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How an RF smart plug can sweeten home energy savings by letting energy consumers see power consumption in context

With everything from refrigerators that help with grocery lists to secure doorbells that identify guests now connecting to the internet, energy consumption is soaring. However, solutions are coming online which make it possible to identify the energy usage of each device in relation to the whole and thus determine the most efficient timing and length of operation.

How would such a device work? It would need to be multipurpose, portable, and compact with the ability to measure and control electrical devices from any standard outlet in the home. It would need the ability to monitor energy consumption, turn appliances on or off, schedule loads for appliances such as clothes dryers and hot water heaters, and set alerts using easy smartphone apps.

Figure 1: One example of device that facilitates power consumption measurement and control is the STMicroelectronics RF Smart Plug.

Features necessary to successfully keeping an eye on power use and responding with actions that help maintain energy efficiency include:

  • A meter design with wireless connectivity
  • Bluetooth Low Energy (BLE) 4.1 connectivity for the control and metering panel as well as smartphone connectivity to enable the control of household appliances via an energy consumption dashboard
  • The ability to control the loads of some appliances, including AC Induction fan speed, heaters, and incandescent lamps
  • Scheduling
  • A Near Field Communication (NFC) interface so that storing the logs to configure the design is possible
  • Isolated USB interface for GUI and calibration: may be required during the development and calibration
  • Rated voltage: 240 VAC, Rated Current: 12A (TYP)
  • Power rating: Up to 2400W / 12 Amps
  •  Power consumption of plug: 0.7 Watts (Max.)

Controlling Consumption
One example of a reference design that encompasses the above features is STMicroelectronics’ RF Smart Plug for IoT home automation applications. It contains all the core functionality required for secure wireless communication, keeping energy consumption data in the user’s control.


Figure 2: The STPM32 metering IC tracks power, voltage, current, and power factor parameters using wireless connectivity over SUB-GHz or BLE.

The smart plug is designed with the STM32L443 Microcontroller (MCU), based on the Arm Cortex-M4 core. This low-power MCU features crystal-less USB, 100 DMIPS, and an operating voltage from 1.71 to 3.6V.

With wireless connectivity over SUB-GHz or BLE, the ability to measure energy parameters like power, voltage, current, and power factor is provided using the STPM32 metering IC (Figure 2). The MCU communicates with BLE using a Serial Peripheral Interface (SPI), with the help of the Triode Alternating Current Switch (TRIAC) to achieve load control. An isolated USB port and dual EEPROM (M24LR) allow communication with the Graphical User Interface (GUI) and Near Field Communication (NFC).

Two Modes
The RF Smart Plug has two modes. In the first mode, the device acts as a BLE peripheral device, which can be connected for control along with monitoring. In the second mode, the device is a manufacturer-specific beacon, which is connectable. It advertises its metering parameters so that multiple Android smartphone brands can monitor the plug, but only the specific bonded device will be able to connect. This application runs at 48 MHz for optimal performance, while power consumption is in between 40 to 50mA.

Figure 3 highlights all the sections of the RF Smart Plug, showing how form factor and reliability challenges are met. This makes for a small, portable device, usable in any outlet in the home.

Figure 3: The two RF Smart Plug modes include: Bluetooth Low Energy peripheral device; connectable manufacturer-specific beacon.

The non-isolated buck converter (Figure 4) is designed using STMicroelectronics’ VIPER06Xs. It is used with Pulsed Wave Modulation (PWM) operation at 30 kHz with frequency jittering for lower Electromagnetic Interference.

(EMI), with standby power less than 30 mW. This type of power supply suits low current applications that demand a small form factor.

Figure 4: Non-Isolated Buck converter

In our design, the output of supply is set at 4.5V. STMicroelectronics’ LDO LD3905 voltage regulator is used to power up all the analog and digital sections with output voltage and current at 3.3V and 500mA, respectively.

Current Control
A Triode Alternating Current Switch or TRIAC is a three-terminal component used to control the current. It gives AC switching for various electrical system applications. In addition, it can change the duty cycle of the AC voltage applied to the lights/load being controlled. Figure 5 shows the status of AC voltage at 50% duty cycle and use of a Zero Cross Detection (ZCD) for a dimming reference point to fire the TRIAC.


Figure 5: TRIAC at 50% duty cycle and ZCD

Figure 6: Functioning Principle

Figure 6 explains the GATT and GAP role, and Figure 7 describes the State Machine of the RF Smart Plug.

Figure 7: RF Smart Plug State Machine

Android Application
An Android application designed by STMicroelectronics can be used for the Turn On/Off of the Load, Scheduling, and Dimming features. The user can also view the metering parameters using the same application. The Android App can be downloaded by following the steps shown in Figures 8 and 9.


Figure 8: RF Smart Plug Android App

Figure 9: RF Smart Plug Android App Settings

This Smart Plug reference design is a system solution for energy management and saving with wireless connectivity over SUB-GHz or BLE. It provides the ability to measure energy parameters like power, voltage, current, and power factor using the STM32 metering IC. With all the core functionality required for secure communication, it provides a fast and flexible alternative to make homes smarter, money-wise, and safer.

Salil Jain is currently working as a Technical Leader with STMicroelectronics Pvt. Ltd., and has seven years of experience as a designer, architect, and project leader of electronics systems for multi-disciplinary domains (HW & SW). His main area of interests are BLE Mesh, Metering, and Healthcare. He is the coordinator of IEEE under Special Education Initiative and is also the STM32 Microcontroller training member under e4i (Electronics For India) initiative.

Alok Kumar Mittal is currently working as a Group Manager with STMicroelectronics Pvt. Ltd. and has more than 20 years of experience in Embedded Systems Design. He has worked on applications around USB, mass-storage, energy metering, and wireless communication. His main responsibility includes architecture design of system solutions and their development, including integration with software applications. He has six patents granted to his credit. Currently, his area of interest is the BLE (Bluetooth Low Energy) Mesh solution.


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