Power module on an Arduino Uno

// Power module: an MB102 breadboard power supply runs a servo off a 9 V
// battery, so the servo's current never comes through the Uno, and the Uno
// keeps an eye on the module's 3.3 V rail.
//
//   MB102     on the left end of the breadboard; a 9 V battery on its jack
//             top rails 5 V (jumper), bottom rails 3.3 V (jumper)
//   servo     red -> top + rail   brown -> top - rail   signal -> pin 9
//   Uno       GND -> top - rail (one ground for everything)   A0 -> bottom + rail
//
// A servo moving draws hundreds of milliamps in bursts, enough to pull a USB
// port's 5 V down and reset the board. The kit's answer is the power module:
// its own regulator, its own battery, and only the ground and the signal
// shared with the Uno.
//
// The sketch sweeps the servo and prints the 3.3 V rail as the Uno reads it.
// Click the module to switch it off and watch the rail fall to nothing and
// the servo stop where it is.

#include <Servo.h>

const int SERVO_PIN = 9;
const int RAIL_PIN = A0;

Servo arm;

void setup() {
  Serial.begin(9600);
  arm.attach(SERVO_PIN);
  Serial.println("Power module: servo on the 5 V rail, watching the 3.3 V rail");
}

void loop() {
  for (int angle = 0; angle <= 180; angle += 45) {
    arm.write(angle);
    delay(400);
    long millivolts = (long)analogRead(RAIL_PIN) * 5000 / 1023;
    Serial.print("servo ");
    Serial.print(angle);
    Serial.print(", 3.3 V rail ");
    Serial.print(millivolts);
    Serial.println(" mV");
  }
}