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
700mA is fine, but 13.5V will allow only 50% charging of your battery, if possible raise it to 14.3V, and allow the charging through a period of minimum 12 hours
Very good site for a lay person like me. I need a pwm circuit with these requirements. Input 12 v 7 ah battery. The circuit needs 12 volt 2 ampere square wave output. The circuit can pull more current upto 5-6 amp. But at this current the gadget gets very hot. So I need to limit the current at 1.5 to 2 amp. I tried to use ic 555 with two pots of 100 k and 50 k with zd 4148. But somehow the current is far less. Can you post me it on my mail address. Thanks a lot in advance.
Dear Swagatam, in your circuit design above could you tell me tell value of capacitors is it 1 microfarad or 0.1 micro,? which one is the true ?.
Thanks more.
Dear kadhim, they are 0.1uF
Good morning Swang.
Hope your are well in this very peculiar period.
I’m Claude and you already provide me with your helpfull advices.
The drawing in this topic is exactly what I need with a small difference.
I want to move the servo from horizontal position to 85° “vertical” positon. What are the modifs (with the electronic components) to perform this?
Would it be possible to go from 0° (horizontal) to any “vertical” position (maxi 90°) using a variable resistor (which one on the drawing) for example ?
When you said “bakward” and “forward” it means “clock-wise” and “anti-clockwise” , is that correct ?
Thank for your help, have a good day.
Claude (from France)
Good Morning papaciela, I am always happy to help, however, I don’t think I will able to help you for this specific application, because I have no practical experience with servo motors, and moreover the article above was written and submitted by an external author.
Hi Swag,
Ok, but may be you can tell me if my thought is correct:
– if I put a NC contact on the line between R5 and positive side of the servo. Using a device mounted on the servo axle, when this device press this contact and make it to open, it will cut the 6 VDC supply to the servo and the servo will stop rotating. Is my thinking correct as electronically speaking ?
By any chance, have you a mail contact with the author of this drawing ?
Thanks for your answer any way.
Claude
Hi Papaciela,
The N/C contact will connect the supply with the servo, when the relay is not powered. When it is powered, the N/C will open and the N/O will connect.
You can refer to the following article for more info
How Relay Works
I might have lost the contact details of the contributor
Hallo Swagatam! My problem with this circuit is, that my servos rotate 60° in either direction. I damaged one servo with this circuit, because its motor was still working while the estimated final point was not yet reached. How can I adjust your circuit to any angle between o° and 60°? I need servos for switching turnouts on my railroad layout. Thank you for your patience and efforts. Best regards from Spain
Peter
Hello Peter, I think the values of the R2 and R4 can be experimented to decrease the rotational angle of the motor. It may be difficult to calculate the precise values of the resistors…a trial and error process can be more helpful
Hello, I have built the circuit up on vero board 3 times checking there was no shorts, the breaks were in the right places, the components are right values.
I checked everything with a multimeter but the servo motor would not work. I changed the motor but still no luck.
I wanted to use this system on a model railway to use the servos to change the points on the track. I am replacing the solonoid system ones as they are so clunky. I have been doing electronics for many years but this has stumped me why it won’t work. I didn’t want to buy the pre assembled proper ones for the railway as its the fun of building it yourself.
Hope you can suggest why I have gone wrong.
Hello, the above circuit was successfully tested by the author of the article, so I am not sure might be wrong with your construction. You can connect an LED in series with the resistor R3, and check whether it illuminates or not while the buttons are pressed. This will prove the response of the IC 555 and if there’s any problem with your design. Remember only the black dots indicate the interconnections. for example pin6 connects with pin2 and not with pin1