Adjusting the 50K potentiometer used in the circuit can control the speed of the motor.
The maximum output of the 555 IC is 200mA due to which we have used a BD135 transistor to drive higher current load of upto 1A. For driving higher current than 1A you can use other high current transistors like TIP31, 2N3055 etc. The transistor may become hot during the operation of the circuit therefore it is essential to use a suitable heatsink with it.
This simple DC motor control or PWM circuit using 555 IC can be used to control the speed of a DC motor. The circuit is very simple and can be built in very short time if all parts are available with you. The heart of the circuit is a NE555 timer IC which is working as a astable multivibrator here.