7. Cold Chain Alarm

Monitor a DS18B20 and activate a buzzer when temperature exceeds a PxServ-configured maximum.

Level: Intermediate · Time: 45 minutes

This is educational. Food, pharmaceutical, or regulated cold chains require calibrated equipment, uninterrupted records, independent alarms, and validated procedures.

Requirements and wiring

ESP32, DS18B20, 4.7 kΩ resistor, active buzzer
OneWire and DallasTemperature libraries

Part	ESP32
DS18B20 DQ	GPIO 4; 4.7 kΩ from DQ to 3V3
DS18B20 VDD / GND	3V3 / GND
Buzzer SIG	GPIO 23

Complete sketch


#include <PxServ.h>
#include <OneWire.h>
#include <DallasTemperature.h>
 
const char* WIFI_SSID = "your_wifi_ssid";
const char* WIFI_PASSWORD = "your_wifi_password";
const char* PXSERV_API_KEY = "your_pxserv_api_key";
const int ONE_WIRE_PIN = 4;
const int BUZZER_PIN = 23;
 
PxServ client(PXSERV_API_KEY);
OneWire oneWire(ONE_WIRE_PIN);
DallasTemperature sensors(&oneWire);
float maxTemperature = 8.0;
 
void setup() {
  Serial.begin(115200);
  pinMode(BUZZER_PIN, OUTPUT);
  digitalWrite(BUZZER_PIN, LOW);
  sensors.begin();
  PxServ::connectWifi(WIFI_SSID, WIFI_PASSWORD);
}
 
void loop() {
  PxServ::Callback threshold = client.getData("cold-chain/maximum");
  if (threshold.status == 200) maxTemperature = threshold.data.toFloat();
 
  sensors.requestTemperatures();
  float temperature = sensors.getTempCByIndex(0);
  if (temperature == DEVICE_DISCONNECTED_C) {
    digitalWrite(BUZZER_PIN, HIGH);
    client.setData("cold-chain/status", "sensor-error");
    delay(5000);
    return;
  }
 
  bool alarm = temperature > maxTemperature;
  digitalWrite(BUZZER_PIN, alarm ? HIGH : LOW);
  client.setData("cold-chain/temperature", String(temperature, 2));
  client.setData("cold-chain/threshold", String(maxTemperature, 1));
  client.setData("cold-chain/status", alarm ? "alarm" : "normal");
  delay(10000);
}

Test

Set the threshold below room temperature to trigger an alarm, then above it to clear. Disconnect the sensor and confirm sensor-error forces the local alarm on.

Reference: DS18B20 manufacturer page