🌙 OpenDewControl

An open-source Arduino firmware and ASCOM driver for managing dew heater straps in astrophotography setups.
It provides 3 independent PWM channels, an auxiliary output, and supports both manual and automatic dew prevention modes — with persistent configuration in EEPROM and a simple, human-readable serial protocol.

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▶️ Hardware prototype

This is a concept prototype using a Nano 328P controller and a common, ready to use, mosfet board.

⚙️ Features

• 3 independent PWM heater channels (0–100%)
• 1 auxiliary digital ON/OFF output
• DHT22 sensor for temperature and humidity
• Automatic dew control using the Magnus dew point equation
• Manual and automatic modes
• Global dewMargin (safety temperature offset above dew point)
• Individual ΔTmax to calibrate each channel
• Master ON/OFF switch for all outputs
• 10% discretization (rounded upward)
• EEPROM storage and versioning
• Human-readable serial protocol, ideal for ASCOM or standalone drivers
• Designed for Arduino Uno / Nano

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🧠 Theory of Operation

🔹 Dew Point Calculation

The dew point is computed from ambient temperature (°C) and relative humidity (%) using the Magnus formula:

Tdew = (b * α) / (a - α)
α = (a * T) / (b + T) + ln(RH / 100)

where
a = 17.27, b = 237.7, T = ambient temperature (°C), and RH = relative humidity (%).

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🔹 Open-Loop Heater Control

Each heater strap has a maximum temperature rise (ΔTmax) above ambient when powered at 100%.
Assuming an approximately linear relationship between PWM power and heat output:

ΔT ≈ (PWM% / 100) * ΔTmax

We define a target heater temperature slightly above the dew point:

Target = Tdew + dewMargin

and compute the required PWM:

PWM% = ceil( ((Target - Tamb) / ΔTmax) * 100 )

Then the PWM is discretized in 10% steps, always rounded upward to ensure adequate heating.

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🔹 Open-Loop Control — Pros & Cons

Advantages

• No extra temperature sensors required on the straps
• Simple wiring
• No risk of extra probes failure

Limitations

• Heater temperature is estimated, not measured
• Requires experimental calibration of ΔTmax per strap
• May vary slightly with airflow or ambient changes

In practice, open-loop control is accurate enough for most astrophotography applications when ΔTmax and dewMargin are correctly calibrated.

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🧩 EEPROM Parameters

All parameters are saved to EEPROM and automatically restored at boot.
If EEPROM data are invalid or missing, safe defaults are loaded.

Parameter	Type	Default	Description
EEPROM_VERSION	int	1000	Firmware structure version
dewMargin	float	3.0 °C	Global dew offset above dew point
ΔTmax1..3	float	5.0 °C	Temperature rise per channel at 100% power (to be measured)
mode	byte	MANUAL	Operation mode

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🧭 Operating Modes

Mode	Description
MANUAL	User directly controls PWM per channel
AUTO	PWM calculated automatically from dew point, ΔTmax, and dewMargin

• Mode persists until changed manually.
• No automatic fallback between AUTO and MANUAL.
• In both modes, MASTER must be ON for outputs to be active.

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🪫 Master & AUX Outputs

• MASTER: globally enables or disables all PWM and AUX outputs.
• AUX: separate ON/OFF channel, useful for relays, fans, or accessories.

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🔌 Serial Command Protocol

All commands are case-insensitive and end with the # character. Responses are in the format COMMAND=VALUE# or a multi-value string for status/config requests.

🔹 Connection Handshake

The firmware uses an auto-reset mechanism common to Arduino boards.

1. When a serial connection is opened by a driver or terminal, the Arduino board automatically resets.
2. After completing its setup() routine, the firmware sends a READY# signal.
3. The driver must wait for this READY# signal before sending any other commands. This ensures the device is fully initialized and ready to communicate.

