Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.

A method for identifying a mechanical impedance of an electromagnetic load may include generating a waveform signal for driving an electromagnetic load, the waveform signal comprising a first tone at a first driving frequency and a second tone at a second driving frequency. The method may also include during driving of the electromagnetic load by the waveform signal or a signal derived therefrom, receiving a current signal representative of a current associated with the electromagnetic load and a back electromotive force signal representative of a back electromotive force associated with the electromagnetic load. The method may further include determining amplitude and phase information of the current signal responsive to the first tone and second tone, determining amplitude and phase information of the back electromotive force signal responsive to the first tone and second tone, and identifying parameters of the mechanical impedance of the electromagnetic load based on the amplitude and phase information of the current signal and the amplitude and phase information of the back electromotive force signal.

Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.

A calling device of a MRI system may include a reader/writer and a call maker. The reader/writer can send a fixed-frequency electromagnetic wave. The call maker may include a main body, an antenna, an operating member, and an answering controller. The antenna can generate an induced current after receiving the fixed-frequency electromagnetic wave. The operating member can switch between first and second positions relative to the main body. The answering controller is configured such that: when the operating member is in the first position, a first radio-frequency signal is sent through the antenna with energy obtained from the induced current generated by the antenna, and when the operating member is in the second position, a second radio-frequency signal is sent through the antenna with energy obtained from the induced current generated by the antenna. The reader/writer can send different prompt signals according to the first and second radio-frequency signals.

Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.

A calling device of a MRI system may include a reader/writer and a call maker. The reader/writer can send a fixed-frequency electromagnetic wave. The call maker may include a main body, an antenna, an operating member, and an answering controller. The antenna can generate an induced current after receiving the fixed-frequency electromagnetic wave. The operating member can switch between first and second positions relative to the main body. The answering controller is configured such that: when the operating member is in the first position, a first radio-frequency signal is sent through the antenna with energy obtained from the induced current generated by the antenna, and when the operating member is in the second position, a second radio-frequency signal is sent through the antenna with energy obtained from the induced current generated by the antenna. The reader/writer can send different prompt signals according to the first and second radio-frequency signals.

Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.

Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.

A service request system comprises: a call bell device transmitting service request information in response to a customer's input; a first gateway device receiving the service request information through a local area communication; a service server connected to the first gateway device over an Internet and, in an online status, receiving the service request information from the first gateway device through the Internet; a staff terminal connected to the service server over the Internet and receiving the service request information from the service server through the Internet; a local server connected to the first gateway device and the staff terminal over the local area communication, monitoring availability of the Internet and, in an offline state, receiving the service request information of the call bell device from the first gateway device and transmitting the service request information of the call bell device to the staff terminal through the local area communication.

Doorbells can send data to each other to enable a first doorbell user to warn a second doorbell user regarding a suspicious visitor. A first user can indicate a first trait of a visitor via a first remote computing device. The first user can create a user group to enable the members of the user group to use their doorbells to take pictures of suspicious visitors and to send the pictures of the suspicious visitors to other members of the user group.