Intercom System with Telephone-Based Door Entry

Figure 1 

The circuit provides a simple illustration of a door phone intercom system having a two-way communication using the features of LM386 amplifier.

• Intercom – short for intercommunication, is an electronic communication system made of microphones and loudspeakers; used for the purpose of announcements, cooperation, association, direction, or private conversations
• LM386 – a low voltage audio power amplifier designed for use in low voltage consumer applications; its features include low distortion, self-centering output quiescent voltage, ground referenced input, voltage gains from 20 to 200, low quiescent current drain, wide range of supply voltage, minimum external parts and battery operated
• Dual Pole Dual Throw (DPDT) Switch – provides a normally open and normally closed contact for each pole which make and break two separate circuits

The door phone circuit consists of speakers rated at 8 ohms that is used as an output speaker and as a microphone. This conversion of microphone to speaker and vice versa is made possible when the variation on the voice coil, made by the audio waves arriving at the speaker, creates a few electrical signals. This signals that are passing to the amplifier are being converted to audio signals coming out of the speaker. Since the speaker has low impedance, the BC109C transistor functions to provide the same amount of low impedance input while its amplification operates at the common base mode to provide a reliable voltage gain, while the current gain modification is permitted by biasing its own direct current.

The role of LM386 in the circuit is to provide power to the speakers operating in non-inverting mode. As a power amplifier, it increases the voltage gain of the speakers. The overall gain of the circuit is set by the 5K preset while the volume of the speakers is being controlled by the 10K ohm potentiometer. A potentiometer is a manually adjustable resistor used to control the potential difference or voltage across a circuit.

To allow a 2-way communication where one talks while the other listens, the switch is manually operated. The dual pole dual throw switch is responsible for reversing the operations of the loudspeakers between end points. The normal setup of an intercom requires long cables and it is possible because of the low impedance and less voltage gain of the circuit.

Intercoms have been utilized in many places like in residential and commercial area, in local area paging systems, in area of rescue assistance systems, in apartment entry systems using hardwire, in access control of telephone entry systems, emergency call systems and nurse call systems. In the world of intercom systems, generally intercoms are being used in video systems, integrated security, baby intercom, commercial systems, security systems, telephone systems, wireless systems, home intercom and other services including installation, repair and maintenance.

LM386 may be used in several applications such as power converters, small servo drivers, ultrasonic drivers, line drivers, TV sound systems, portable tape player amplifiers, AM-FM radio amplifiers and intercoms.

Source:www.zen22142.zen.co.uk/Circuits/Audio/doorint.htm