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W1209 12V -50° to 110°C Digital Thermostat Temperature Controller Module Installation and Operating Instructions

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W1209 -50C to 110C Digital Thermostat Temperature Controller Module Installation and Operating InstructionsEstas instrucciones están disponibles en español: W1209 Instrucciones de instalación y funcionamiento del módulo controlador de temperatura del termostato digital

The W1209 is a low cost yet highly functional thermostat controller. This module intelligently controls power to most types of electrical device based on the temperature sensed by the included high accuracy NTC remote temperature sensor.  

The cooler, heater or fan is connected to a power supply through the relay contacts K0 and K1, and the temperature on/off setpoints are programmed into the W1209 using the onboard Set, Increase and Decrease buttons and display. The relay contacts are then open and closed, acting as a power switch for the cooler/heater, based on the temperature measured by the NTC temperature sensor (included with the module) and the setpoints programmed into the W1209 controller. 3 switches are used to configure the parameters including on & off trigger temperatures. The on-board relay can switch up to a maximum of 125V AC or 14V DC at 20A. The temperature is displayed in degrees Centigrade via its 3 digit display and the current relay state by an onboard LED.

Note that the power supply must be connected to the K0 / K1 relay contacts, the relay is not connected to the 12V support power. If your device operates on 12V, the same power supply can be used for both (see the second interconnection drawing below), otherwise, a separate power supply should be used to power your device as shown in the interconnect drawing below. 

Interconnection drawing when using a 12VDC power supply to power the W1209 and a separate power supply to power the load: 

Interconnection drawing when using a single 12VDC power supply to power both the W1209 and the load:

Using the W1209 With a 12V Battery And Alternator or Generator

The maximum input support voltage should not exceed 12V. In applications such as an automobile or RV where the battery voltage can exceed 12V, a regulator circuit such as a 1N5349A zener diode and a series resistor in the range of 20 Ohm 1W should be used to limit the voltage to 12V. The schematic below shows how the resistor and diode should be connected:

12V Resistor/Diode Regulator Circuit

Alternately, a 12V to 9V buck voltage converter with the input to the converter connected to the battery and the output to the W1209 will provide a stable, regulated 9V DC supply for the W1209.

All program settings are stored in non-volatile memory and are retained when power is removed from the module.   

W1209 Example Operating Settings

 1) Set P0 to C for cooling, P1 (hysteresis) to 2 degrees, and setpoint to 30C, with a fan wired to the fan’s power supply with one of the connections (either hot or ground) connected through the K0/K1 relay contacts. When the temperature measured by the sensor rises to 30C (1), the fan will turn on to cool. When the temperature drops below 28C (30C setpoint – 2C hysteresis (1)), the fan will turn off.  

2) Set P0 to H for heating, P1 to 1 degree, and setpoint to 15C, with a heating element wired to its power supply through the K0/K1 relay contacts. When the temperature drops to 15C, the heater will turn on. When the temperature rises above 16C (15C setpoint + 1C hysteresis), the heater will turn off.  

W1209 Specifications

  • Temperature Control Range: -50° ~ 110°C
  • Resolution at -9.9°to 99.9°:  0.1°C; 1°C over  99.9°
  • Measurement Accuracy: 0.1°C; Control Accuracy: 0.1°C
  • Refresh Rate: 0.5 Seconds
  • Input Support Power (DC): 9V – 12V
    The maximum input support voltage should not exceed 12V. In applications such as an automobile or RV where the battery voltage can exceed 12V, a regulator circuit such as a 1N5349 Zener diode and series resistor should be used to limit the voltage to 12V
  • Measuring Input: NTC (10K 0.5%)
  • Waterproof Sensor: 0.5M cable
  • Output: 1 Channel Relay Output, 20A max current

Power Consumption:

  • Static Current: ~35mA; Maximum Current: ~65mA

Environmental Requirements:

  • Temperature: -10° ~ 60°C; Humidity: 20-85%


  • 48mm x 40mm x 14mm (LxWxH)
W1209 Module Dimensions

Displaying the current temperature:

If you apply power to the module without the sensor attached, it will display “LLL”. When you plug in the sensor, it will display the current temperature in degrees C by default. When in any other mode, making no input for approximately 5 seconds will cause the thermostat to return to this default display.

Setting the trigger temperature:

To set the trigger temperature press the button marked ‘SET’. The display will flash. You can now set a trigger temperature in 0.1°C increments using the ‘+’ and ‘-‘ buttons. If no buttons are pressed for approximately 2 seconds the trigger temperature will be stored and the display will return back to the current temperature.


Table of Operating Parameters and Values

See a detailed description for each parameter below.

