## 4 Smart Home Control System Prototype and Performance Evaluation  
### Prototype of VLC Lightbulb  
The detailed layout design of the PCB for the proposed AC-powered VLC lightbul is presented in Figure 6. Figure 6a presents the layout design of the power management board and LED string, which consists of the AC-DC converter circuit and 24 LEDs connected in series. The diameter of this PCB board is 9 cm for fitting the size of a standard lightbulb enclosure. The layout design of the BLE control board, which comprises the DC-DC converter circuit and BLE control circuit, is shown in Figure 6b. The size of this PCB board is 3 \(\times\) 5 cm, and this compact design targets the removal of the traditional large external modulator for practical applications. Moreover, heat sinks are applied to the bottom of LDO1 and the power MOSFET for heat emission. As for the manufacturing cost, this VLC lightbulb prototype is cost-effective, and the cost can be reduced in large-scale manufacturing since the prices of adopted components and PCB boards decrease dramatically with large-scale order numbers. This relatively low cost of the VLC lightbulb also guarantees its possibility for real-world applications. The proposed high-power VLC lightbulb is fabricated and retrofitted in a commercial 9 cm diameter AC lightbul enclosure with a standard E27 socket for practical implementation, as shown in Figure 7. The AC-DC converter circuit and LED string are combined on one PCB board while the BLE control board is affixed on its back side. Both PCBs are enclosed in the lightbulb shell. The measured results of this designed VLC lightbulb are summarized in Table 3. To support the operation of 24 LEDs connected in series, the designed total operating voltage for the LEDs is 72 V. As presented in Figure 4, the simulated operating voltage of this VLC lightbulb presented is 74 V. The measured actual operating voltage of LEDs is 74.2 V, which is close to the design target and consistent with the measured result. This voltage is high enough to maintain the LEDs working in the suitable region. Another important factor for estimating the working states of this VLC lightbulb is the driving current of LEDs. According to the datasheet, the driving current of a single LED under the operating voltage of 3 V is 130 mA. In the simulation, the average driving current of this LED series is 130 mA. The measured current through the LED string is 139 mA, which  
Figure 6: Details of PCB for the designed VLC LED lightbulb: (**a**) layout of LED and power management board; (**b**) layout of BLE control board. is also consistent with the simulation result. As for the total electric power that this VLC consumed, our design target is 10 W for guaranteeing the indoor illumination requirements as the illumination is proportional to the electric power. The measured electric power is 10.3 W and meets the design target. With this electric power, the measured illuminance at a distance of 1.5 m is 362.5 lx, which is higher than the illumination requirement of 300 lx in the domestic environment. The typical ceiling height in residential buildings is around 3 m and requires more than 300 lx for illumination. To meet these real-life application requirements, we studied the relationships between the diffuser diameter and VLC link distance as well as illuminance and distance. The setup for measuring the VLC link distance is shown in Figure 8, and the corresponding measured results are presented in Figure 9. It can be noticed that the VLC link distance is proportional to the diffuser diameter. Thus, a 20 cm lamp housing is applied outside of the lightbul to reach up to a 3.4 m VLC link distance. The measured illuminance at 1.5 m from the lightbul is 362.5 lx, which also meets the requirement for residential lighting.