How Intercom Systems Work: Step-by-Step Guide to Secure Access

Intercom systems are communication and access control solutions that let you see who is at your door, talk to visitors, and remotely unlock the entrance. Understanding how an intercom works reveals a straightforward sequence: a visitor dials an apartment number, the system initiates a call to the resident, voice communication is established, and after the resident presses the unlock button, the electromagnetic lock is temporarily released.

This familiar process involves several devices working together. The call panel receives the visitor's commands, the controller manages the system, the handset or monitor ensures communication with the apartment, and the electromagnetic lock keeps the door securely closed. Additional components include a power supply, communication lines, and a door closer to return the door to its original position.

Let's break down the intercom system step by step-from pressing the call button to the moment the electromagnetic lock releases and the visitor can enter.

What Is an Intercom and What Does the System Consist Of?

An intercom is a set of devices that connect visitors and residents while managing the entrance door. The intercom system is more than just the keypad panel at the entrance; that panel is only one part of a comprehensive system.

Outside, a call panel is installed. It features a keypad or apartment buttons, microphone, speaker, and in video intercoms, a camera. Visitors use this panel to enter the apartment number and communicate with the resident.

The intercom controller processes commands. It identifies which apartment to call, sends the signal to the correct internal device, and receives the unlock request. Depending on the system, the controller may be inside the call panel or installed separately.

Inside the apartment, there is usually a handset or monitor. This device alerts the resident to a call, enables conversation, and allows the resident to press the unlock button. Modern systems may use touch panels or mobile apps instead of a separate handset.

All components are connected by communication lines that transmit control commands, voice signals, and video in case of video intercoms. A dedicated power supply provides energy to the electronics and the door lock.

The entrance door is typically equipped with an electromagnetic lock. As long as power is supplied to the lock's coil, the magnet firmly holds a metal plate, keeping the door closed. When the controller receives an unlock command from the resident, the lock's power is cut off briefly, allowing entry.

Once the person enters, the door closer automatically returns the door to the closed position. The electromagnet again attracts the metal plate, and the entrance remains locked until the next authorized command.

What Happens After Pressing the Call Button?

When a visitor enters the apartment number and presses the call button, the call panel sends this command to the controller. For the system, the apartment number is not just a number-it's the address of a specific internal device connected to the intercom network.

The controller identifies the intended apartment and transmits the signal via the communication line. In older analog systems, apartments might be connected by a coordinate wiring scheme, while modern digital intercoms assign a unique address to each device.

Once the signal is received, the handset or monitor inside the apartment rings. Simultaneously, the system activates the correct communication channel, so once the resident picks up or presses the answer button, both parties can converse.

Other apartments are not connected to this conversation. The controller selects only the module corresponding to the entered number, keeping the call private.

If the resident doesn't answer, the controller ends the call after a set time and frees the line. The call panel is then ready for another apartment number or command.

How Does the Intercom Transmit Voice in Both Directions?

After the resident answers, the intercom switches to talk mode. The call panel's microphone converts the visitor's voice into an electrical signal, sent over the line to the apartment's handset or monitor.

The internal device's speaker converts the signal back into sound. When the resident replies, the process works in reverse: the handset's microphone captures the voice, and the system transmits it to the call panel, where it is played through the built-in speaker at the entrance.

Depending on the design, the intercom may use duplex or semi-duplex communication. Duplex systems allow both parties to speak simultaneously, like a regular phone. Simpler systems may prioritize one direction, so if both talk at once, one voice may be quieter or fade.

Video intercoms transmit video alongside voice. The door opening principle remains unchanged: video simply lets the resident see the visitor, while the unlock command is sent separately.

Voice communication is for decision-making, not for directly controlling the lock. After speaking, the resident either ends the call or presses the unlock button, prompting the system to move to the next stage-controlling the electromagnetic lock.

How Does the Intercom Unlock the Door?

After the conversation, the resident presses the unlock button on the handset or monitor. This button doesn't directly supply power to the lock-it sends a control command to the intercom controller.

The controller receives the signal and briefly changes the state of the circuit powering the electromagnetic lock, usually using a relay or electronic switch. While the lock is powered, its magnet holds the metal plate on the door. When power is cut, the magnetic force is released.

This is why the intercom unlocks the door not mechanically, but by temporarily disabling the electromagnet. At this moment, the visitor can push or pull the door to enter.

The unlock period lasts only a few seconds-enough to open the door but not long enough to leave the entrance unsecured. Afterward, the controller restores power to the lock.

When the door closes, the door closer returns it to the frame, the metal plate reattaches to the electromagnet, and the door is locked again. The entire cycle takes just a few seconds and repeats every time the door is authorized to open.

How Does the Intercom's Electromagnetic Lock Work?

The electromagnetic lock consists of a housing with a coil and a metal plate attached to the door. When electric current passes through the coil, it creates a magnetic field inside the lock, powerfully attracting the plate and keeping the door closed.

Unlike mechanical locks, there's no bolt physically entering a strike plate. The holding force is generated solely by the electromagnet, so to unlock, it's enough to stop supplying power to the coil.

After a command from the controller, the lock's voltage is temporarily cut off. The magnetic field disappears almost instantly, allowing the door to be opened with no resistance. After a few seconds, power is restored, but the door is only fully secured once the door closer returns it to the closed position and the plate contacts the magnet again.

This setup explains why many entrance electromagnetic locks open when the power goes out. They are fail-safe: without electricity, the magnet cannot hold the door. This is crucial for safety, as in an emergency, power failure, or evacuation, the entrance must not remain locked.

To keep the intercom functioning during brief power outages, the system may include a backup power supply. However, the principle of the electromagnetic lock remains unchanged: electricity is needed only to keep the door locked.

Conclusion

The intercom serves two main purposes: communication with visitors and access control for the building. After dialing an apartment number, the controller directs the call to the corresponding device, enables a voice conversation, and upon the resident's command, temporarily unlocks the entrance.

In systems with an electromagnetic lock, unlocking occurs by cutting off power to the coil. The magnet releases the metal plate, allowing the door to open; the door closer then returns it to the closed position and the lock is powered again.

The entire intercom process can be summarized as a single chain: call → communication → controller command → electromagnet deactivation → door opening → re-locking. The coordination of these elements turns an ordinary entrance door into an efficient access control system.