The Cheapo Dew Controller, or CheapoDC, is a low cost, easy to build DIY dew controller based on an ESP32-C3 mini. Parts required include the ESP32-C3 mini, one or two MOSFET modules, a 12V to 5V buck converter, some protoboard, a couple of RCA sockets, a 12V barrel socket and wire. Cost of the parts should be less than $20 for a unit that controls 2 dew heater straps. More hardware details can be found in the Hardware section.

A primary goal was to keep the build simple with minimal components. This is done by leveraging the ESP32 WiFi capability to query one of the open weather service APIs. Either the OpenWeather API or the Open-Meteo API may be used to retrieve ambient temperature, humidity and dew point for the controller's geographic location. This is then used to calculate CheapoDC's power output. No additional components, such as temperature or humidity probes, are required. The responsiveness and aggressiveness of the controller can be adjusted through several configuration settings. CheapoDC works best with internet connectivity to support the weather service queries but it can also be used in a limited way without internet access.

Although the CheapoDC supports two dew strap outputs, the output levels are not individually controlled. Two separate MOSFET modules are used but the PWM output for the two ESP32 pins driving the MOSFETs are tied to the same PWM channel.

When working in Automated mode, the controller will periodically calculate the power output to the dew straps using the calculations below. The default output calculation or update period is 1 minute.

• Set Point = SP
  • as set using the Set Point Mode selection.
• Reference Temperature = RT
  • as set using the Temperature Mode selection.
• Track Point = TP
  • the calculated temperature point where the Tracking Range starts. A Reference Temperature less than or equal to the Track Point will cause the controller to use Maximum Output.
• Track Point Offset = TPO
  • a temperature offset applied to the Set Point when calculating the Track Point relative to the Set Point. The Track Point Offset may be set from -5.0°C to 5.0°C. The default is 0.0°C.
• Tracking Range = TR
  • the temperature range starting at the Track Point where the controller output ramps up from Minimum Output at the high end of the range to Maximum Output at the low end of the range. The range may be set to values from 4.0°C to 10.0°C. The default is 4.0°C.
• Power Output = PO
  • the percentage of power the controller is outputting to the dew straps. It varies from the Minimum Output setting to the Maximum Output setting.
• Minimum Output = MinO
  • the minimum percent power setting for the controller to output when not in the Off Controller Mode. Defaults to 0%.
• Maximum Output = MaxO
  • the maximum percent power setting for the controller to output when at full output. Defaults to 100%.

$TP = SP + TPO$

$IF$ $(RT <= TP )$ $THEN$ $PO= MaxO$

$IF$ $(RT >= (TP + TR))$ $THEN$ $PO = MinO$

$IF$ $(RT <(TP + TR))$ $THEN$ $PO = MinO + (MaxO - MinO) * ( 1 - ((RT - TP)/TR))$

Example 1 is primarily a reference image to illustrate the variables defined above.

• Power Output curve is shown relative to the Track Point and the Tracking Range. While the Reference Temperature is greater than the high end of the Tracking Range the Power Output is set to Minimum Output. The Power Output ramps up linearly through the Tracking Range from the Minimum Output to the Maximum Output as the Reference Temperature drops.
• This example shows a Reference Temperature, RT = 8°C, which is greater than the upper end of the Tracking Range, causing Power Output to be set to Minimum Output. In this case Minimum Output is set to 10%.
• The upper end of the Tracking Range, at 7°C, is determined from the Set Point (SP = -2°C) plus the Track Point Offset (TPO = 4°C), creating a Tack Point, TP = 2°C, plus the Tracking Range (TR = 5°C).

NOTE: The Set Point and the Reference Temperature values may vary from one output calculation to the next with each weather update.

Example 2 shows changing the values of the controller configuration variables can affect the Power Output calculation.

• The Set Point, SP = -2°C, and the Reference Temperature, RT = 8°C, the same as in Example 1. However changing the Track Point Offset, now TPO = 2°C, and the Tracking Range, now TR = 10°C, has changed the Power Output as well as flattening the Power Output curve.
• The Maximum Output has been increased, MaxO = 100%, to allow full power output to be reached at 0°C.

The following modes of operation are used by CheapoDC to determine overall controller operation as well as which values to assign to the Set Point and Reference Temperature when calculating output.

The Controller Mode selects the overall operating mode of the dew controller.