CodeDescriptionSetting RangeDefault
P0Heating/Cooling ModeH/CC
P1Hysteresis (Return difference) Degrees0.1C to 40C2C
P2Set Upper Temperature Limit (Degrees)-50C to 110C110C
P3Set Lower Temperature Limit (Degrees)-50C to 110C-50C
P4Temperature Offset Correction (Calibration) Degrees-15C to +15C0
P5Trigger Delay Start (Minutes)0-10 Minutes0
P6/P7High Temperature Alarm (Degrees)-50C to +110C+100C
P8Factory Reset (Can also press + and – for 5 seconds)C/HC

To set any parameter first press the ‘SET’ button for at least 5 seconds. The display should now display ‘P0’. This represents parameter P0. Pressing the ‘+’ or ‘-‘ buttons will cycle through the various parameters (P0 to P6). Pressing the ‘SET’ button while any of the parameters are displayed will allow you to change the value for that parameter using the ‘+’ and ‘-‘ buttons (see below). When finished setting a parameter press the set button to exit that option. If no buttons are pressed for approximately 5 seconds the thermostat will exit the parameter options and will return back to the default temperature display.

Setting the cooling or heating parameter P0:

The parameter P0 has two settings, C and H. When set to C (default) the relay will energize when the temperature setpoint is reached. Use this setting if connecting to an air-conditioning system. When set to H the relay will de-energize when the temperature is reached. Use this setting if controlling a heating device.

Setting the hysteresis parameter P1:

This sets how much change in temperature must occur before the relay will change state. For example, if you set the hysteresis value at 1C and the desired temperature on the thermostat at 20°C, the relay will be deactivated at 21°C (20°C+1°C) (1). The relay is activated again when the temperature drops to 20°C. Setting this hysteresis to a higher value helps stop the thermostat from continually triggering when the temperature drifts around the trip temperature.

The hysteresis setting is used to avoid constantly triggering the device on and off (oscillating) around the trigger temperature. This is how home thermostats typically work since it is hard on home heating and cooling systems to be constantly cycled on and off. On the other hand, something like an aquarium heater is fine to be operated with no hysteresis in order to hold at as constant of a temperature as possible.

(1) NOTE: Depending upon the firmware version of your module, the hysteresis is either subtracted from, or added to, the temperature setpoint. For example, when P0 = “C”, hysteresis (P1) is set to “2” and the setpoint is 28, either the relay will activate at 28 and deactivate or 26 (setpoint – hysteresis), or it will activate at 30 (setpoint + hysteresis), and deactivate at 28. A quick test will confirm which firmware version your module has.  

Setting the upper limit of the thermostat parameter P2:

This parameter limits the maximum trigger temperature that can be set. It can be used as a safety stop to avoid an excessively high temperature from being set by a user of the module.

For example, if you are using the W1209 to control the temperature of an aquarium, you might set P2 to 30C, to reduce the possibility of the temperature being set too high and killing the fish.

Setting the lower limit of the thermostat parameter P3:

This parameter limits the minimum trigger temperature that can be set. It can be used as a safety to stop an excessively low trigger temperature from accidentally being set by the user.

Setting temperature offset correction (calibration) parameter P4:

Should you find there is a difference between the displayed temperature and the actual temperature (for instance if the temperature probe is on a long run of cable) you can make minor corrections to the temperature reading with this parameter. Calibration allows you to adjust the temperature in 0.1° increments either positive or negative.

Setting the trigger delay parameter P5:

This parameter provides a delay between when the setpoint temperature is reached and when the relay is energized or de-energized. This setting can range from 0-10 minutes in 1-minute increments. For example, If you set 1 minute, the relay is activated until one minute after reaching the set temperature.

Setting the high-temperature alarm parameters P6 and P7:

Setting the alarm to ON through parameter P6, and setting a temperature value for this parameter via P7, will cause the relay to switch off when the temperature reaches this setting. The display will also show ‘HHH’ or “—” to indicate an alarm condition. The relay will not re-energize until the temperature falls below this value. The default setting for P6 is OFF.

NOTE: Some firmware versions set the temperature alarm value through P6, and do not have a P7 parameter. Other firmware versions set the alarm version with P7 and do not use P6. If you are unable to set the value with one of the parameters, try the other one.

Reset to Factory Defaults P8

Some firmware versions of the W1209 include a P8 parameter that resets it to factory defaults. If P8 is available in your model, the default value is C. To reset it to factory defaults, change the value to H using the + or – button. Press and hold the Set button. When it returns to the default temperature setting, it will be reset to factory defaults.

Not all firmware versions include P8. If yours does not, to reset the W1209, press and hold both + and – buttons simultaneously. After 5 seconds it will reset to factory defaults.


W1209 Temperature Controller Tested on Chinese Electronic Products Tested blog (includes a W1209 circuit schematic):

For commonly asked questions and answers about the use and operation of the W1209, see the W1209 Temperature Controller Q&A article. 

For instructions on how to assemble the W1209 acrylic plastic enclosure (case), click here: W1209 Temperature Controller Case Assembly Instructions.

This video by Robojax provides good examples of connecting the W1209 controller to a power supply and load and setting its parameters:

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