Pick and place arm on a PCA9685, on an Arduino Uno

// Pick and place arm on a PCA9685: a four-servo arm on the 16-channel servo
// board, driven with the Adafruit PWM Servo Driver library.
//
//   PCA9685  SDA to A4, SCL to A5, VCC to the Uno's 5V, GND to GND
//            V+ from a separate 5 V supply for the servos
//   arm      BASE on channel 0, SHOULDER 1, ELBOW 2, GRIP 3
//
// The board makes the servo pulses itself, fifty times a second on every
// channel, so the Uno only speaks when a joint should move: two wires for up
// to sixteen servos, and no pulse timing in the sketch at all.
//
// A move is a list of four angles. glide() walks every joint there together
// in small steps, which is how a real arm avoids slamming its gears and
// swinging what it carries.

#include <Wire.h>
#include <Adafruit_PWMServoDriver.h>

Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();

// 544 to 2400 us for 0 to 180 degrees: the Arduino Servo library's window,
// which the servos in the arm answer to.
const int MIN_US = 544;
const int MAX_US = 2400;

// Channels 0 to 3 are the base, shoulder, elbow and claw, in that order.
const int OPEN = 110, SHUT = 25;

int now[4] = {90, 90, 90, OPEN};

void setAngle(int channel, int degrees) {
  pwm.writeMicroseconds(channel, map(degrees, 0, 180, MIN_US, MAX_US));
}

void glide(const char *what, int base, int shoulder, int elbow, int grip) {
  Serial.println(what);
  int target[4] = {base, shoulder, elbow, grip};
  for (int step = 1; step <= 25; step++) {
    for (int j = 0; j < 4; j++) {
      setAngle(j, now[j] + (target[j] - now[j]) * step / 25);
    }
    delay(30);
  }
  for (int j = 0; j < 4; j++) now[j] = target[j];
  delay(250);
}

void setup() {
  Serial.begin(115200);
  if (!pwm.begin()) {
    Serial.println("no PCA9685 at 0x40: check SDA, SCL and VCC");
  }
  pwm.setPWMFreq(50);
  for (int j = 0; j < 4; j++) setAngle(j, now[j]);
  Serial.println("pick and place arm: ready");
}

void loop() {
  glide("over the part", 150, 90, 90, OPEN);
  glide("reach down", 150, 130, 60, OPEN);
  glide("grip", 150, 130, 60, SHUT);
  glide("lift", 150, 90, 100, SHUT);
  glide("swing across", 30, 90, 100, SHUT);
  glide("lower", 30, 130, 60, SHUT);
  glide("let go", 30, 130, 60, OPEN);
  glide("home", 90, 90, 90, OPEN);
  delay(500);
}