The circuit functioning may be understood through the following points:
A continuously variable DC voltage needs to be applied externally at pin #11 of the IC to generate proportionately varying PWM pulses. These pulses are further processed and are effectively used to control the connected motor speed right from zero to maximum.
Referring to the figure we see, a dual timer IC 556 forms the heart of the circuit. As the name suggests, the IC consists of two discrete timer sections. This dual feature of the IC has been perfectly exploited here to generate the required PWM pulses.
One half (left hand side) of the IC has been wired as an astable multivibrator. The configuration is used to generate stable and constant oscillations of around 100 Hz.
The above pulses suitably determine the required PWM frequency.
Transistor T5 here performs the function of a constant current source to charge C3.
T5 along with R4 and C3 forms a constant sawtooth wave generator.
The other half (right hand side) of the IC is configured as a voltage comparator.
A continuously variable DC voltage applied at the control input pin #11 of this half of the IC and the different levels of voltage at this point are compared by the generated sawtooth voltage as explained above.
The above operation results in a perfect PWM, which becomes available at pin #9 of the IC.
6 gates from the IC4049 are used for buffering the PWM output before it can be amplified.
Power transistors T1, T2 and T3, T4 are all used to amplify the received PWM signal appropriately for driving the connected DC motor whose speed needs to be regulated.
These transistors are able to handle loads up to 6 Amps quite comfortably. Diodes D1-D4 are all kept to ground for any generated back EMF (inductive surges) by the motor and thus ensure safe operation of the transistors.
A single flick to the switch S1 will enable a “screeching" halt and an immediate subsequent reversal of the motor spin direction, on either side, depending upon the position of S1. This feature may be difficult to find in many other DC motor speed control circuits.