How do I read DS18B20 probes and a DHT sensor?

1-Wire probe discovery by ROM code, DHT11/DHT22 temperature and humidity, both logged on a cron

The DS18B20 is the waterproof stainless-steel probe you use for a fridge, a boiler, or a fermenter: several of them share one GPIO, each answering to its own 64-bit ROM code. The DHT11/DHT22 is the cheap plastic block that gives you temperature and humidity from a single pin. This recipe wires up both, discovers the probes on connect, and logs every reading once a minute.

The commands in this recipe need firmware 0.2.0 or newer. When the server knows a device’s firmware is older, they fail fast with a firmware_incompatible error telling you to reflash - see Firmware version reporting.

Wiring (Pico W)

Sensor pin	Pico W pin
DS18B20 VDD (red)	3V3 (pin 36)
DS18B20 GND (black)	GND (any)
DS18B20 DATA (yellow)	GP4 (pin 6)
DHT VCC	3V3 (pin 36)
DHT GND	GND (any)
DHT DATA	GP15 (pin 20)

The DS18B20 data line needs a 4.7 kOhm resistor between DATA and 3V3. Without it the bus never releases and every search comes back empty - it is the single most common reason this recipe “doesn’t work”. Multiple probes all connect to the same three wires; you do not need one GPIO each. Most DHT breakout boards already include their own pull-up.

On an ESP32 the same code works: swap deviceType and pick any free GPIO. On the ESP32-C61 the 1-Wire bus uses the UART1 peripheral (that chip has no RMT), so leave UART port 1 unused there.

`devicesdk.ts`

import { defineConfig } from "@devicesdk/cli";

export default defineConfig({
  projectId: "probe-monitor",
  devices: {
    probes: {
      className: "ProbeMonitor",
      main: "./src/devices/probeMonitor.ts",
      deviceType: "pico-w",
      wifi: { ssid: "YOUR_WIFI_SSID", password: "YOUR_WIFI_PASSWORD" },
    },
  },
});

`src/devices/probeMonitor.ts`

import { DeviceEntrypoint, type DeviceResponse } from "@devicesdk/core";

const ONEWIRE_PIN = 4;
const DHT_PIN = 15;

export class ProbeMonitor extends DeviceEntrypoint {
  crons = { sample: "*/1 * * * *" }; // every minute UTC

  async onDeviceConnect() {
    // Walk the bus once and remember which probes are out there. Do this on
    // every connect, not once ever: probes get added, removed, and swapped.
    const reply = await this.env.DEVICE.onewireSearch(ONEWIRE_PIN);
    if (reply.type !== "onewire_search_result") {
      console.error("1-Wire search failed - check the 4.7k pull-up on GP4.");
      return;
    }

    const roms = reply.payload.roms;
    if (roms.length === 0) {
      console.error("No DS18B20 found on GP4.");
      return;
    }

    console.log(`Found ${roms.length} probe(s): ${roms.join(", ")}`);
    await this.env.DEVICE.kv.put("roms", roms);
  }

  async onCron() {
    const roms = (await this.env.DEVICE.kv.get<string[]>("roms")) ?? [];

    for (const rom of roms) {
      // Address one specific probe. Each read takes ~750 ms for the
      // conversion, so a handful of probes is fine on a one-minute cron.
      const reply = await this.env.DEVICE.onewireReadTemperature(
        ONEWIRE_PIN,
        rom,
      );
      if (reply.type !== "onewire_temp_result") continue;

      const { celsius } = reply.payload;
      console.log(`${rom}: ${celsius.toFixed(2)} C`);
      await this.env.DEVICE.emitState(`probe_${rom}`, celsius);
    }

    // The DHT returns both measurements in one command.
    const dht = await this.env.DEVICE.dhtRead(DHT_PIN, "dht22");
    if (dht.type === "dht_read_result") {
      const { celsius, humidity_pct } = dht.payload;
      console.log(`Room: ${celsius} C, ${humidity_pct}% RH`);
      await this.env.DEVICE.emitState("room_temperature", celsius);
      await this.env.DEVICE.emitState("room_humidity", humidity_pct);
    }
  }

  // Surface firmware-side failures instead of silently logging nothing.
  async onMessage(message: DeviceResponse) {
    if (message.type === "command_error") {
      console.error(`Sensor error: ${message.payload.error}`);
    }
  }
}

One probe, no ROM code

If you only ever have a single DS18B20 on the bus, skip the search entirely and omit the rom argument - the firmware falls back to Skip ROM:

const reply = await this.env.DEVICE.onewireReadTemperature(ONEWIRE_PIN);

This is shorter, but it breaks the moment a second probe joins the bus: both answer at once and the readings collide. Use the ROM code as soon as you have more than one.

Using a DHT11 instead

The DHT11 speaks the same command with a different model string:

const dht = await this.env.DEVICE.dhtRead(DHT_PIN, "dht11");

DHT11 reports whole degrees and whole percent, and its usable range is roughly 0-50 C / 20-90% RH. The DHT22 reports tenths, handles negative temperatures, and covers -40 to 80 C. Both are enforced to a minimum of 2 seconds between reads on the same pin by the firmware - ask sooner and the command fails, because the sensors return corrupt frames when polled faster.

What this demonstrates

Discovering 1-Wire devices by ROM code with onewireSearch, so several probes share one GPIO.
Addressing an individual probe with onewireReadTemperature(pin, rom).
Reading temperature and humidity in one shot with dhtRead(pin, model).
Caching the discovered ROM codes in KV so onCron doesn’t re-search every minute.
Publishing each reading with emitState for dashboard watchers and Home Assistant.

Going further

Declare the entities in devicesdk.ts so they appear in Home Assistant - see the HA recipe.
Alert when a probe leaves a range - see the Discord recipe.
Show live readings on an OLED - see the OLED recipe.
Full method signatures live in the Device API reference.