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Lennard Electronics Ltd

Unipolar Stepper Motor Driver

How it works:

The 4017 receives pulses from the timebase (my Quartz controlled 60Hz/1Hz/1ppm circuit). The 4017 has 10 outputs which go high one at a time for each pulse. In this circuit the 5th output of the 4017 is connected to it's reset pin, so the 4017 only counts to 4.

The 2nd and 4th Outputs from the 4017 go to a 4066 (Quad Bilateral Switch). The 4066 switches the outputs to either the yellow or blue driver transistors. Swapping the signal between blue and yellow reverses the motor. The 1st and 3rd outputs go straight to the red and brown drive transistors. So, in forward mode, the sequence is: Red, Yellow, Brown, Blue. In reverse mode, the sequence is: Red, Blue, Brown, Yellow.

The 4069 is a hex inverter. Only 2 inverters are used, these go to the control pins of the 4066 to switch two switches off and the other two on.

Maximum Speed with this motor was achieved with a clock pulse of 180Hz. Note: The coils and flyback diodes should be placed on the collector side of the transistors - one day I'll fix the schematic... :)

To make this type of motor turn around, you have to provide pulses as follows:

coil 1: Pulse

coil 2: Pulse

coil 1: Pulse in reverse polarity

coil 2: Pulse in reverse polarity

coil 1: Pulse


Four pulses required, so the 4017 can be used again.

The working circuit I came up with is shown below. This one doesn't have a reverse function - but is easy to add.

How it works:

The 4017 counts the pulses at it's input. For each count, one output from the 4017 goes high at a time. pin 10 is connected to the reset pin of the 4017, so it only counts to 4.

The outputs from the 4017 go to four NPN/PNP transistor pairs. When an output is high, it turns on the NPN transistor. As a PNP transistor's operation is the reverse of a NPN, an inverter is used to send the base of the PNP transistor low to turn it on. With both transistors on, current flows through the coil. Depending on which pair is turned on, the curent will flow one way or the other.