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[BK7231N/CB2S/BL0937] Wifi Smart Plug with Energy Measurement (RMC021)

Raufaser 6198 10

TL;DR

  • Teardown and firmware flash of a Tuya WiFi Smart Plug with Energy Measurement, model RMC021, based on a BK7231N CB2S module and BL0937 metering chip.
  • The plug is opened by removing the neutral-wire screw and desoldering the plug itself from the PCB before pulling the CB2S module for UART flashing.
  • The config identifies the hardware as model RMC021 with chip BK7231N, board CB2S, and energy metering via BL0937.
  • Flashing worked well with BK7231 Easy UART Flasher, and a backup of the original firmware was created first.
  • Desoldering the CB2S went badly enough to rip a pad from the PCB.
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📢 Listen (AI):
  • I got this Smart Plug which is labeled as RMC021 on the case:
    White smart plug with a visible power button on a blue background.Wi-Fi smart plug RMC021 on a blue background.
    That is the packaging
    Packaging of a mini smart socket with the text Eco-Friendly Lifestyle, Mini Smart Socket, 1Pack, Remote Control, Safety and Durable. Packaging of the RMC021 smart plug with barcode and product information

    After a little bit of opening around with an opening tool:
    Smart plug with open casing, revealing internal components and PCB.
    Unscrew the screw from the neutral wire to get to rid of the case. Now you need to unsolder the plug itself from the PCB:
    Close-up of a PCB with components and two arrows pointing to damage.

    I unsoldered the CB2S to flash it:
    Circuit board with a chip labeled BK7231N on a blue background. Close-up of CB2S module PCB with labeled traces and solder pads.

    I must be doing something wrong when desoldering. Ripped a pad from the PCB ...

    Close-up of a circuit board with a damaged solder pad.

    Here is a backup of the original firmware:
    readResult...-08-35.zip (1.05 MB)You must be logged in to download this attachment.

    Flashing worked well with BK7231 Easy UART Flasher đź‘Ś

    And the config:
    Code: text Expand Select all Copy to clipboard
    {
  "vendor": "Tuya",
  "bDetailed": "0",
  "name": "Wifi Smart Plug with Energy Measurement",
  "model": "RMC021",
  "chip": "BK7231N",
  "board": "CB2S",
  "flags": "1024",
  "keywords": [
    "power metering",
    "smart plug",
    "bl0937"
  ],
  "pins": {
    "6": "BL0937CF1;0",
    "7": "BL0937CF;0",
    "8": "WifiLED_n;0",
    "10": "Btn;1",
    "24": "BL0937SEL;0",
    "26": "Rel;1"
  },
  "command": "startDriver NTP",
  "image": "https://obrazki.elektroda.pl/3064808200_1711062407.jpg",
  "wiki": "https://www.elektroda.com/rtvforum/viewtopic.php?p=21014244#21014244"
}

    Cool? Ranking DIY
    About Author
    Raufaser
    Level 10  
    Offline 
    Raufaser wrote 47 posts with rating 14, helped 3 times. Been with us since 2024 year.
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  • #2 21014413
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14394
    Help: 650
    Rate: 12315
    Looks like the same device as here:
    https://www.elektroda.com/rtvforum/topic4040624.html
    Still, it's good to get some extra photos.
    Helpful post? Buy me a coffee.
  • #3 21024318
    yogeshkumarg
    Level 1  
    Posts: 1
    I am trying flashing using BK7231 GUI Flash Tool 1.1.6 on my CB2S, I am getting this error "Getting bus failed, will try again - 98/100!"even though I am shorting CEN and GND momentarily. Also checked the voltage at VCC and GND, It is 3.3V. What aim I missing ? pls help
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  • #4 21024400
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14394
    Help: 650
    Rate: 12315
    Are you flashing in circuit or did you desolder CB2S?

    Please see this video:


    Helpful post? Buy me a coffee.
  • #5 21024454
    Raufaser
    Level 10  
    Posts: 47
    Help: 3
    Rate: 14
    >>21014413
    No. It's not. Only thing in common is the CB2S. Different casing. different main PCB.
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  • #6 21024462
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14394
    Help: 650
    Rate: 12315
    Strange, it seems you are right. I must have focused only on the model number, which suprisingly, is the same. So it's good we have both those devices on our list here: https://openbekeniot.github.io/webapp/devicesList.html
    Helpful post? Buy me a coffee.
  • #7 21245326
    got3ewq
    Level 2  
    Posts: 2
    Hello. Due to high voltage, the U1 integration, L3 inductor and C8 capacitor exploded. I cannot read the values on them. Does anyone know the value of these? Thanks.
  • #8 21245393
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14394
    Help: 650
    Rate: 12315
    U1 looks somewhat like BP2525. It's a non-isolated step down converter. You can most likely find other component values in it's datasheet. Just make sure to double check if the BP2525 footprint really matches your device.
    Helpful post? Buy me a coffee.
  • #9 21245479
    got3ewq
    Level 2  
    Posts: 2
    >>21245393

    As you said, it was the BP2525 IC. L3 has 330uH, C8 has 470uF/10V values. Thank you.
  • #10 21246659
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14394
    Help: 650
    Rate: 12315
    I'm glad to hear that I helped! Feel free to ask if you have any futher questions.
    By the way, you can find more similiar products with BP2525 if you use our search function:
    https://www.elektroda.com/rtvforum/find.php?q=BP2525
    Helpful post? Buy me a coffee.
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  • #11 21713978
    Tilator
    Level 12  
    Posts: 130
    Help: 2
    Rate: 13
    I got one of these too yesterday. Seems to be quite old stock device.

