Arduino Code

The code below is for the bird project. You can change the chirp function for one of the other sounds defined on the Animal Sounds page.

The code reads the photocell and if the value is less than a certain amount (you pet the animal, lowering the light level), the animal makes sounds and moves. You can change the sensitivity of the photocell (800) to a value between 200 and 1100 depending on ambient light you expect.

The register/interrupt code allows the servo to be refreshed periodically to have it stay where it is commanded to. The Arduino Servo library does not work for Trinket/Gemma so the Adafruit Softservo library is used.

Click on the link for the SoftServo library and select the "Download ZIP" button to get the library code.  Install the library into the Arduino libraries directory. See All About Installing Arduino Libraries for a guide.

Please ensure your Arduino IDE is version 1.6.7 or above to support Trinket optimally.  You can download the Arduino IDE at

The code below may be copied into a new project window in the Arduino IDE.

Be sure to set your Board type as Adafruit Trinket 3V 8 MHz, and Programmer as USBtinyISP in the Arduino IDE under the Tools menu. If you do not see Trinket as an option, add the Adafruit AVR boards in the Board Manager. This code will not run on the Trinket M0. To use an M0 board you must use the CircuitPython example on the next page.
  Adafruit Animal - control code for toy animal animation
  Required library is the Adafruit_SoftServo library
  available at
  The standard Arduino IDE servo library will not work with 8 bit
  AVR microcontrollers like Trinket and Gemma due to differences
  in available timer hardware and programming. We simply refresh
  by piggy-backing on the timer0 millis() counter
  Required hardware includes an Adafruit Trinket microcontroller
  a servo motor, a piezo speaker, a photocell, and a resistor
  As written, this is specifically for the Trinket although it should
  be Gemma compatible.  
#include <Adafruit_SoftServo.h>  // SoftwareServo (works on non PWM pins)
#define SERVO1PIN 0    // Servo control line (orange) on Trinket Pin #0
#define SPEAKER   1    // Piezo Speaker on GPIO #1
#define PHOTOCELL 1    // CdS photocell on GPIO #2 (A1)
Adafruit_SoftServo myServo1;  // create servo object
int16_t  servoPosition;       // servo position  

void setup() {
  // Set up the interrupt that will refresh the servo for us automagically
  OCR0A = 0xAF;            // any number is OK
  TIMSK |= _BV(OCIE0A);    // Turn on the compare interrupt (below!)
  servoPosition = 90;            // Tell servo to go to midway
  myServo1.attach(SERVO1PIN);    // Attach the servo to pin 0 on Trinket
  myServo1.write(servoPosition); //  and move servo
  delay(15);                     // Wait 15ms for the servo to reach the position
  pinMode(SPEAKER,OUTPUT);       // Set Trinket/Gemma Digital 0 to output
                                 //  to drive the piezo buzzer (important)
void loop()  {
  uint16_t light_reading;  // value read from photocell voltage divider
  if(servoPosition != 0)
     servoPosition = 0;          // if it's up, go down & visa versa
     servoPosition = 180;
  light_reading = analogRead(PHOTOCELL);  // Read Analog pin 1 for the changing
                                          // voltage from the CdS divider
  if(light_reading < 800) {               // if the photocell is dark enough, we're petting
    chirp();                              //    so make sound ...
    myServo1.write(servoPosition);        //    and tell servo to move
  delay(1000);   // wait a second between checks/chirps/movements (changable)

// Generate the Bird Chirp sound
void chirp() {   
  for(uint8_t i=200; i>180; i--)

// Play a tone for a specific duration.  value is not frequency to save some
//   cpu cycles in avoiding a divide.  
void playTone(int16_t tonevalue, int duration) {
  for (long i = 0; i < duration * 1000L; i += tonevalue * 2) {
     digitalWrite(SPEAKER, HIGH);
     digitalWrite(SPEAKER, LOW);
// We'll take advantage of the built in millis() timer that goes off
// to keep track of time, and refresh the servo every 20 milliseconds
// The SIGNAL(TIMER0_COMPA_vect) function is the interrupt that will be
// called by the microcontroller every 2 milliseconds
volatile uint8_t counter = 0;
  // this gets called every 2 milliseconds
  counter += 2;
  // every 20 milliseconds, refresh the servos!
  if (counter >= 20) {
    counter = 0;
This guide was first published on Nov 21, 2013. It was last updated on Nov 21, 2013. This page (Arduino Code) was last updated on Dec 12, 2018.