Dewcontroller

Dew Controller for Celestron C9.25

Environment-driven dew heater controller for the Celestron C9.25 corrector plate dew ring.

This project uses an ESP32-S3 (Seeed XIAO ESP32S3), an SHT40 temperature/humidity sensor, and a PWM-controlled heater channel to reduce dew risk while limiting unnecessary heat.

What It Does

• Computes dew point from ambient temperature and relative humidity.
• Uses spread control: spread = Ta - Td.
• Looks up heater output from a configurable spread-to-power table.
• Applies a global maximum power cap for safety.
• Uses ring thermistor only as an over-temperature safety clamp.
• Hosts a built-in web UI for status, configuration, logs, and manual PWM test.

Repository Layout

• firmware/dewcontroller/dewcontroller.ino - Main firmware and web UI.
• firmware/TempDewSensorTest/ - Sensor test sketch.
• firmware/i2cverify/ - I2C verification sketch.
• firmware/ioverify/ - GPIO/IO verification sketch.
• data/ - Calibration and logged data.
• hardware/dewcontroller/ - KiCad PCB/schematic and fabrication files.
• hardware/mechanical/ - Mechanical model files.

Hardware

• MCU: Seeed XIAO ESP32S3
• Env sensor: SHT40 on I2C
• Heater output: PWM channel driving dew ring power stage
• Ring thermistor: analog input, safety cutoff only

Firmware Behavior

• Control loop runs from ambient conditions, not ring temperature tracking.
• Max power default is limited (90%) to reduce risk of over-heating optics.
• If ring temperature exceeds cutoff, PWM is clamped.
• Thermal log is stored in SPIFFS and pruned automatically.
• If local SHT40 reads fail, firmware can fall back to weather API values.

Connect to the Device in a Browser

The controller always serves its web interface on HTTP port 80.

Option 1: Connected to Your Home/Observatory Wi-Fi (STA mode)

1. Power the controller.
2. Wait for it to join your configured Wi-Fi network.
3. In a browser, open one of these:
   • http://DewController.local (mDNS)
   • http://<device-ip> (router-assigned IP)

If mDNS does not resolve on your network, use the IP address shown in your router client list.

Option 2: Setup / Fallback Access Point (AP mode)

If Wi-Fi credentials are missing or connection fails, the device starts an AP:

• SSID: DewHeaterSetup
• Device IP: typically 192.168.4.1

Steps:

1. Connect phone/laptop to Wi-Fi network DewHeaterSetup.
2. Open http://192.168.4.1 in your browser.
3. In the web UI, enter your Wi-Fi SSID/password under settings and save.
4. The device will reboot/reconnect and become available on your normal network.

Shared Telemetry Service for Other Telescope Devices

This controller exposes GET /status.json as a machine-to-machine telemetry service.

Example URL:

• http://DewController.local/status.json

Other devices mounted on the telescope can poll this endpoint to get live environmental data. The Flat Panel dew heater is expected to use this service for ambient temperature and humidity inputs.

Recommended fields for client devices:

• ambient - Ambient temperature in C (string value)
• humidity - Relative humidity in % (string value)
• dewpoint - Calculated dew point in C (string value)
• dewspread - Dew-point spread (ambient - dewpoint) in C (string value)
• time - Device timestamp string
• version - Firmware version

Example:

{
   "ambient": "11.8",
   "humidity": "83.2",
   "dewpoint": "9.0",
   "dewspread": "2.8",
   "time": "2026-08-30 21:12:44",
   "version": "v5.2.0"
}

Typical polling interval for clients is 1-5 seconds, depending on how quickly they need to react.

Web UI Endpoints (for integrations)

• GET / - Main UI
• GET /status.json - Live status JSON
• GET /log - Runtime log
• POST /toggle - Enable/disable heater
• POST /settings - Update configuration
• POST /pwmtest - Manual PWM test mode
• POST /clearlog - Clear runtime log
• POST /clearlog_thermal - Clear thermal CSV log
• GET /downloadlog - Download thermal log

Build and Flash

Use Arduino IDE (or Arduino CLI) with ESP32 board support.

1. Install ESP32 board package.
2. Select board compatible with XIAO ESP32S3.
3. Install required libraries used by firmware:
   • Adafruit SHT4x
   • ArduinoJson
4. Open firmware/dewcontroller/dewcontroller.ino.
5. Build and upload.

Intended Use Notes

• Designed for dew prevention on a Celestron C9.25 corrector plate dew ring.
• Start with conservative max power and verify ring temperature behavior in field conditions.
• Confirm wiring polarity, current capacity, and fuse protection before first light.

Version

Current firmware version string in source: v5.1.0