I'm currently building a robotic arm. I'm using an Adafruit servo shield, 7 servos, Arduino uno, and a 5V 10A power supply.

Diagram of Robotic arm

I found a program that controls the arm with mouse movement using a processing sketch, Fixbot.

However I'm using the Adafruit servo shield, and I've got no clue how to convert the program. I've found the center points for the servos, so I've managed to make the robot stand upright using my own code.

If someone could give me some pointers as to how to convert from the on-board PWM outputs to the servo shield outputs for the linked program, and how to set servo max/min, I would highly appreciate it.

  • This site uses a question/answer format. A "question" along the lines of "please give me some pointers as to how to do X" is rather broad.
    – Nick Gammon
    Nov 6, 2015 at 6:16
  • What aspect needs converting? More importantly, what does it need converting to? The Arduiino code for the Fixbot is wonderfully straightforward; it reads a run of integers (by parsing a string, blah blah) and then writes those integers to the servos. Where is the problem occurring? Nov 11, 2015 at 18:46

1 Answer 1


I use a generic servo shield, and the servo PWM outputs (3 pin inline connector) on the shield are labeled accordingly (0 - 15). Your Adafruit servo shield should be the same, see below.

Adafruit Servo shield

The link that you provide also has the Arduino version, see below. You should use that, instead of the Processing version.

The Arduino library Servo.h, handles all of the "Servo PWM to Arduino pin conversions" that you ask for. So, looking at the Fixbot code, it would seem that all you would need to do is change the five pins (servo PWM outputs) to which the servos are attached in setup(), according to which servo PWM outputs you attach each servo to.

#include <Servo.h>

String command;

Servo seg0;
Servo seg1;
Servo seg2;
Servo seg3;
Servo seg4;

void setup() {
  seg0.attach(8);            /* <--- Change Me !!! */
  seg1.attach(9);            /* <--- Change Me !!! */
  seg2.attach(10);           /* <--- Change Me !!! */
  seg3.attach(11);           /* <--- Change Me !!! */
  seg4.attach(12);           /* <--- Change Me !!! */

void loop() {
  if(Serial.available() > 0) {
    char inChar = Serial.read();
    command += inChar;
    if(inChar == '\n') {
      // clear command
      command = "";

void performCmd(String cmd) {
  int comma1 = cmd.indexOf(',');
  int comma2 = cmd.indexOf(',', comma1+1);
  int comma3 = cmd.indexOf(',', comma2+1);
  int comma4 = cmd.indexOf(',', comma3+1);
  int cmd0 = stringToInt(cmd.substring(0, comma1));
  int cmd1 = stringToInt(cmd.substring(comma1+1, comma2));
  int cmd2 = stringToInt(cmd.substring(comma2+1, comma3));
  int cmd3 = stringToInt(cmd.substring(comma3+1, comma4));
  int cmd4 = stringToInt(cmd.substring(comma4+1, cmd.length()));

int stringToInt(String str) {
  String tmp = String(str);
  return str.toInt();

Regarding your question about the maximum and minimums, again, these will be handled by the Servo.h library.


Change the five servo attach lines in the setup() routine, to reflect which servo ports your servos are connected to.

Power Requirements

As an aside, remember that you will need to supply external power (+5V) to the Adafruit Servo board, via the blue screw terminal block, in order to have sufficient power (current) to actuate the servos. In the photo above the screw terminal block is not shown, but would be mounted half way up on the left hand side of the PCB (where it says 5-6V Servo). It can be seen in this photo, at the top:

Adafruit Servo Shield, showing blue screw terminal

If you do not supply external power, then the Arduino board, by itself will not be able to supply enough power. Even though the code may be running, the lack of power may make it appear that there is no output, as the servos remain immobile, due to the lack of current.

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