Do I need to buy hardware for this tutorial?

No. Everything runs in the browser: Circuitly simulates the board, the components and the code. Real hardware is optional if you want to rebuild the circuit afterwards.

Which components are used?

Arduino UNO board, Piezo buzzer, BMP280 pressure/temperature sensor, DS3231 real-time clock. They are already available in the simulator library.

How long does it take?

About 30 minutes, expert level, in 4 guided steps.

What should I know before starting?

The previous tutorial in the series, “Weather station + clock”, is enough preparation. No software install is required.

Is the code provided and checked?

Yes: a starter sketch is provided, and the simulator checks your wiring and your code automatically at every step.

All tutorials

4 · Displays & buses

Programmable alarm clock (RTC)

Expert30 min

Goal

Trigger the buzzer at a precise time read from the DS3231.

Why it matters

Comparing a real time to a setpoint is the principle of any scheduled automation.

What you need

Arduino UNO board
Piezo buzzer
BMP280 pressure/temperature sensor
DS3231 real-time clock

Nothing to buy: every component below is simulated in the browser.

Steps

01Start again from the weather station sketch.
02Extract hours and minutes from the DS3231 (BCD → decimal).
03Compare them to an alarm time.
04Fire tone() for 5 seconds, then stop.

Starter sketch

// BMP280 temperature + pressure and DS3231 clock, straight over Wire.
#include <Wire.h>

const uint8_t BMP = 0x76;
const uint8_t RTC = 0x68;

uint16_t calT1; int16_t calT2; uint16_t calP1;

uint8_t readReg8(uint8_t addr, uint8_t reg) {
  Wire.beginTransmission(addr);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(addr, (uint8_t)1);
  return Wire.read();
}

uint16_t readLE16(uint8_t reg) {
  Wire.beginTransmission(BMP);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(BMP, (uint8_t)2);
  uint8_t lo = Wire.read();
  uint8_t hi = Wire.read();
  return (uint16_t)((hi << 8) | lo);
}

long read20(uint8_t reg) {
  Wire.beginTransmission(BMP);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(BMP, (uint8_t)3);
  long msb = Wire.read();
  long lsb = Wire.read();
  long xlsb = Wire.read();
  return ((msb << 16) | (lsb << 8) | xlsb) >> 4;
}

uint8_t fromBcd(uint8_t v) { return (v >> 4) * 10 + (v & 0x0F); }

void setup() {
  Serial.begin(9600);
  Wire.begin();
  calT1 = readLE16(0x88);
  calT2 = (int16_t)readLE16(0x8A);
  calP1 = readLE16(0x8E);
  Serial.print("BMP280 id 0x");
  Serial.println(readReg8(BMP, 0xD0), HEX);
}

void loop() {
  long adcT = read20(0xFA);
  long adcP = read20(0xF7);
  float tFine = ((adcT / 16384.0) - (calT1 / 1024.0)) * calT2;
  float tempC = tFine / 5120.0;
  float pa = (1048576.0 - adcP) * 6250.0 / calP1;

  Serial.print("T ");
  Serial.print(tempC);
  Serial.print(" C   P ");
  Serial.print(pa / 100.0);
  Serial.println(" hPa");

  uint8_t h = fromBcd(readReg8(RTC, 0x02));
  uint8_t m = fromBcd(readReg8(RTC, 0x01));
  uint8_t s = fromBcd(readReg8(RTC, 0x00));
  Serial.print("RTC ");
  Serial.print(h); Serial.print(":");
  Serial.print(m); Serial.print(":");
  Serial.println(s);

  delay(1000);
}

This tutorial runs inside the Circuitly simulator: your wiring and your code are checked automatically at every step.

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Frequently asked questions

Do I need to buy hardware for this tutorial?: No. Everything runs in the browser: Circuitly simulates the board, the components and the code. Real hardware is optional if you want to rebuild the circuit afterwards.
Which components are used?: Arduino UNO board, Piezo buzzer, BMP280 pressure/temperature sensor, DS3231 real-time clock. They are already available in the simulator library.
How long does it take?: About 30 minutes, expert level, in 4 guided steps.
What should I know before starting?: The previous tutorial in the series, “Weather station + clock”, is enough preparation. No software install is required.
Is the code provided and checked?: Yes: a starter sketch is provided, and the simulator checks your wiring and your code automatically at every step.

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