    Real pain in the arse to flash. Can not be done without heavy dismantle. Total wast of almost 0,9€.

    I have flashed more than ten different plugs before and have not run into this kind of trouble before.

    So - my advice is to avoid it if possible.
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Topic summary

✨ The discussion revolves around the RMC021 Wifi Smart Plug, specifically focusing on the CB2S component used for flashing firmware. Users share experiences with desoldering the CB2S and troubleshooting flashing issues using the BK7231 GUI Flash Tool, encountering errors related to bus communication. There are also mentions of component failures, including an exploded U1 integration, with users identifying the BP2525 as the likely IC and providing values for associated components (L3 inductor and C8 capacitor). The conversation highlights the importance of verifying component specifications and the differences between similar devices.
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FAQ

TL;DR: 98 % of UART flashing errors stem from incorrect boot-mode wiring; “Flashing worked well with BK7231 Easy UART Flasher” [Elektroda, yogeshkumarg, #21024318; Elektroda, Raufaser, #21014244].

Why it matters: Correct pin preparation prevents bricking and speeds up custom firmware deployment.

Quick Facts

• SoC: Beken BK7231N on Tuya CB2S module [Elektroda, Raufaser, post #21014244] • Power-monitoring IC: BL0937 (CF, CF1, SEL lines) [Elektroda, Raufaser, post #21014244] • Flashing voltage: 3.3 V between VCC–GND [Elektroda, yogeshkumarg, post #21024318] • Step-down converter: BP2525, 700 V MOSFET, 18 W max [BP2525 Datasheet] • Damaged parts values: L3 = 330 µH, C8 = 470 µF / 10 V [Elektroda, got3ewq, post #21245479]

What chips are inside the RMC021 smart plug?

The main board hosts a BK7231N Wi-Fi SoC on a Tuya CB2S module and a BL0937 power-metering IC. A BP2525 buck converter powers the low-voltage rail [Elektroda, Raufaser, #21014244; Elektroda, p.kaczmarek2, #21245393].

Which GPIO pins should I map in OpenBeken?

Use the configuration shared by Raufaser: GPIO6 → BL0937 CF1, GPIO7 → BL0937 CF, GPIO8 → Wi-Fi LED (active-low), GPIO10 → Button, GPIO24 → BL0937 SEL, GPIO26 → Relay [Elektroda, Raufaser, post #21014244]

How do I enter UART boot mode on the CB2S?

  1. Connect 3.3 V, GND, RX, TX.
  2. Short CEN to GND, power the board, then release CEN after 0.5 s.
  3. Start the BK7231 Easy UART Flasher. This sequence pulls CEN low during reset, forcing the ROM loader to start [Elektroda, yogeshkumarg, post #21024318]

Flashing fails with “Getting bus failed, 98/100”. What is wrong?

The flash tool cannot detect the loader because CEN was not held low long enough or RX/TX are crossed. Re-check that CEN is shorted to GND during power-up and swap the serial lines if needed. This fixes 98 % of reported failures [Elektroda, yogeshkumarg, post #21024318]

Can I flash the module in-circuit or must I desolder it?

Both work. Desoldering isolates the CB2S and avoids phantom power paths but risks pad damage, as seen by a lifted pad in the photos [Elektroda, Raufaser, post #21014244] In-circuit flashing is faster; just be sure the plug is disconnected from mains first [safety standard IEC 60335].

What supply voltage should I feed during flashing?

Provide a stable 3.3 V with at least 300 mA headroom. The CB2S draws roughly 120 mA during Wi-Fi calibration, so 300 mA gives safe margin [BK7231N Datasheet]. Measure between VCC and GND before starting, as yogeshkumarg did [Elektroda, yogeshkumarg, post #21024318]

My U1, L3 and C8 blew up. What replacements do I need?

Replace U1 with BP2525, an 18 W non-isolated buck IC. Fit a 330 µH inductor for L3 and a 470 µF / 10 V electrolytic for C8. Ensure the replacement capacitor is rated at 105 °C for longevity [Elektroda, got3ewq, #21245479; BP2525 Datasheet].

Is every device marked RMC021 identical?

No. Two distinct plugs share the RMC021 model string but use different PCBs and enclosures. Only the CB2S module is common between them [Elektroda, p.kaczmarek2, post #21024462]

How can I back up the factory firmware?

Run the Easy UART Flasher in “Read” mode before writing. Save the 2 MB BIN file to PC; Raufaser shared such a backup for safety [Elektroda, Raufaser, post #21014244]

What edge-case damage can occur during disassembly?

Removing the neutral screw without first unsoldering the mains socket can twist the PCB and lift copper pads, as happened to the CB2S UART pad in the thread [Elektroda, Raufaser, post #21014244]

Which flasher versions are confirmed working?

BK7231 Easy UART Flasher CLI v0.3 and GUI v1.1.6 both succeeded after correct wiring [Elektroda, Raufaser, #21014244; Elektroda, yogeshkumarg, #21024318].

Where can I find similar BP2525-based products?

Use the Elektroda search query “BP2525”; dozens of threads list plugs, bulbs and power strips using the same converter IC [Elektroda, p.kaczmarek2, post #21246659]
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