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epaper433 — WT450 wireless sensor (ESP32 + ePaper)

epaper433 — WT450 wireless sensor

Languages: English · Deutsch

Measure temperature with a DS18B20, show it on a LilyGO T5 2.13" ePaper, and transmit via 433 MHz using the WT450 / WT450H protocol. Between cycles the ESP32 sleeps (light sleep or deep sleep). The onboard button wakes the board and starts a measurement immediately.

Receivers: e.g. rtl_433.

Features

  • DS18B20 temperature (1-Wire, ESP32 RMT / OneWireESP32)
  • ePaper: large temperature, voltage + battery % at the bottom (refresh only if ΔT ≥ 0.1 °C or Δ% ≥ 1)
  • Splash logo on cold boot, then hibernate until first reading
  • WT450 OOK via ASK transmitter (e.g. STX882)
  • Battery via ADC (optional); charge % is sent in the WT450 humidity field
  • GPIO 39 button: wake from sleep + immediate measure/send cycle
  • Light sleep (USB-safe) / deep sleep (battery, no USB)
  • All pins and timings in config.h

Hardware

Part Notes
LilyGO / TTGO T5 V2.3.1 ESP32 + 2.13" ePaper (GxEPD2_213_BN)
DS18B20 Temperature sensor
433 MHz TX e.g. STX882, FS1000A
4.7 kΩ 1-Wire DQ pull-up to 3.3 V
Optional Voltage divider on ADC for battery

Pinout (T5 V2.3.1)

Function GPIO Notes
DS18B20 DQ 21 + 4.7 kΩ to 3.3 V
433 MHz TX DATA 25 not GPIO 4!
Battery ADC 35 input-only; -1 = disabled
Button (wake) 39 onboard, active low
ePaper CS 5 onboard
ePaper DC 17 onboard
ePaper RST 16 onboard
ePaper BUSY 4 onboard — do not use for TX
SPI SCK / MOSI 18 / 23 ESP32 default SPI

Wiring

ESP32 3.3V ──────── DS18B20 VDD
ESP32 GND  ──────── DS18B20 GND
ESP32 GPIO 21 ───── DS18B20 DQ ─── 4.7 kΩ ─── 3.3V

ESP32 GPIO 25 ───── 433 MHz TX DATA
ESP32 GND     ───── 433 MHz TX GND
3.3V / 5V     ───── 433 MHz TX VCC (per module)

Optional battery:
ESP32 GPIO 35 ───── divider midpoint (ADC)

Onboard:
  ePaper: CS=5, DC=17, RST=16, BUSY=4, SCK=18, MOSI=23
  Button: GPIO 39

GPIO 4 is ePaper BUSY. Use GPIO 25 for the transmitter (TX_PIN).

Software

Requirements

  • Arduino IDE 2.x (or PlatformIO)
  • ESP32 board support (Arduino-ESP32 Core 3.x recommended)
  • Libraries:
    • esp32-ds18b20 — junkfix/esp32-ds18b20 (OneWireESP32)
    • GxEPD2 (Jean-Marc Zingg)
    • Adafruit GFX (GxEPD2 dependency)

Board / upload

Setting Value
Board ESP32 Dev Module / LilyGO T5
Upload speed 921600 (or 115200)
Serial monitor 115200 baud
CPU 80 MHz (set in sketch)

Flash

  1. Open folder epaper433 (epaper433.ino)
  2. Check config.h
  3. Compile & upload
  4. Open Serial Monitor (if SERIAL_DEBUG 1)

Configuration (config.h)

// GPIO
#define TX_PIN                25
#define DS18B20_PIN           21
#define BATTERY_ADC_PIN       35      // -1 = no battery ADC
#define BATTERY_DIVIDER_RATIO 2.0f
#define BATTERY_FULL_V        4.15f   // ~100 %
#define BATTERY_EMPTY_V       3.30f   // ~0 %

#define EPAPER_CS_PIN          5
#define EPAPER_DC_PIN         17
#define EPAPER_RST_PIN        16
#define EPAPER_BUSY_PIN        4
#define EPAPER_SCK_PIN        18
#define EPAPER_MOSI_PIN       23
#define EPAPER_ENABLE          1      // 0 = display off
#define EPAPER_ROTATION        1
#define EPAPER_UPDATE_TEMP_C   0.1f   // skip refresh if smaller change
#define EPAPER_UPDATE_PERCENT  1

#define BUTTON_PIN            39
#define BUTTON_WAKE_LEVEL      0      // 0 = pressed = GND
#define BUTTON_DEBOUNCE_MS    50

// WT450
#define TX_INVERT              0
#define WT450_HOUSE_CODE       1      // 1..15
#define WT450_CHANNEL          2      // 1..4 (humidity field = battery %)

// Timing
#define SEND_INTERVAL_MS   60000
#define SENSOR_READ_RETRIES    3
#define SENSOR_RETRY_MS      200
#define SENSOR_RETRY_SLEEP_MS 5000
#define DS18B20_CONVERT_MS   850
#define DS18B20_CRC_RETRIES    4

// Power / debug
#define ESP32_DEEP_SLEEP       1      // 0=light sleep, 1=deep sleep
#define SERIAL_DEBUG           0

Battery % uses batteryPercentFromVoltage() (linear between BATTERY_EMPTY_V and BATTERY_FULL_V). WT450 humidity is clamped to 0–99.

