Overview
The W1209 is a stand-alone digital thermostat module — give it 12V DC, plug in the included NTC thermistor probe, and you have a fully programmable temperature controller with a 7-segment LED display, three buttons, and a 20A relay output. Set the temperature setpoint, the hysteresis, and whether the relay turns on when the temperature rises above or falls below your target — and the module does the rest.
It's perfect for incubators (chicken egg hatching, yogurt-making, fermentation), reptile enclosures, mini greenhouses, refrigerator temperature monitoring, beer brewing, soldering pre-heater plates, and any project where you'd otherwise reach for an expensive PID controller. The waterproof probe lets you sense liquid temperatures or stick it inside a freezer.
No programming required — three buttons (SET, +, −) configure all parameters which are stored in EEPROM. Hold SET to enter the menu, scroll through P0 through P7 with + and −, then SET to edit each value. The relay can switch loads up to 240V AC at 10A or 20A on the relay datasheet.
At a Glance
Specifications
| Parameter | Value |
| Operating Voltage | 12V DC |
| Quiescent Current | ~25 mA (relay off) |
| Active Current | ~65 mA (relay on) |
| Temperature Range | -50°C to +110°C |
| Display Resolution | 0.1°C (between 0 and 100°C; 1°C outside) |
| Accuracy | ±1°C typical |
| Sensor | NTC 10kΩ (B=3950) on a 0.5m waterproof cable |
| Relay Type | SRD-12VDC-SL-C, SPDT, 250V AC 10A or 20A spec |
| Modes | Heating (C) or cooling (H) — software selectable |
| Hysteresis | 0.1°C - 15°C user-set |
| Memory | EEPROM (settings survive power-off) |
| Dimensions | ~48 × 40 × 14 mm |
Pinout Diagram
Wiring Guide
Power and Sensor Connections
Plug the NTC probe into the 2-pin sensor interface (the small JST connector). Connect 12V DC to the +12V/GND screw terminals. The display lights up immediately and shows the current temperature.
| Terminal | Connection |
|---|---|
| +12V | 12V DC power supply (+) |
| GND | 12V DC power supply (-) |
| NTC probe (JST) | Plug in the included probe |
| K0 / K1 | Relay common and switched contact (see Load tab) |
Load Wiring (the device you're controlling)
The relay is a dry-contact switch — it doesn't supply power, it just makes/breaks a circuit. Wire your load through the relay contacts in series with its power source. K0 and K1 are the relay's two switched terminals.
| Use Case | Wiring |
|---|---|
| 12V heating pad | +12V → K0, K1 → pad +, pad - → GND |
| 120/240V AC heater | Live wire through K0-K1 in series with heater (use proper enclosure!) |
| USB fan (cooling mode) | +5V from USB → K0, K1 → fan +, fan - → USB GND |
Programming the Setpoint
The W1209 has 8 menu items (P0-P7). Press and hold SET for ~3 seconds to enter the menu. Use + and − to navigate, SET to edit, + and − to change the value, SET again to save.
| Menu | Meaning |
|---|---|
| P0 | Mode: C (heating, relay ON when temp < setpoint) or H (cooling) |
| P1 | Hysteresis (deadband) - temp difference before relay switches back |
| P2 | Maximum allowed setpoint |
| P3 | Minimum allowed setpoint |
| P4 | Calibration offset (correct probe error) |
| P5 | Start delay (minutes before relay can turn on after power-up) |
| P6 | High-temp alarm threshold |
| P7 | Reserved/lock |
Code Examples
The W1209 is a stand-alone device — it doesn't need an Arduino or any code. Connect 12V power, plug in the probe, set your target temperature with the buttons, and wire your load through the relay. It runs autonomously.
Programming Example: 37°C Egg Incubator
1. Power up the W1209 with 12V DC.
2. Tap SET briefly. Display shows current setpoint.
3. Press + or − to set 37.5°C. Press SET to save.
4. Hold SET ~3 seconds to enter parameter menu.
5. + to scroll to P0. Press SET. Use + or − to choose "C" (cooling=OFF means heater stays on
below setpoint). Press SET.
6. + to scroll to P1. Press SET. Set hysteresis to 0.5°C. Press SET.
7. Hold SET 3 seconds to exit menu.
8. Wire heating pad through K0-K1.
9. Watch the green relay LED — it turns on when temperature drops below 37°C
and turns off when it rises above 37.5°C (37 + 0.5 hysteresis).