I have to make a control system that maintains a constant speed of a small car prototype using a DC Gearmotor regardless of external disturbances such as slopes (small slopes).

For the development of this project, the control system will consist of a PID controller that affects the speed of a DC motor. I am not able to use a microcontroller.

I will measure the real RPM of the motor by using an optical sensor which is going to give me pulses, and then I'll use the lm2917 that will transform pulses to a certain voltage value going back to the sum point (operational amplifier adder circuit). And this is how I will do the feedback of the system.

My question is: What is the transfer function of the feedback stage? (Optical sensor and the lm2917)

Thank you.

  • \$\begingroup\$ Hi, I don't know why you reversed parts of my edit, and I'm not going to edit your text to fix it again. However as you can see on the manufacturer's website here, the device is called LM2917 (capital "LM"). Wherever you are seeing "lm2917" is incorrect - the manufacturer decides how they want to write the model number, and as you can see from the link I've given, it's clear how the manufacturer wants it written. \$\endgroup\$
    – SamGibson
    Apr 11, 2019 at 18:38

1 Answer 1


As a Tachometer opto IC, the transfer function is easy if you follow the datasheet.

\$ \dfrac{V_{OUT}}{f_{IN}} = V_{CC} \cdot R1 \cdot C1\$

with limits on Vcc and current range thru R1.

  • \$\begingroup\$ Thank you! that would be the TF of the LM2917. \$\endgroup\$ Apr 9, 2019 at 21:30
  • \$\begingroup\$ That is what you asked for isn't it? \$\endgroup\$ Apr 9, 2019 at 21:38

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