This document describes a bipolar stepper motor driver circuit that provides forward and reverse control. The circuit uses an LM555 timer to generate clock pulses that are fed into a 74LS194 shift register. The shift register output determines the direction of motor rotation. An L293D H-bridge driver takes the shift register outputs and controls the stepper motor windings to rotate the motor clockwise or counterclockwise depending on the shift register state. A potentiometer controls the stepping rate and switches determine the direction.

1. BIPOLAR STEPPER MOTOR DRIVER CIRCUIT
BSECE 41A1
PRESENTED BY: Cauan, Sarah Krystelle P.
PRESENTED TO: Engr. Jan Berden D. Bermejo
DATE: Oct. 12, 2011

2. ABSTRACT
Stepper motors are used in any application which requires a discrete rotational
movement. The performance of stepper motor is strongly dependent on the driver circuit. There
are two basic winding arrangements in two phase stepper motor, the unipolar and the bipolar.
This project contains the discussion and features of simple bipolar stepper motor driver. It
provides the forward and reverse control function. The concept of this project got from a simple
running lights circuit.
The circuit contains three different IC which are the LM555, 94LS194, and the L293D.
The stepping rate of the stepper motor is controlled by the 1 Megaohm potentiometer while the
switch determined the direction of the motor. The LM555 astable oscillator produces clock
pulses that are directed to PIN 11 of the 74194 shift register. Each time the output of the LM555
timer goes high, the high state at the pins 12, 13, 14, and 15 of 74194 is shifted either up or down
by one place.
The output shift determines the direction of the rotation of motor. Switch S1 and S2,
which are connected to the switch control the direction. When the S1 is active, the motor will
rotate clockwise, on the other hand, when S2 is active, the motor will rotate counter-clockwise.
The four outputs of the 74194 are fed to one of the driver segments of a LS293D - H Bridge
driver IC (IC 3). The output will be dependent on the input on LS293D segment.

3. CIRCUIT DESIGN COMPONENTS
1 MΩ Potentiometer
The potentiometer controls the rate of the bipolar stepper motor.
NE555N
Operation of 555 Timer
The pin connections are as follows: 1) ground, 2) trigger, 3) output, 4) reset, 5) control, 6)
threshold, 7) discharge, and 8) positive supply voltage.

4. The operation of the 555 timer revolves around the three resistors that form a voltage
divider across the power supply, and the two comparators connected to this voltage divider
(See Figure 2). The IC is quiescent so long as the trigger input remains at positive supply
voltage and the threshold input is at ground.
The three resistors in the voltage divider all have the same value, so the comparator
reference voltages are one-third and two-third of the supply voltage, whatever that may be. The
control voltage input at pin 5 can directly affect this relationship.
The internal flip-flop changes state when the trigger input at pin 2 is pulled down
below one-third of positive supply voltage. When this occurs, the output changes state to
positive supply voltage and the discharge transistor (pin 7) is turned off. The trigger input can
now return to positive supply voltage; it will not affect the state of the IC.
However, if the threshold input (pin 6) is now raised above two-third of positive supply
voltage, the output will return to ground and the discharge transistor will be turned on again.
When the threshold input returns to ground, the IC will remain in this state, which was the
original state. (Source: http://www.play-hookey.com/digital/timer_555.html)
Operating Modes of 555 Timer
There are three operating modes of 555 – monostable, astable, and bistable modes. A
monostable mode circuit produces one pulse of a set length in response to a trigger input.
A bistable mode has two stable states, high and low. Taking the Trigger input low makes
the output of the circuit go into the high state. Taking the Reset input low makes the output of the
circuit go into the low state. This type of circuit is ideal for use in an automated model railway
system where the train is required to run back and forth over the same piece of track.

5. The output of astable continually switches between high and low without any
intervention. This type of circuit could be used to give a mechanism intermittent motion by
switching a motor on and off at regular intervals. (Source: http://www.555-timer-
circuits.com/operating-modes.html)
10 pF Ceramic Capacitor
The 10 pF capacitor is used to reduce the electrical noise with the pin 5 connected to
ground.
DM74LS194N
This integrated circuit is used to operate the direction of the bipolar stepper motor. This is
commonly known as 4 Bit Universal Shift Register.

6. CIRCUIT DIAGRAM