For wired Robo car !!!
Use the back EMF voltage to regulate your speed control power transistor. A
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What do i need to do that ?
Most speed control is accomplished by using a mechanical tachometer attached to a motor shaft to control electronics to maintain a set motor speed. However a spinning motor is also a tachometer if the power is momentarily disconnected and the back EMF ( Electro-Magnetic-Force ) voltage measurement is used to control the power to the motor with a group of op-amps. Linear Technology has an IC that can do this for you. A
This has be covered before several tims - resistors waste energy in heat and the motor will be very poor at regulating it's speed.You need to control the voltage in a dynamic way.For the rest much depends on the motor you wan to control.
Don't. Its a very bad way to control them. Steve