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

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The W1209 Temperature Controller is a compact, cost-effective, and versatile thermostat module designed for precise temperature-based control of electrical devices. Equipped with a high-accuracy NTC thermistor temperature sensor, the W1209 allows users to automate the operation of cooling or heating systems based on real-time temperature readings.

This module features a built-in relay that can switch power to connected devices such as fans, heaters, heat lamps or coolers. The user sets the desired temperature thresholds—both activation (on) and deactivation (off) points—via the onboard interface, which includes a 3-digit digital display and three tactile buttons: Set, Increase, and Decrease.

When the sensed temperature crosses the defined threshold, the W1209 activates or deactivates the relay accordingly, effectively functioning as an automatic power switch. The current temperature is continuously displayed in degrees Celsius, and an onboard LED indicates the relay’s operational status, providing clear, real-time feedback.

W1209 ELECTRICAL CONNECTION

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. 

Wiring Steps

1. Power the W1209:
   • Connect the positive (+) terminal of the 12V power supply to the VCC pin.
   • Connect the negative (–) terminal to the GND pin.
2. Connect your device to the relay:
   • The relay terminals are labeled K0 (normally open), K1 (common).
   • For most setups, wire one lead of your device to K0, and the other to your power source.
   • Connect K1 to the matching terminal of your power source (to complete the circuit when the relay is closed).

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:

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
• Relay Rating: 20A at 14V DC, 16A at 24V DC, 7A at 125V AC
• 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%

Dimensions

• 48mm x 40mm x 14mm (LxWxH)

Displaying the current temperature

If you apply power to the module without the sensor attached, it will display “LLL” or “888”. 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.

SETTING THE OPERATING PARAMETERS

Table of Operating Parameters and Values

See a detailed description for each parameter below.

Code	Description	Setting Range	Default
P0	Heating/Cooling Mode	H/C	C
P1	Hysteresis (Return difference) Degrees	0.1C to 40C	2C
P2	Set Upper Temperature Limit (Degrees)	-50C to 110C	110C
P3	Set Lower Temperature Limit (Degrees)	-50C to 110C	-50C
P4	Temperature Offset Correction (Calibration) Degrees	-15C to +15C	0
P5	Trigger Delay Start (Minutes)	0-10 Minutes	0
P6/P7	High Temperature Alarm (Degrees)	-50C to +110C	+100C
P8	Factory Reset (Can also press + and – for 5 seconds)	C/H	C

How to set the W1209’s Parameters

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, with P0 = “C”, hysteresis (P1) set to “2” and the setpoint to 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 at which the W1209 will operate. 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.

When the actual temperature reaches the upper limit temperature, the display will show ‘HHH’ to indicate an overtemperature condition and the relay will not energize. The relay will not re-energize until the temperature falls below this value.

Setting the lower limit of the thermostat parameter P3

This parameter limits the minimum trigger temperature at which the W1209 will operate. It can be used as a safety to stop an excessively low trigger temperature from accidentally being set by the user.

For example, if you are using the W1209 to control a cooler and you don’t want the temperature to drop below freezing, you might set P3 to 2C, to reduce the possibility of the temperature being set too low and freezing the items in the cooler.

When the actual temperature reaches the lower limit temperature, the display will show ‘LLL’ to indicate an under-temperature condition and the relay will not energize. The relay will not re-energize until the temperature rises above this value.

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. When the actual temperature reaches the alarm temperature, the display will 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 TROUBLESHOOTING

Below are common errors and problems encountered using the W1209 temperature controller, and their solutions:

The relay is not applying voltage to my heating or cooling device (fan, heater, TEC, lamp, etc.). The device is not turning on:

If the red LED is turning on:

• Your power supply or battery must be connected in series with your heating or cooling device through the relay contacts K0 and K1 on the W1209’s terminal strip. Note that the relay is not automatically connected to the W1209’s power, you must wire it through the K0 and K1 contacts. See “W1209 ELECTRICAL CONNECTION” above.
• When the LED turns on, can you hear the relay click? If yes, see the step above regarding the electrical connection. If no, the relay may have failed. Replace the W1209 or the relay.

If the red LED is NOT turning on:

• The setpoint criteria are not being met. For example, if you are in Heating mode, the actual temperature has not dropped below the setpoint, or in Cooling mode, the actual temperature has not increased above the setpoint.
• The hysteresis value is too large. Reduce the hysteresis – see Parameter P1 above.

The display shows “888” or “LLL”:

• The temperature sensor (NTC thermistor) is not connected. Plug an 10K NTC thermistor into the W1209’s connector.
• For “LLL” display: The actual temperature is below the lower limit setpoint. See Parameter P3 above.

The display shows “HHH” or “—“:

• The actual temperature is above the upper limit setpoint. See Parameter P2 above.
• The actual temperature is above the upper temperature alarm setpoint. See Parameters P6 and P7 above.

The W1209 turns on (energizes the relay) a couple of degrees above (or below) the setpoint that I entered. For example, the setpoint is 28C, but it’s turning on at 30C, not 28C:

• Depending upon the firmware version of your module, the hysteresis is either subtracted from, or added to, the temperature setpoint. For example, with P0 = “C”, hysteresis (P1) set to “2” and the setpoint to 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.  

The W1209 turns my heating or cooling device on and off too frequently:

• Increase the hysteresis. See Parameter P1 above.

The W1209 lets the temperature increase (or decrease) too much before it turns on my heating or cooling device:

• Decrease the hysteresis. See Parameter P1 above.

When I use a setpoint at a negative temperature, the hysteresis doesn’t work. For example:
Cooling Mode
Setpoint -1C
Hysteresis 2C:

• We have observed that the W1209 has a problem when the difference between the setpoint and the hysteresis, as shown in the example above, crosses the zero point. Here is a workaround that has worked for some of our customers:
  
  Parameter P4 lets you set an offset “calibration” temperature. So you can “trick” the W1209 by setting an offset temperature to make the setpoint above zero, while the actual temperature at which it is switching is below zero. For example, if you want to set it to an actual -1C with a 2 degree hysteresis, set the P4 offset to -6, then set the setpoint to +5C. With a 2 degree hysteresis, even though the setpoint is set to +5, because of the -6 degree calibration offset, it will switch at an actual -1C.

I can’t change the display from Celsius (C) to Fahrenheit (F):

• The W1209 is only available in Celsius (C). It can’t be switched to Fahrenheit (F).

OTHER W1209 SUPPORT & REFERENCE INFORMATION

W1209 Temperature Controller Tested on Chinese Electronic Products Tested blog (includes a W1209 circuit schematic): https://chinese-electronics-products-tested.blogspot.com/p/w1209-temperature-controller-tested.html

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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