How to Run a Servo Motor using IC 555

In this project we are going to learn the basic specifications of a servo motor and also how to operate a Servo Motor using a 555 timer IC, and a couple of push buttons.

By Ankit Negi

WHY SERVO?

Servo Motors are used in variety of fields. These are mainly used as actuators in those areas where we need a precise movement to control output load.

Best example is a RC car. Let's see you want movement of 45 degree, not more not less. In that case you can't use a simple DC motor because it will overshoot the desired position every time you power it up.

And thus we need a Servo Motor to achieve this task as it will not only make a precise 45 degree rotation but will also stop smoothly at the desired position.

FEW TECHNICAL POINTS ONE MUST KNOW:

A) Before buying or using a servo one must know what's inside it and how it works. a servo motor is made up of three key components:

1. A DC motor
2. 1 Potentiometer, either analogue or Digital
3. Control circuit

B) There are total 3 wires that come out of a Servo Motor:

1. RED: To positive of supply
2. BLACK: TO negative of supply
3. ORANGE OR YELLOW: Connected to a reference voltage i.e., a pwm source

C) Servo Motor can rotate 90 degrees in either direction, covering maximum 180 degrees i.e., either 90 degrees clockwise or 90 degree anticlockwise from its neutral position.

To rotate the motor clockwise, on time period of clock pulse must be greater than 1.5 milliseconds and to rotate it anticlockwise on time period must be less than 1.25 milliseconds but frequency should lie between 50 to 60 Hertz.

And thus we are going to use a 555 timer to generate such clock pulses for us.

COMPONENTS REQUIRED FOR THIS PROJECT:

1. SERVO MOTOR
2. 555 TIMER
3. 6 VOLT BATTERY
4. TWO PUSH-BUTTONS
5. RESISTORS: 1K, 4.7K, 33K, 10K, 68K, all 1/4 watt 5%
6. ONE TRANSISTOR (BC547)
7. TWO CAPACITORS of 0.1uf

CIRCUIT DIAGRAM SHOWING HOW TO RUN A SERVO MOTOR USING IC 555:

Make connections as shown in the above shown circuit diagram.

Connect positive and negative pin of motor to positive and negative terminal of the battery respectively. And connect signal or reference pin to the collector terminal of the transistor.

CIRCUIT WORKING:

1. When forward push button is pressed-

When this case arises then 68 K resistor get connected between discharge and threshold pin. Now initially capacitor is not charged so pin 2 is at 0 volt which is less than 1 by 3 of applied voltage.

This resets the flip flop inside the 555 and gives logic 1 at the output terminal at which base of the transistor is connected.

This causes transistor to turn on and conduct current directly to ground due to which signal pin of motor get zero volt as this pin is directly connected to Collector terminal.

Since capacitor start charging when output is 1, the output becomes 0 as soon as voltage across capacitor becomes greater than 2 by 3 of applied voltage as it is directly connected to threshold pin.

Now transistor will be off and signal pin will get logic 1.

In this way pwm signals are generated at the reference pin of motor. Now in this case on time period of generated pulse is greater than 1.5 milliseconds, which you can calculate by the duty cycle formula for 555. And thus we get 90 degree clockwise rotation of motor as explained in above paragraph.

1. When backward push button is pressed-

When this case arises then 10 K resistor get connected between discharge and threshold pin which is less than 68k ohm resistor. Thus in this case the on time period of pulse is lesson than 1.5 milliseconds, which you can calculate by the duty cycle formula for 555.

Now the pwm is generated at the reference pin of motor the same way as in the above case. And thus we get 90 degree anticlockwise rotation of motor as explained in above paragraph.

**in both the cases frequency is between 40 to 60 hertz