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

Raufaser 6357 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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  • 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 👌

    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 16, helped 3 times. Been with us since 2024 year.
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  • #2 21014413
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14589
    Help: 654
    Rate: 12610
    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.
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  • #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
  • #4 21024400
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14589
    Help: 654
    Rate: 12610
    Are you flashing in circuit or did you desolder CB2S?

    Please see this video:


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  • #5 21024454
    Raufaser
    Level 10  
    Posts: 47
    Help: 3
    Rate: 16
    >>21014413
    No. It's not. Only thing in common is the CB2S. Different casing. different main PCB.
  • #6 21024462
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14589
    Help: 654
    Rate: 12610
    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: 14589
    Help: 654
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    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.
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  • #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.
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  • #10 21246659
    p.kaczmarek2
    Moderator Smart Home  
    Posts: 14589
    Help: 654
    Rate: 12610
    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.
  • #11 21713978
    Tilator
    Level 12  
    Posts: 131
    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: If you have this RMC021 plug, 3.3V on VCC is not enough by itself, and “avoid it if possible” is fair advice for beginners. This FAQ helps OpenBeken users open, back up, repair, and flash the BK7231N/CB2S version with BL0937 metering while avoiding pad damage and bus-detection failures. [#21713978]

Why it matters: This thread identifies the exact RMC021 hardware, pin map, repair parts, and failure points, so you can decide early whether the plug is worth reflashing or repairing.

Alternative What the thread reports Best use
BK7231 Easy UART Flasher Flashing “worked well” after removing the CB2S Clean off-board flashing
BK7231 GUI Flash Tool 1.1.6 One user hit “Getting bus failed… 98/100!” on CB2S Troubleshooting or recovery attempts
In-circuit flashing Specifically questioned as a possible cause of failure Only if the board does not interfere
Desoldered module Required heavy dismantling, but avoids board-side interference Most reliable on this plug

Key insight: The hardest part is not software. This RMC021 is difficult because the CB2S sits in a plug that needs major disassembly, and rough desoldering can rip PCB pads off.

Quick Facts

  • The confirmed OpenBeken map uses BK7231N on a CB2S board, with BL0937 on GPIO6, GPIO7, and GPIO24, relay on GPIO26, button on GPIO10, and Wi-Fi LED on GPIO8. [#21014244]
  • A failed flash attempt showed 3.3V between VCC and GND, but the tool still looped at “Getting bus failed, will try again - 98/100!”, which points to more than just supply voltage. [#21024318]
  • The mains power section was identified as a BP2525-based non-isolated step-down stage, and one repaired unit used L3 = 330uH and C8 = 470uF / 10V. [#21245479]
  • Long-term usability matters: one later buyer called the plug “a total wast of almost 0,9€” because flashing required heavy dismantling compared with more than 10 other plugs they had reflashed. [#21713978]

How do you open the RMC021 WiFi smart plug and remove the PCB without damaging the case or mains connections?

Open the case carefully with a thin opening tool, then remove the neutral-wire screw before trying to free the housing. After that, desolder the plug pins from the PCB so the board can come out cleanly. The thread shows that forcing the board before removing those mains connections risks mechanical damage during teardown. [#21014244]

What is the correct way to desolder a CB2S module from the RMC021 smart plug for flashing without ripping pads off the PCB?

Desolder the CB2S only after the main PCB is fully accessible and support the module while heating all joints evenly. A safe sequence is: 1. remove the PCB from the case, 2. free the plug hardware from the board, 3. lift the CB2S only when solder is fully molten. One teardown still ripped a pad, so this plug gives very little margin for rough removal. [#21014244]

Why does BK7231 GUI Flash Tool 1.1.6 show the error "Getting bus failed, will try again - 98/100!" on a CB2S even when VCC is 3.3V and CEN is briefly shorted to GND?

Because 3.3V power and a brief CEN-to-GND reset do not guarantee that the CB2S is actually reachable on the bus. In this thread, the failure appeared at 98/100 even with 3.3V present, and the immediate follow-up question was whether flashing was being attempted in circuit. That strongly suggests board-side connections can block communication even when supply voltage looks correct. [#21024400]

When flashing a CB2S, what differences should I expect between in-circuit flashing and desoldering the module first?

Expect in-circuit flashing to be less predictable on this plug and desoldered flashing to be cleaner but more labor-intensive. The thread directly questions in-circuit flashing as a cause of bus errors, while a removed CB2S flashed successfully with BK7231 Easy UART Flasher. The trade-off is clear: less disassembly means more interference risk, but full removal means harder rework and possible pad damage. [#21024400]

Which pin configuration works for the RMC021 BK7231N/CB2S smart plug with BL0937 energy measurement in OpenBeken?

