Simple intercom. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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A popular intercom today is the intercom. It allows residents of a house or cottage to conduct remote negotiations with a visitor who has pressed the bell button at the entrance or near the gate. A relatively simple intercom design is proposed in this article. The intercom consists of a main amplifier with a handset and a power source, which are located in the apartment in a convenient place for use, and a microphone with an amplifier and a dynamic head, installed on the front door or gate.
The main amplifier circuit is shown in fig. 1. The BM1 microphone and the BA1 dynamic head are located in the handset, from which the parts that were previously in it are removed. In the position of the contacts of the SA1 switch shown in the diagram and the pressed button SB1, the landing, the space near the gate or the visitor's response is being listened to. When the switch knob is moved to the opposite position, you can send a message to the visitor or ask questions. The volume of the visitor's answers is regulated by a variable resistor R1. Clamps "A" - "G" are connected to similar ones on an external amplifier (Fig. 2). It is advisable to use a three-wire harness in a shielding braid that acts as a conductor - "B".
(click to enlarge)
The amplifier uses available parts. Transistor VT4 can be any of the KT814 series, the rest - any of the KT361 series. Variable resistor R1 - SP5-2V or another suitable in size and resistance, fixed resistors - MLT-0,125. In the main amplifier, a dynamic head with a power of 0,25 W is used, in the remote one - 0,5 W. Microphone VM1 - electromagnetic DEMSh-1, VM2 - electret MKE-3. Capacitors - any type. Power source - battery "Krona".
The parts of the main amplifier are mounted on a board made of one-sided foil material, shown in fig. 3, and the remote amplifier - in fig. 4. In the author's version, the main amplifier board, power supply and parts not mounted on the board are placed in the body of the old telephone (the parts have been removed from it, of course). A variable resistor, a switch and a button are fixed on the front panel of the device. The connection of the handset with the details of the amplifier is made with a shielded wire.
(click to enlarge)
To accommodate the parts of the remote amplifier, a case was made of foil fiberglass. The sensitivity of the intercom is such that when the remote amplifier is located above the gate at a height of two meters and the length of the connecting line is 15 m, the connection is quite loud. If necessary, the greatest sensitivity and volume of communication is achieved by selecting resistors R2, R5, R8.
This operation is carried out, of course, before installing a remote amplifier, but with a connected communication line of the appropriate length. You can't do without an assistant. At the time of tuning the amplifiers, these resistors can be replaced by chains of a variable resistor that is slightly larger than that indicated in the diagram, and a constant resistor with a resistance of 10 ... 20 kΩ connected in series with it.
Author: A. Gridnev (RA6JGZ), Vladikavkaz; Publication: cxem.net
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