how to create a small brake for a toy car ?

[platonas]

The way to short the motor's leads is simple.
Sypply/connect the motor via a change over contacts of a relay. Common contacts (of the two different sets of contacts) are going to the motor, the normally open contacts N.O. are going to the supply (+ and -) and the normally close contacts (N.C.) are connected together. When the relay is energised, the motor is on the supply. When poewr is removed from relay, the motor is shorted.

 

[puviarasu]

If you use a stepper motor there is no need for any extra hardware for bricking only by changing the switching scheme we can convert the stepper motor in to a brake. In ordinary DC motor the way to stop from high speed is by removing the power supply and shorting the leads by using SCR or transistor. It will not work at low speed as the emf generated is low due to acceleration if you want to do in slow speed then you have to go for mechanical or polarity reversing technique.