• Automatic

  • Uses the Set Point Mode and Temperature Mode settings to calculate the controller output based on the Power Output calculations above. In Automatic mode the Power Output calculation will be run periodically based on the Update Output Every setting. This may be set to a value from 1 to 20 minutes. The default is 1 minute. Automatic mode may be used with or without internet access.

• Manual

  • Controller output is manually controlled by setting the Dew Controller Output either through the Web UI or the API. When in Manual mode the the periodic Power Output calculation is suspended. Using Manual control is an option if internet access for OpenWeather API queries is not available.

• Off

  • Controller output is set to Zero.

The Set Point Mode selects what will be used as the SetPoint for calculating Power Output.

• Dew Point

  • Uses the Dew Point as the Set Point used for calculating output. This is the default selection.

• Temperature

  • Uses a Temperature Set Point value input via the Web UI or API as the Set Point for calculating output. If the CheapoDC is being used without internet access then this mode allows a Set Point to be defined when a the DeW Point cannot be determined through the OpenWeather API.

• Midpoint

  • Uses the midpoint between the current Temperature Mode temperature value and the Dew Point value as the Set Point for calculating output. The impact of selecting this mode is to effectively double the Tracking Range and further flatten the ramp of the Power Output curve.

The Temperature Mode selects how the Reference Temperature will be determined for calculating the Power Output.

• Weather Query

  • Uses the Ambient Temperature returned by the OpenWeather API weather query as the Reference Temperature for calculating the controller output. This requires that the CheapoDC have internet access.

• External Input

  • Use the External Input temperature set through the Web UI or API as the reference temperature for calculating the controller output. This may be the preferred mode when using the controller with KStars/Indi. The CheapoDC Indilib driver can use a temperature probe attached to a focuser as the external input. This is also the mode that should be used when the CheapoDC does not have access to the internat and the OpenWeather API.

One of the goals with the CheapoDC is to for it to be relatively easy to assemble. Although some soldering will be required the number of connections is minimized by using common modules.

Component list:

• ESP32-C3 SuperMini (Other ESP32 modules should work but this one is very small and low priced) Example: [https://www.aliexpress.com/item/1005005967641936.html?spm=a2g0o.order_list.order_list_main.10.3b2c1802dRy3Tw]
• Buck converter to reduce 12V to 5V. I used an LM2596 Module. You could also use an MP1584EN Module. These modules often have an adjustable output. You'll need to use an Volt meter to adjust the output to 5 volts before hooking it up.
• Two dual-MOSFET Modules to handle output to the dew straps while being able to be triggered by the 3.3V levels from the ESP32 PWM pins.
• A resettable fuse that can handle 5A. An example 5A PPTC
• Some assorted hardware:
  • 12VDC 5.5mm x 2.1mm socket. Common socket size used for Astronomy.
  • Switch that will work for 12VDC at 5A.
  • Two RCA sockets. RCA plugs are commonly used for dew straps.

Assembled in a 6 inch long piece of 2"x1" aluminum channel. This also has Pin 8, which is the status LED, connected to an extra LED on the left side for external visibility. The ESP32-C3 is also mounted in some protoboard to make the wiring easier. If you plan to use a small case like this, use silicone hookup wire. The extra flexibility of silicone wire will save a lot of frustration. I used 22 awg wire for the 5V and 3.3V ESP32 connections and 18 awg wire for the 12V connections.

Two completed CheapoDCs from the outside.

How to build the CheapoDC firmware can be found here. One of the pre-build configuration items is to pick and configure a weather service.

CheapoDC can leverage one of two open weather service APIs to retrieve current temperature, humidity and dew point for your location. The service to use is specified in the CDCDefines.h file and only one service may be support at a time. The selected service is built into the firmware.

The OpenWeather API is the default configuration. In order to use the OpenWeather API a registered account and API key is required. The account is free and allows for up to 60 queries/minute and 1,000,000 per month. Weather updates can occur from 5 to 20 minutes apart. Doing an API call every 5 minutes is more than adequate for dew control and even with 2 or 3 CheapoDC's sharing a key should have no issue at the free account level. Register and get your API key here.

Use of the Open-Meteo API does not require any registration for current weather queries. The free level allows for 10,000 API calls per day. Weather updates seem to be regular at a 15 minute interval. AT a 5 minute query interval several CHeapoDCs can be running at the same time without issue.

Both services require a location using Latitude and Longitude which you can set using the CheapoDC Web UI or INDI Lib driver. OpenWeather provides the name of the weather station closest to your coordinates in its response. Open-Meteo does not. If run side-by-side with the same co-ordinates they provide slightly different results. You may want to check which service provides the best results for your location.

