FAQ
TL;DR: Relay handles 20 A @ 14 VDC [Elektroda, TechEkspert, post #17177090]; "works fine" when driving a CO pump [Elektroda, AndyNW, post #17198520] Fans love the W1209 for its −50 °C…+110 °C span and ₹300-level price. Why it matters: cheap, hackable control beats mechanical thermostats.
Quick Facts
• Temp span: −50 °C to +110 °C, 0.1 °C step [Elektroda, TechEkspert, post #17177090]
• Hysteresis adjustable 0.1–15 °C via P1 [Elektroda, TechEkspert, post #17177090]
• Supply: 12 V DC, 16 mA idle / 65 mA relay on [Elektroda, TechEkspert, post #17177090]
• Relay contacts: 20 A @ 14 VDC or 20 A @ 125 VAC (expect ≈5 A @ 240 VAC) [Elektroda, TechEkspert, #17177090; Elektroda, vodiczka, #17190393]
• Board MCU: N76E003AT20 or STM8S003F3P6, both re-flashable [Elektroda, TechEkspert, #17179031; miszczo997, #17316242]
1. What temperature range can the W1209 measure and control?
The on-board firmware accepts set-points from −50 °C to +110 °C. The same limits apply to displayed readings and to the P2 (upper) and P3 (lower) safety boundaries [Elektroda, TechEkspert, post #17177090]
2. How are negative temperatures displayed?
The display shows a leading minus sign; resolution drops to 1 °C for two-digit negatives (e.g., −18, −19) [Elektroda, TechEkspert, post #17179031]
3. Is the supplied NTC probe waterproof?
Yes, the NTC is potted in a metal sleeve, but corrosion in aquaria has been reported after 6 months. Slide on heat-shrink or coat with two-part epoxy to seal it fully [Elektroda, pierotez, post #17475828]
4. Can the relay safely switch 230 V AC mains loads?
The PCB relay is rated 20 A @ 125 VAC; users confirm it survives 240 V AC but at lower current (~5 A) [Elektroda, vodiczka, post #17190393] For higher power, drive an external contactor or SSR instead.
5. How accurate is the thermometer and how do I calibrate it?
Uncorrected error is typically ±1 °C from 10–30 °C, worsening beyond 30 °C [Elektroda, vodiczka, post #17187842] Use P4 to add −7 °C…+7 °C offset: 1) Measure with a reference, 2) Hold SET 5 s, pick P4, 3) Adjust until readings match [Elektroda, TechEkspert, post #17208700]
6. What do parameters P0–P6 actually change?
P0: Heat (H) or Cool (C)
P1: Hysteresis 0.1–15 °C
P2/P3: Max/Min set-point limits
P4: Sensor offset −7…+7 °C
P5: Relay delay 0–10 s
P6: Over-temp alarm OFF or 0…110 °C [Elektroda, Freddy, post #17210353]
7. How do I disable the P6 alarm that kills cooling at 110 °C?
Enter menu (hold SET 5 s), scroll to P6, set value to OFF. The relay then ignores the temperature ceiling [Elektroda, TechEkspert, post #17181491]
8. Can I replace the probe with a 3-wire DS18B20 sensor?
No. The firmware expects a 10 kΩ NTC. A DS18B20 will read 3× too high even with two wires connected [Elektroda, Fisqss, #20846901; #20847307]. Swap in any waterproof 10 kΩ NTC instead [Elektroda, vodiczka, post #20847443]
9. How reliable is the board?
Field data: about 20 % of new boards have relay contact issues and occasional MCU failure has been logged (pin 6 shorted) [Elektroda, Kkaz, #21156208; sergiuszf, #18022978]. Quote: "very unreliable thermostat" [Elektroda, Kkaz, post #21156208]
10. Can I re-flash the firmware?
Yes. STM8 versions program via ST-Link V2 and open-source builds like TG9541/W1209 [Elektroda, miszczo997, post #17316277] N76E003 versions need a Nu-Link programmer and full rewrite [Elektroda, TechEkspert, post #17316230]
11. How do I quickly set a target temperature with hysteresis?
- Tap SET; digits blink. 2. Use +/− to choose target, tap SET to save. 3. Hold SET 5 s, pick P1, set hysteresis, save. The relay now toggles at target±hysteresis/2 [Elektroda, TechEkspert, post #17177090]
12. What are common applications shared by users?
Users drive CO pumps, DHW heaters, fireplace blowers, aquarium heaters, PCB etching baths, and lawn-mower acid batteries [Elektroda, multiple posts #17178161, #17183039, #18210292, #17265473]. A 65 mA coil current lets battery systems run efficiently [Elektroda, TechEkspert, post #17177090]
