want to know what values my motors are running at so if I find that a certain speed yields a cool design, then I will be able to find that design again because I know what speed or in the case of DC motors, how much power they are getting which translates to their rpm

Just remember that the rpm is not only dependent on the PWM value... it's also very much dependent on the momentary load the motor has. Higher load is lower rpm for the same supplied power.

Theoretically the rpm should stay constant because I’m not changing the load. It’s small spinning disk with an arm extending out. The only thing that might affect the load is the arm not being centered and “pulling” the motor one way or another, but that should be minimal and also absorbed by the mount I have for the motor