Arduino on 12 V with an LM2596, on an Arduino Uno

// An Arduino on a 12 V adapter, through an LM2596 buck converter.
//
//   12 V    the adapter's jack into the LM2596 module's IN+ and IN-
//   5V      the module's OUT+ into the Uno's 5V pin, OUT- to GND
//   A0      the adapter's 12 V through 10k, with 4.7k from A0 to GND
//
// SET THE TRIMMER FIRST. A module comes set to whatever the factory left it
// at, often well over 5 V. On the bench, turn the brass screw with a meter on
// OUT+ before the Uno is on it: the 5V pin goes straight to the chip, with no
// regulator to save it. Here the module's volts property is the trimmer.
//
// WHY A BUCK. A 7805 would do the same job by burning the difference: at
// 200 mA it takes 200 mA from the adapter and turns (12 - 5) x 0.2 = 1.4 W
// into heat. The LM2596 switches 150,000 times a second and trades volts for
// amps instead: 5 V at 200 mA is 1 W, and at about 85 % that is under 100 mA
// from 12 V.
//
// THE DIVIDER. A0 reads 0 to 5 V, and 12 V would wreck it, so 10k and 4.7k
// bring it down to 4.7 / 14.7 of the adapter: 3.84 V, which reads 785 of 1023.
// The sketch undoes the divider and prints the adapter's volts.
//
// THINGS TO TRY. Swap the adapter for 9 V and watch the reading follow it
// while 5V stays put. Swap it for 5 V: the LM2596 needs about a volt across
// itself, and the Mokxi Uno, which also has its USB power, holds its own 5V.
//
// Press Run.

const int SENSE_PIN = A0;
const float DIVIDER = (10000.0 + 4700.0) / 4700.0;  // the 10k and 4.7k

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Arduino on 12 V through an LM2596");
}

void loop() {
  int raw = analogRead(SENSE_PIN);  // 0 to 1023 for 0 to 5 V
  float pin = raw * 5.0 / 1023.0;
  float adapter = pin * DIVIDER;

  Serial.print("A0 ");
  Serial.print(raw);
  Serial.print(", adapter ");
  Serial.print(adapter, 1);
  Serial.println(" V");

  digitalWrite(LED_BUILTIN, HIGH);
  delay(500);
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);
}