The posted OpenBeken configuration maps GPIO6 to BL0937CF1, GPIO7 to BL0937CF, GPIO24 to BL0937SEL, GPIO26 to Rel, GPIO10 to Btn, and GPIO8 to WifiLED_n. It also identifies the device as model RMC021 with chip BK7231N and board CB2S. Those six pin assignments are the working map shared for this exact plug. [#21014244]

What is BL0937 and how does it measure power, voltage, and current in a smart plug like the RMC021?

"BL0937 is an energy-measurement chip that handles power metering in this smart plug, using dedicated CF, CF1, and SEL signal lines to expose measurement data to the BK7231N controller." In this RMC021, the OpenBeken config ties those lines to GPIO6, GPIO7, and GPIO24, which is why BL0937 is the key part behind the plug’s energy-measurement feature. [#21014244]

What is the CB2S module and how is it related to the BK7231N chip in Tuya-based smart plugs?

"CB2S is a plug-in Wi-Fi module board that carries the BK7231N chip, letting the smart plug separate its radio-controller section from the main power PCB." In this thread, the device config explicitly lists chip = BK7231N and board = CB2S, and users discuss removing that CB2S module for direct flashing. [#21014244]

How do I back up the original firmware from a BK7231N CB2S smart plug before flashing OpenBeken?

Back up the firmware before writing anything else, because this exact RMC021 thread includes an original-firmware backup file from the CB2S module. The practical workflow is simple: remove the CB2S, connect it to the flasher setup, read out the contents, and save the dump first. That backup gives you a recovery path if the new flash fails or pin mapping needs rework. [#21014244]

What steps are needed to flash this RMC021 plug with BK7231 Easy UART Flasher after removing the CB2S module?

Use the off-board CB2S module, not the fully assembled plug. The shortest path is: 1. remove the PCB and desolder the CB2S, 2. connect the module to BK7231 Easy UART Flasher, 3. flash and then apply the posted OpenBeken pin config. The user who documented this exact plug states that flashing worked well with BK7231 Easy UART Flasher after module removal. [#21014244]

In what ways is this RMC021 device different from the other smart plug thread that uses the same model number but a different PCB and casing?

It is not the same device internally, even though the model number overlaps. The thread explicitly states that the only common part is the CB2S, while the casing and main PCB are different. That means you should not assume shared photos, teardown steps, or pin-compatible board details just because both plugs are labeled RMC021. [#21024454]

BK7231 Easy UART Flasher vs BK7231 GUI Flash Tool 1.1.6 — which is better for recovering or flashing a CB2S module?

For this plug, BK7231 Easy UART Flasher has the better reported outcome. One user says it flashed the desoldered CB2S successfully, while another using BK7231 GUI Flash Tool 1.1.6 reported repeated “Getting bus failed” errors up to 98/100. That does not prove the GUI tool never works, but this thread gives the Easy UART Flasher the only confirmed success case. [#21014244]

How can I identify the U1 power-supply IC in this plug when the marking is damaged, and what clues point to BP2525?

Identify U1 by matching its role and footprint to a non-isolated mains step-down controller, because that is the clue used here. The repair discussion says U1 “looks somewhat like BP2525” and specifically describes BP2525 as a non-isolated step-down converter. That combination of function and package match was enough for the later repair to confirm the BP2525 identification. [#21245393]

What replacement values should be used for the exploded U1, L3 inductor, and C8 capacitor in the RMC021 smart plug power supply?

Use BP2525 for U1, 330uH for L3, and 470uF / 10V for C8. Those exact values were confirmed by the user who repaired the blown power section after the original parts exploded under high voltage. This is the most concrete component-level repair data in the thread for the RMC021 supply stage. [#21245479]

What safety precautions should I follow when repairing a non-isolated BP2525-based mains step-down section in a smart plug?

Treat the section as live mains circuitry and verify the IC footprint before installing parts. The thread explicitly calls BP2525 a non-isolated step-down converter and advises double-checking that the BP2525 footprint really matches the device. That means you should not assume low-voltage isolation anywhere in that supply stage during diagnosis or repair. [#21245393]

Why do some users consider the RMC021 a difficult smart plug to reflash, and what alternative plugs are easier to work with?

Users call it difficult because it needs heavy dismantling, the CB2S is awkward to remove, and pad damage can happen during desoldering. One buyer said they had flashed more than 10 different plugs before, yet still found this one unusually troublesome and “avoid it if possible” was their conclusion. The thread does not name an easier specific alternative, only that many other plugs were easier than this RMC021. [#21713978]
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