As indicated the defaults service is OpenWeather API. But if you do not want to share information with yet another internet data collector then configure Open-Meteo.

CheapoDC comes with a Web UI that supports basic web authentication. The ID and password for the web authentication is set in the CDCdefines.h file as part of the build configuration. Default is "admin" for both. TLS (or HTTPS) is not supported so the security is minimal. The intention is to provide a deterrent to someone on your network from easily doing a Web OTA upgrade to the firmware or uploading files to the LittleFS partition.

The Web UI has 4 main pages, a dashboard, a configuration page, a device management page and a file management page.

CheapoDC can be configured to support Web Sockets for the CheapoDC API. By default Web Socket support is Disabled but if it is Enabled then the configuration page will use the Web Sockets API to display and update configuration data. Otherwise, by default the configuration page will use the basic Web API utilizing the HTTP POST method.

The Device Management page provides the ability to do Over-The-Air (OTA) firmware updates. It also allows for remote Reboot of the CHeapoDC.

The CheapoDC uses LittleFS for file storage on the ESP32. Although LittleFS supports directories as well as files CheapoDC uses a flat structure and all files are managed at the root. The file management page supports upload, download and delete functions.

The Status LED is used to provide information about the current status of the CheapoDC. Status blinking lasts for 10 seconds. It will blink as WiFi access attempts are made. If a Station mode connection is successfully made to an access point then the status LED will slow blink (1 second cycle). If no connection is made then the CheapoDC will go into Access Point mode. The status LED will then fast blink (200ms cycle).

The status LED will also blink for 10 seconds after a power output changes and after a controller configuration change. The 10 second blink period may be modified by changing CDC_STATUS_LED_DELAY in CDCdefines.h.

The CheapoDC provides API access to all configuration and data items available through the Web UI. There is no authentication support in the API but the API also does not support firmware OTA updates or file management. These can only be done through the Web UI.

CheapoDC supports three API mechanisms:

1. TCP API using JSON syntax
2. Basic Web API utilizing HTTP POST
3. Web Sockets API using JSON syntax

The APIs use the same commands which are listed in the top of CDCommands.cpp. Commands are 2 to 4 character strings. For each command there is a map indicating:

• the number of the command,
• whether or not the value associated with the command is to be saved or loaded using the CDCConfig.json file,
• units used for the value. Units are HTML or JSON formatted and may be:
  • None: empty string.
  • Degrees Celsius: "°C"
  • Decimal Degrees: "°"
  • Percent: "%"
  • Milliseconds: "msec"
  • Seconds: "sec"
  • Minutes: "min"
  • JSON ENUM, for enumerated values like Controller Mode.
    • ie: "{"Mode":["Automatic","Manual","Off"]}"

The table below provides a list of the commands but the code is the final correct source of truth here.

• String maximum lengths are identified in the table.
• Floats are truncated to 2 decimal places.
• All commands support getter API methods.
• Commands supporting a setter method are identified.
• The QN command is a setter only command. It requires a value that is not Null. "NA" works.
• Using an invalid command or trying a setter on a command not supporting a setter will return an error.
• Strictly speaking command values are always treated as Strings since the values are always enclosed in quotes in the JSON. Conversion to appropriate type is handled internally.

The TCP API uses JSON formatted commands over TCP on port 58000. JSON strings must always be terminated with a newline character "\n". "cmd" in the following formats is the 2 to 4 character command identified in CDCommands.cpp.

• Send: {"SET":{"cmd":"value"}}
• Response: {"RESULT":"value"}
  value = 0 for success
  value = -1 for failure

• Send: {"GET":"cmd"}
• Response: {"cmd":"value"}
  a failure will return a response of {"RESULT":"-1"}

The Web API uses POST and "x-www-form-urlencoded" with name value pairs.

• Send: cmd=value
• Response: HTTP status code of 200 on success.

• Send: get=cmd
• Response: cmd=value
  success has HTTP status code of 200.

The Web Sockets API uses the same JSON formatted Send/Response strings as the TCP API. Strings do not need to be terminated with a newline character.

NOTE: Read the information about modifying the message queue size in AsyncTCP.cpp and AsyncWebSocket.h.

An INDI driver is now available in the master branch and should be available in the next INDI release (2.0.7 in April 2024). Details should become available on the INDI drivers site.