# Design and Implementation of a Smart Intercom System through Web Services on Web of Things
## 3 Hardware Architecture  
This system uses an embedded computer built on a single circuit board. This is a complete computer built on a single circuit board with a microprocessor, memory, input/output (I/O), and other functions; it is the main working platform for the intercom. The hardware of this system is built into the embedded system, including an MCU (microcontroller unit), a wireless module, an SD card (secure digital card) slot, a touch panel, a camera, a relay, an electronic lock, a buzzer, and a speaker. The use of microprocessors can be used to open doors and activate buzzers to sound warnings. However, to make it easier for residents to communicate with visitors, the system uses an instant messaging platform as a bridge. The hardware architecture is shown in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Type** & **Deploy** & **Deploy** & **Deployed** & **Web** & **Computer** & **Internet** & **Embedded** \\  & **Apps** & **VoIP** & **Chatbot** & **Integration** & **Vision** & **Access** & **System** \\ \hline VoIP & V & V & - & - & - & V & V \\ intercom [11, 12] & & & & & & & \\ Mobile & V & - & - & - & - & V & - \\ intercom [15] & & & & & & & \\ Smart & V & - & V & V & V & V & V \\ intercom & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Intercom use technology list.  
Although the stability of a wired network connection is higher than that of a wireless network, considering that the intercom is usually installed outside the apartment or in the lobby of a building, there may not be a wired network interface. The wireless transmission module of the embedded system is used in the intercom, and the system can be restarted remotely in case of errors.  
As the content of the standby screen displayed on the touch panel is similar to that of a traditional intercom, visitors can easily use the system without training. Once the visitor presses the floor number on the touch panel, the system will send system information to the resident through the MCU of the embedded system according to the resident ID set by the system. The resident receives messages and photos of the visitor through the instant messaging software of the smartphone, which will decide whether to open the door, activate an alarm, or send a picture or text messages after confirmation by the resident. The visitor's photo will be sent to the cloud through the wireless transmission module and forwarded to the resident. When a visitor's photo is received, it can be clearly identified, and the resident who wants to open the door can do so by clicking \"Open Door\" on the menu of the instant messaging software. If the resident cannot be clearly identified, he or she can click the \"Watch\" button, and the system will take the photo back from the camera on the intercom.  
The traditional intercom system does not allow the resident to know that a visitor is coming to the house when the resident is not home. In this system, if the resident does not respond immediately, they can still use the instant messaging software of the smartphone to know who has visited. This system not only reduces the cost of handsets and wiring indoors and outdoors but also reduces the cost of maintenance for future line failures.  
This system uses a cathode lock, which requires an external power supply and locks the door without power. However, the power supply of the embedded system is not enough to drive the cathode lock; so, it needs to rely on the small current output of the embedded system to control the switch with a higher current. When the relay is energized, it can open the lock body to open the door, thereby controlling the opening and closing of the door. If a resident finds an outsider, then he or she can press the \"Alarm\" button in the instant messaging software, which will immediately activate the buzzer to emit an alarm sound to draw the attention of neighbors and make the outsider leave.