user33243: We're complete beginners with Arduino and we're working on a project that moves a pointer based on a color sensor reading.
We used two servo motors each: one performs a specific task, the small servo works pretty fine. The problem is with the larger one.
We used this servo motor model: (towardpro MG996R), Arduino Uno and a color sensor. Because the motor is large we connect it to a 9 V power supply. 
But when we run the code it just vibrates in place and the arm won't move :(
Is there anything else we need to add to the circuit like a resistor or some thing?
#include <Servo.h>
Servo largeServo;

void setup() {
  largeServo.attach(6);
  largeServo.write(0); 
}

void loop() {
  largeServo.write(0);
  delay (2000);
  largeServo.write(30);
  delay (2000);
  largeServo.write(60);
  delay (2000);
  largeServo.write(90);
  delay (2000);
}


user43663: To complement the other answers, also make sure that the Arduino itself is being powered properly!
I was attaching Arduino Uno to a nearly out of battery laptop via USB cable, resulting in a jittering servo. As soon as I've put the laptop on charge, the servo started working smoothly!

user19867: As already suggested in the comments you need a more powerful current source than your battery.
No device work purely on Voltage, they all operate on Power (watts) or, if you prefer, on a combination of Voltage and Current (ampere).
You are feeding 9V into your beefy motor, but the amount of current your battery can provide is pretty limited due to the battery internal resistance and limited capacity.
Replace the battery with a proper power source capable to deliver 10W or more (1+ ampere at 9 volts equals 9+ watts) and your motor will start to operate and it will also get warm while operating.

user76704: According to (MG996R)'s datasheet
You must connect a 6 volts - 2.5 amps power supply for it's best performance (max. Torque)
Maximum voltage of this servomotor is 6 volts
And be careful, exceeding that voltage can be cause of some damage to it's control board
Try 6 volts 2.5 amps power supply and if doesn't work maybe your servomotor's control-board is damaged by over-voltage.

33243: <p>We're complete beginners with Arduino and we're working on a project that moves a pointer based on a color sensor reading.</p>

<p>We used two servo motors each: one performs a specific task, the small servo works pretty fine. The problem is with the larger one.</p>

<p>We used this servo motor model: (towardpro MG996R), Arduino Uno and a color sensor. Because the motor is large we connect it to a 9 V power supply. </p>

<p>But when we run the code it just vibrates in place and the arm won't move :(</p>

<p>Is there anything else we need to add to the circuit like a resistor or some thing?</p>

<pre><code>#include &lt;Servo.h&gt;
Servo largeServo;

void setup() {
  largeServo.attach(6);
  largeServo.write(0); 
}

void loop() {
  largeServo.write(0);
  delay (2000);
  largeServo.write(30);
  delay (2000);
  largeServo.write(60);
  delay (2000);
  largeServo.write(90);
  delay (2000);
}
</code></pre>
 43663: <p>To complement the other answers, also make sure that the Arduino itself is being powered properly!</p>
<p>I was attaching Arduino Uno to a nearly out of battery laptop via USB cable, resulting in a jittering servo. As soon as I've put the laptop on charge, the servo started working smoothly!</p>
 19867: <p>As already suggested in the comments you need a more powerful current source than your battery.</p>

<p>No device work purely on Voltage, they all operate on Power (watts) or, if you prefer, on a combination of Voltage and Current (ampere).</p>

<p>You are feeding 9V into your beefy motor, but the amount of current your battery can provide is pretty limited due to the battery internal resistance and limited capacity.</p>

<p>Replace the battery with a proper power source capable to deliver 10W or more (1+ ampere at 9 volts equals 9+ watts) and your motor will start to operate and it will also get warm while operating.</p>
 76704: <p>According to (MG996R)'s datasheet
You must connect a 6 volts - 2.5 amps power supply for it's best performance (max. Torque)
Maximum voltage of this servomotor is 6 volts
And be careful, exceeding that voltage can be cause of some damage to it's control board
Try 6 volts 2.5 amps power supply and if doesn't work maybe your servomotor's control-board is damaged by over-voltage.</p>
