text: I was building a big project and wanted to move the nozzle of a squirting flower, and the only servos I had were micro servos which did not have enough power to move the mechanism, so I decided to build something, a Jumbo Servo.
  As mentioned in element14 Essentials: Motor Control 1 a servo consists of a motor with some kind of position feedback and a control circuit. I decided that a potentiometer and an arduino should be capable of this. I found a 12v gearbox motor in the spares drawer and after a little cleaning and lubrication got that running. There was a gear on the output shaft that could be used for driving a the potentiometer. I added a mounting plate and determined where I could add a potentiometer. A 3D printed gear was added to slip onto the pot and mesh with the output gear. Also something to drive the motor would be needed and I had a suitable L298 H-Bridge module which fitted the spec.
  So that my jumbo servo would look the part, I made a wooden case and wooden servo horn. A 3D printed output shaft was also added.
  Next up was the electronics. I decided to go with I2C for communication rather than the analogue PWM which is typically used by a servo. This allows me to stop the motor when the servo has reached it's destination rather than constantly seeking position, it also means that I can read back the position to the controlling software and avoid that twitch you get when you first power on a servo.
  To test this I used two arduinos and used the examples from Arduino to work out what code was needed. Because I was using a long cable (for I2C at least) I added some low value pullups on the servo end.
  Here's my first draft of the code.
  #include <Wire.h> const int slaveAddress = 8; const int sensePin = A0; const int drivePin1 = 3; const int drivePin2 = 4; const int speedPin = 5; //Needs to support PWM short registers[5]; int readCmd; int blink = 0; enum readRegisters { Target = 0, Running = 1, Position = 2, Speed = 3, Direction = 4 }; enum commands { CmdStop = 1, CmdAngle = 2, CmdSpeed = 3, }; void setup() { stop(); pinMode(drivePin1, OUTPUT); pinMode(drivePin2, OUTPUT); pinMode(speedPin, OUTPUT); pinMode(13, OUTPUT); //Onboard LED pinMode(sensePin, INPUT); Wire.begin(slaveAddress); // join i2c bus with address #8 Wire.onReceive(receiveEvent); // write data Wire.onRequest(requestEvent); // requests to read data } void loop() { digitalWrite(13, blink); blink = !blink; registers[Position] = analogRead(sensePin); if (registers[Running]) { if (registers[Direction] == 1) { if (registers[Position] >= registers[Target]) { stop(); } } if (registers[Direction] == -1) { if (registers[Position] <= registers[Target]) { stop(); } } } } void stop() { digitalWrite(drivePin1, LOW); digitalWrite(drivePin2, LOW); analogWrite(speedPin, 0); registers[Running] = false; } // function that executes whenever data is received from master // this function is registered as an event, see setup() void receiveEvent(int bytesReceived) { char command; int value; int lowB; int highB; switch (bytesReceived) { case 1: //Read request register readCmd = Wire.read(); break; case 3: command = Wire.read(); // receive byte as a character //Pi is low endian lowB = Wire.read(); highB = Wire.read(); value = (highB << 8) | lowB; switch (command) { case CmdStop: stop(); break; case CmdAngle: registers[Target] = value; registers[Running] = true; if (registers[Target] > registers[Position]) { digitalWrite(drivePin1, LOW); digitalWrite(drivePin2, HIGH); registers[Direction] = 1; } else { digitalWrite(drivePin2, LOW); digitalWrite(drivePin1, HIGH); registers[Direction] = -1; } break; case CmdSpeed: registers[Speed] = value; analogWrite(speedPin, registers[Speed]); break; } break; default: for (int a = 0; a < bytesReceived; a++) { Wire.read(); // throw buffer away so we can read again } } } // function that executes whenever data is requested by master // this function is registered as an event, see setup() // Don't add serial print to this routine, it will error!! void requestEvent() { short value = registers[readCmd]; Wire.write((uint8_t *)&value, sizeof(value)); }
  I've got myself an AdaFruit "ItsyBitsy" which is basically a small form factor Arduino Leonado. So the next step is to wire this in, instead of the Arduino.
  I'll also need to do some simple calibration to turn the 0-1023 value into a degrees value. Because of the gearing on the pot the servo can do a bit more than 180 degress so I should be able to set it up with a safe limit so I don't over drive the pot.
  Have added a breakout board for the ItsyBity and completed some further testing with some LEDs in place of the H-Bridge,
  I then did some testing with the H-Bridge. I used back to back LEDs to check that the circuit was wired correctly and the software was working. After a few loose connections I got that working. However, swapping the motor in caused my 2A PSU to shutdown.
  So I plugged these into the bench power supply (thanks again Secret Santa) and set the voltage and current limit. Luckily I happened to have a slightly smaller motor that had the same size shaft and screw holes that took a lot less current. Next up I'll swap that in.
  The gearbox I used is available on CPC.