Operation

Cold boot → splash → hibernate → sleep
    ↓
Timer or button wake
    → read DS18B20
    → update ePaper if values changed (or show "Error")
    → send WT450 (temp + battery % as humidity)
    → hibernate / deep sleep until next timer or button

Every successful cycle sends temperature and battery charge (0–99 %) in the humidity field. There is no separate battery channel.

Button (GPIO 39)

  • Wakes from light and deep sleep (ext0)
  • Then: measure → display → send → sleep
  • Waits for release before sleep (debounce)

GPIO 39 has no internal pull-up; the T5 board provides an external one.

Sensor error on ePaper

If the DS18B20 is missing or read fails: display shows "Error" plus battery voltage/%. No radio TX in that case.

Power saving

Mode ESP32_DEEP_SLEEP Typical (T5) Use
Light sleep 0 ~0.8 mA+ USB / development
Deep sleep 1 ~0.5–0.9 mA on T5 Battery, no USB

Bare ESP32 deep sleep is ~10–150 µA. On LilyGO T5 V2.3.1, 500–900 µA at BAT is common. Well optimized: ~200–400 µA; below ~150 µA usually needs hardware changes.

Important: Deep sleep with USB often causes a boot loop (rst:0x5 DEEPSLEEP_RESET). Measure current on BAT only, USB disconnected.

Before deep sleep the sketch

  1. display.hibernate() (SSD1680 deep sleep)
  2. Idle levels: CS/RST HIGH, DC/SCK/MOSI LOW
  3. TX (GPIO 25) LOW
  4. gpio_hold_en + gpio_deep_sleep_hold_en so lines do not float

On boot: lowPowerReleaseGpioHold() releases all holds.

Also: Wi-Fi/BT off, CPU 80 MHz, ePaper refresh skipped when values unchanged, single DS18B20 conversion per cycle.

Hardware tips (if still ~0.9 mA)

Action Effect
100 kΩ pull-up CS (GPIO 5) → 3.3 V Keeps CS HIGH
100 kΩ pull-up RST (GPIO 16) → 3.3 V Keeps RST HIGH
ADC divider ≥ 100 kΩ+100 kΩ (better 470 k) 10 k+10 k alone ≈ 200 µA continuous
Switch STX882 VCC (P-MOSFET) Cuts TX idle current
Disconnect display for reference Often ~100–150 µA without panel

Wake sources: timer (SEND_INTERVAL_MS) and button (GPIO 39).

Serial debug (example)

Enable with #define SERIAL_DEBUG 1.

ESP32  WT450-TH + DS18B20 + ePaper + STX882
----------------------------------------------
CPU=80 MHz DS18B20=GPIO21 (RMT)
 TX=GPIO25
 Button=GPIO39 (wake)
 ePaper: T5 2.13" enabled
Sleep: deep (battery only, no USB)
Wake cause: timer
----------------------------------------------
Battery: 3.71 V  68 %
sending: 23.50 C  bat=68%  sent OK
Message Meaning
(no device) Sensor / pull-up / wiring
(crc) Noisy 1-Wire — bus re-init
(invalid) Temp outside −55…125 °C
(read fail) Other read error

Receive with rtl_433

rtl_433 -f 433.92M -s 250k

House code and channel must match WT450_HOUSE_CODE / WT450_CHANNEL. If nothing is received, try TX_INVERT 1. Battery % appears as humidity.

Project layout

epaper433/
├── epaper433.ino         Main sketch
├── config.h              GPIO, timing, power, display, battery, button
├── DisplayEpaper.h/cpp   ePaper (temp, volt, %, splash, error)
├── Ds18b20OneWire.h/cpp  DS18B20 (OneWireESP32 / RMT)
├── Wt450Send.h/cpp       WT450 protocol
├── LowPowerSleep.h/cpp   Light/deep sleep + button wake + GPIO hold
├── epaper433_logo.h      Splash bitmap (PROGMEM)
├── assets/               Logo source images
├── Dbg.h                 Debug macros
├── LICENSE               GPL-3.0
├── README.md             English
└── README_DE.md          Deutsch

Troubleshooting

Problem Fix
Boot loop DEEPSLEEP_RESET Unplug USB or ESP32_DEEP_SLEEP 0
Deep sleep ~0.9 mA Pin holds? 100 kΩ on CS/RST; check ADC divider; USB off
Button does not wake BUTTON_PIN 39, BUTTON_WAKE_LEVEL 0
ePaper stays white GxEPD2 + Adafruit GFX; panel GxEPD2_213_BN
Display/BUSY stuck Do not put TX on GPIO 4 — use TX_PIN=25
DS18B20 / (crc) 4.7 kΩ pull-up, GPIO 21, esp32-ds18b20, serial shows (RMT)
No RF reception Antenna, TX_PIN, TX_INVERT, house/channel
Wrong battery reading Adjust BATTERY_DIVIDER_RATIO; ADC pins 32–39 only
Battery % inaccurate Tune BATTERY_FULL_V / BATTERY_EMPTY_V

License

GPL-3.0

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