FAQ
TL;DR: SM2135 drives 5 LED channels at up to 20 mA each; “We already know how to set the colors” [Elektroda, p.kaczmarek2, post #20106071] One I2C-like CLK/DAT pair sends 7 bytes in 450 µs. Why it matters: Knowing the pin map and current bytes lets you retrofit OpenBeken, Tasmota or ESPHome in minutes.
Quick Facts
• Default current limit: 0x02 = 20 mA per channel [Elektroda, p.kaczmarek2, post #20106071]
• Address byte: fixed 0xC0; multiple chips on one bus unsupported [Elektroda, p.kaczmarek2, post #20106071]
• Command frame: 1 addr + 1 mode + 5 data bytes = 7 bytes + 7 ACKs [Elektroda, p.kaczmarek2, post #20106071]
• Typical standby consumption with OBK: ~0.5 W vs <0.1 W stock firmware [Elektroda, rawilson, post #20108336]
• BK7231T module WBLC9 exposes P7, P8, P9, P16, P24, P26 pins for SM2135 CLK/DAT selection [Elektroda, p.kaczmarek2, post #20315739]
Which two BK7231 pins are usually wired to SM2135 CLK and DAT?
Most GU10/E27 bulbs route SM2135 CLK to P26 and DAT to P24, but LVWIT/WBLC9 variants may swap them; confirm with a multimeter before flashing [Elektroda, p.kaczmarek2, post #20315739]
Why do cold-white and warm-white channels look dimmer after enabling the CW flag?
Enabling “Use separate RGB/CW modes” halves the byte bandwidth per update, so OBK initially sends 0x02 (20 mA) to both RGB and CW. Raise CW current with SM2135_Current, e.g. SM2135_Current 0x02 0x07 to push CW to 45 mA [Elektroda, skrc1, post #20246829]
How do I change the maximum LED current in OpenBeken?
- Go to Options → Short startup command. 2. Add
SM2135_Current RGBhex CWhex (hex 0–7). 3. Reboot. Example: SM2135_Current 0x02 0x07 sets RGB 20 mA, CW 45 mA [Elektroda, p.kaczmarek2, post #20246171]
Can I store the current settings permanently?
Firmware ≥1.14.37 keeps the last SM2135_Current values in flash unless CFG_NOAUTOSAVE is set [Elektroda, p.kaczmarek2, post #20246867]
Does SM2135 support hardware I²C?
No. Its timing deviates from the I²C spec, so Tasmota, ESPHome and OBK use bit-bang GPIO (digitalWrite/digitalRead) for reliability [Elektroda, p.kaczmarek2, post #20106326]
Is it safe to connect several SM2135 chips on the same bus?
Risky. The address byte is fixed at 0xC0, so a second chip would mirror commands, causing unpredictable colours [Elektroda, p.kaczmarek2, post #20106071]
Home Assistant doesn’t auto-discover my bulb—what now?
Add the MQTT light block manually. Use topics cmnd/<device>/led_basecolor_rgb and .../led_enableAll as shown by OBK’s console [Elektroda, hode0055, post #20145727]
How do I get smooth fades like PWM dimming?
Enable flag “Use LED lerp” and, if needed, set led_lerpSpeed <value>. A speed of 255 transitions 0→255 in one second [Elektroda, p.kaczmarek2, post #20299385]
I measure 0 Ω between CLK and DAT pads—is my board dead?
Normally there is no DC short. If continuity persists after desoldering wires, the SM2135 or PCB is damaged [Elektroda, p.kaczmarek2, post #20519757]
What’s the standby power when flashing OpenBeken?
Users report ~0.5 W idle with OBK versus <0.1 W on Tuya stock firmware because the BK7231 Wi-Fi radio stays awake [Elektroda, rawilson, post #20108336]
Expert quote on protocol timing?
“The device generates an ACK every nine clock cycles—data changes only when CLK is low.” [Elektroda, p.kaczmarek2, post #20106071]
Edge case: what if ACK never arrives?
If SM2135_WriteByte returns NAK three times, reset CLK & DAT lines high; a hung bus blocks further writes until power-cycle [Tasmota source].
