Stepper Motor Control

A simple, low-cost
hardwired step per motor control circuit that can be used in low-power
applications, such as moving toys etc is presented here. The circuit
comprises a 555 timer IC configured as an astable multivibrator with
approx. 1Hz frequency. The frequency is determined from the following
relationship:

Frequency = 1/T = 1.45/(RA + 2RB)C Where RA = RB = R2 = R3 = 4.7 kilo-ohm and C = C2 = 100 µF.

The output of timer is used as clock for two 7474 dual ‘D’
flip-flops (IC2 and IC3) configured as a ring counter. When power is
initially switched on, only the first flip-flop is set (i.e. Q output at
pin 5 of IC2 will be at logic ‘1’) and the other three flip-flops are
reset (i.e. their Q outputs will be at logic ‘0’). On receipt of a clock
pulse, the logic ‘1’ output of the first flip-flop gets shifted to the
second flip-flop (pin 9 of IC2). Thus with every clock pulse, the logic
‘1’ output keeps shifting in a ring fashion. Q outputs of all the four
flip-flops are amplified by Darlington transistor arrays inside ULN2003
(IC4) and connected to the stepper motor windings marked ‘A’ through ‘D’
in the figure.

The common point of the winding is connected to +12V DC supply,
which is also connected to pin 9 of ULN2003. The colour code used for
the windings is shown in the figure. When the power is switched on, the
control signal connected to SET pin of the first flip-flop and CLR
pins of the other three flip-flops goes active ‘low’ (because of the
power-on-reset circuit formed by R1-C1 combination) to set the first
flip-flop and reset the remaining three flip-flops. On reset, Q1 of IC2
goes ‘high’ while all other Q outputs go ‘low’. External reset can be
activated by pressing the reset switch. By pressing the reset switch,
you can stop the stepper motor. On releasing the reset switch, the
stepper motor again starts moving further in the same direction.

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