Basically, there are three ways to vary the speed of DC motors:

- use a gear box

- use a series resistor 

Measure the current drawn by the motor and then calculate the value of a series resistor needed to drop the voltage applied to the motor.   Less voltage means the motor will slow down.  The problem with this method is that the current drawn by the motor increases as the load, on the motor increases.   More current means a larger voltage drop across the series resistor and therefore less voltage drop across the series resistor and therefore less voltage to the motor.  The motor now tries to draw even more current, resulting in the motor stalling.

By applying the supply voltage to the motor for a variable amount of time, eliminating the series dropping effect.  This is the method used in this kit.

The circuit uses two timer/oscillators connected as a Pulse Width Modulator.  The chip used is an nmos dual timer/oscillator, NE556.  This IC has two 555 timers in one 14-pin IC package.

One 555 (IC1:B) is configured as an astable oscillator.  The output frequency of the trigger pulses is given by:

f = 1.44 / ((R3 + 2R4)C2), or about 410Hz.

The time period for the high output is given by:

T x HEIGHT = 0.69(R3 + R4) C2 seconds.

And, the low output by T x LOW = 0.69R4C2 seconds.

The second 555 (IC1:A) is configured for Pulse Width Modulation.  It is set up in monostable mode.  It is triggered with the continuous pulse train from the first 555.  However, by also applying a DC voltage to pin 3 the comparator reference levels will be changed from their nominal levels of one-third and two-thirds of the supply voltage.  This has the effect of modulating the pulse width as the control voltage varies.

The control voltage is supplied via transistor Q1, which is configured as an emitter-follower.  This means that the emitter output voltage follows the base input voltage (less 0.6 volt base-emitter drop).  This configuration gives us a low output impedance voltage source with which to drive the control input of the timer.  This makes the control voltage less susceptible to the loading effect of the timer control input.

The output from the timer is a continuous stream of pulses whose width is controlled by the voltage level applied to the control voltage input.  This modulated output drives a power darlington transistor, Q2, which is used to switch the voltage to the DC motor.

The maximum ON time of the output pulse, and therefore maximum motor speed, can be set by changing the value of resistor R1.  Increasing the value lowers the maximum motor speed.  The end stop resistor R1 may be replaced by a link if desired.

The motor is always connected to the 3 pole terminal block, pins + and -.  If the motor is driven with the same power supply as the controller then insert link LK1.   If separate power supplies are used to drive the controller and the motor, then remove link LK1 and connect the positive motor power supply lead to the EXT pin.   Make sure that the ground lines from each supply are tied together.