Overview

The BL0940 is a single-phase electricity metering IC manufactured by Shanghai Belling. Embedded inside Tuya, BlitzWolf, and Sonoff smart plugs and power sockets, it measures RMS voltage, RMS current, active power (W), fast power, and cumulative energy consumption over a 4,800 baud TTL UART interface.

Equipped with dual 16-bit Sigma-Delta ADCs, an internal high-precision temperature compensated voltage reference, and digital signal processing (DSP) logic, the BL0940 delivers Class 0.5 measurement accuracy across a 2000:1 dynamic range. It is natively supported by open-source firmware projects including Tasmota, ESPHome, and OpenBK7231T.

Quick reference

Operating voltage (VDD)3.0 V to 3.6 V DC (3.3 V nominal)
InterfaceTTL UART (4,800 bps, 8-N-1)
Fixed UART Baud Rate4,800 bps
Measurement parametersVRMSV_{RMS}, IRMSI_{RMS}, Active Power (PP), Cumulative Energy (EE), Temperature
ADC architectureDual 16-bit Sigma-Delta ADCs
Measurement accuracyClass 0.5 (±0.5%\pm 0.5\%)
Operating current3.5 mA3.5\text{ mA} active

Pinout (10-Pin SSOP Package)

text
             ┌───┴───┐
         VDD ─┤ 1   10├─ GND
          IP ─┤ 2    9├─ RX / Mode
          IN ─┤ 3    8├─ TX
          VP ─┤ 4    7├─ CF / OverCurrent
         VN  ─┤ 5    6├─ NC / Test
             └───────┘
PinNameTypeDescription
1VDDPowerCore supply (+3.3 V DC)
2IPAnalog InputPositive current sense input (from shunt resistor)
3INAnalog InputNegative current sense input
4VPAnalog InputPositive voltage sense input (from resistor divider)
5VNAnalog InputNegative voltage sense input
7CFDigital OutputPulse frequency output / over-current alarm
8TXDigital OutputUART Transmit output pin (4,800 bps)
9RXDigital InputUART Receive input pin
10GNDPowerGround reference (0 V)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply VoltageVDDV_{DD}3.03.33.6VDC
Supply CurrentIDDI_{DD}—3.55.0mAVDD=3.3 VV_{DD} = 3.3\text{ V}
Current Channel Input RangeVIP−INV_{IP-IN}-50—+50mV PeakDifferential shunt voltage
Voltage Channel Input RangeVVP−VNV_{VP-VN}-200—+200mV PeakDifferential bus voltage
Measurement AccuracyEmeterE_{meter}-0.5%±0.1%\pm 0.1\%+0.5%—Active power range 2000:1
Internal Voltage RefVREFV_{REF}1.181.2181.25VLow temperature coefficient

UART Frame Protocol

The BL0940 streams 22-byte serial telemetry frames continuously or on request over 4,800 baud UART:

  • Header byte: 0x55 (Frame header).
  • Registers read: IRMSI_{RMS} (3 bytes), VRMSV_{RMS} (3 bytes), Active Power PP (3 bytes), Energy Accumulator EE (3 bytes), Status (3 bytes), Checksum (1 byte).

VRMS=Raw VRMS RegisterKvVoltsV_{RMS} = \frac{\text{Raw } V_{RMS} \text{ Register}}{K_v} \quad \text{Volts}

IRMS=Raw IRMS RegisterKiAmperesI_{RMS} = \frac{\text{Raw } I_{RMS} \text{ Register}}{K_i} \quad \text{Amperes}

Pactive=Raw P RegisterKpWattsP_{active} = \frac{\text{Raw } P \text{ Register}}{K_p} \quad \text{Watts}

Wiring

Inside a smart plug, the BL0940 connects directly to an ESP8266, ESP32, or BK7231 Wi-Fi microcontroller:

BL0940 Pin→MCU PinsNotes
VDD3.3VLogic power
GNDGNDCommon ground
TXMCU RX Pin (4800 baud)Serial data from BL0940
RXMCU TX Pin (4800 baud)Serial data to BL0940
Warning

High Voltage Safety Warning:

  • In commercial smart plugs, the BL0940 GND pin is connected directly to Neutral AC mains. Connecting a serial programmer to the UART pins while plugged into wall power will destroy computer USB ports and poses a lethal electric shock hazard. Always use an isolated USB serial adapter or program via OTA.

Example (ESPHome Configuration)

yaml
sensor:
  - platform: bl0940
    tx_pin: GPIO1
    rx_pin: GPIO3
    voltage:
      name: "Smart Plug Voltage"
    current:
      name: "Smart Plug Current"
    power:
      name: "Smart Plug Power"
    energy:
      name: "Smart Plug Energy"

Common mistakes

  • Setting incorrect UART baud rate: BL0940 operates strictly at 4,800 baud. Setting 9,600 or 115,200 baud results in unreadable garbage data.
  • Forgetting calibration constants (Kv,Ki,KpK_v, K_i, K_p): PCB trace layouts and resistor tolerances vary between smart plug manufacturers. Use a resistive load (such as a 60W incandescent bulb) to calibrate voltage, current, and power multipliers in Tasmota/ESPHome.

Notes

  • BL0940 vs CSE7766 vs HLW8012: BL0940 uses 4,800 baud UART; HLW8012 uses pulse frequencies; CSE7766 uses 4,800 baud UART with different packet headers.

Assets & downloads