variable speed DC motor

project on variable speed gear motor

is there any DC motor with variable speed and is there anyway to modify normal DC motor to have variable speed. ( i'm talking about Dc motor used for a small control control car)

Added after 1 minutes:

i also looking for variable speed gear box for dc motor (for the same project)

From places such as for example: **broken link removed** you can get geared dc motors, and for motor controllers you can visit: http://www.discovercircuits.com/M/motor-cont.htm

DC motor control application notes of microchip company
**broken link removed**

If you need variable speed in both directions, you can buy an H Bridge Chip, or build an H Bridge Circuit. Below is a simple schematic showing transistors, but there is no reason you can't replace with MOSFETS, switches, or relays. To turn the motor in one direction, you can saturate A and D. To turn in the other direction, you saturate B and C. If you only need one direction you can omit B and C and replace A with a short.

You can send a PWM signal to D or C to control the speed, but if you do you can't use relays for D and C.

The resistors limit base current, the diodes protect against over voltage due to the motor inductance, and the capacitor (often 1 uF) reduces brush noise.

The transistors need to be able to supply enough current to the motor. If you can stall the motor and drive it for a second without smoking the transistors you did ok.

You can use L293D, it has both direction control and a voltage input for the motors, you can use PWM signals for the input voltages of the motors.