🔹 General Commands

Command	Description	Example	Response
GET_INFO	Get firmware name	GET_INFO#	GET_INFO=MGZ_OpenDewControl#
GET_FWVERSION	Get firmware version	GET_FWVERSION#	GET_FWVERSION=1.0#
RESET	Restart the device	RESET#	RESET=OK#

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🔹 Configuration Commands (SET)

Command	Description	Example	Response
SET_MASTER:TRUE	Turn all outputs ON	SET_MASTER:TRUE#	SET_MASTER=TRUE#
SET_MASTER:FALSE	Turn all outputs OFF	SET_MASTER:FALSE#	SET_MASTER=FALSE#
SET_AUX:TRUE	Enable AUX output	SET_AUX:TRUE#	SET_AUX=TRUE#
SET_AUX:FALSE	Disable AUX output	SET_AUX:FALSE#	SET_AUX=FALSE#
SET_MODE_AUTO:TRUE	Enable automatic mode	SET_MODE_AUTO:TRUE#	SET_MODE_AUTO=TRUE#
SET_MODE_AUTO:FALSE	Enable manual mode	SET_MODE_AUTO:FALSE#	SET_MODE_AUTO=FALSE#
SET_POWER1:X	Set CH1 power (0–100%)	SET_POWER1:50#	SET_POWER1=50#
SET_POWER2:X	Set CH2 power (0-100%)	SET_POWER2:20#	SET_POWER2=20#
SET_POWER3:X	Set CH3 power (0-100%)	SET_POWER3:100#	SET_POWER3=100#
SET_DEWMARGIN:X	Set global dew margin (°C)	SET_DEWMARGIN:2.5#	SET_DEWMARGIN=2.50#
SET_DTMAX1:X	Set ΔTmax for CH1 (°C)	SET_DTMAX1:8.2#	SET_DTMAX1=8.20#
SET_DTMAX2:X	Set ΔTmax for CH2 (°C)	SET_DTMAX2:10.0#	SET_DTMAX2=10.00#
SET_DTMAX3:X	Set ΔTmax for CH3 (°C)	SET_DTMAX3:12.5#	SET_DTMAX3=12.50#

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🔹 Status & Config Readback (GET)

Command	Description	Example	Response
GET_STATUS	Get all real-time status values	GET_STATUS#	`GET_STATUS=MASTER=TRUE
GET_CONFIG	Get all configuration parameters	GET_CONFIG#	`GET_CONFIG=MODE_AUTO=TRUE
GET_MASTER	Get master status	GET_MASTER#	GET_MASTER=TRUE#
GET_AUX	Get AUX status	GET_AUX#	GET_AUX=FALSE#
GET_MODE_AUTO	Get current mode	GET_MODE_AUTO#	GET_MODE_AUTO=TRUE#
GET_DEWMARGIN	Get global dew margin	GET_DEWMARGIN#	GET_DEWMARGIN=3.00#
GET_DTMAX1	Get ΔTmax for CH1	GET_DTMAX1#	GET_DTMAX1=5.00#
GET_DTMAX2	Get ΔTmax for CH2	GET_DTMAX2#	GET_DTMAX2=5.00#
GET_DTMAX3	Get ΔTmax for CH3	GET_DTMAX3#	GET_DTMAX3=5.00#

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🔹 EEPROM Management

Command	Description	Example	Response
SAVE_EEPROM	Save all settings to EEPROM	SAVE_EEPROM#	SAVE_EEPROM=OK#
READ_EEPROM	Reload settings from EEPROM	READ_EEPROM#	READ_EEPROM=OK#

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⚗️ Parameter Calibration

ΔTmax (Maximum Temperature Rise)

1. Power the heater strap at 100% in still air for a few minutes.
2. Measure the temperature difference between the strap and ambient air.
3. Use this value as ΔTmax.

dewMargin

Defines how much higher the heater temperature should be compared to the dew point.
Typical setting: 2–3 °C

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🧮 Example Calculation

Given:

T_amb = 10°C
RH = 90%
dewMargin = 2°C
ΔTmax = 10°C

Then:

Tdew ≈ 8.3°C
Target = 8.3 + 2 = 10.3°C
ΔT = 10.3 - 10 = 0.3°C
PWM = ceil((0.3 / 10) * 100) = 10%

→ Heater runs at 10% duty cycle (1/10 step).

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🧾 License

This project is released under the MIT License

⚠️ Disclaimer

This software provided "as is". The author is not responsible for any hardware or software issue. Use at your own risk.