Intercom for two subscribers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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This device is able to provide operational communication between two subscribers located in different rooms. Its main difference from similar designs is that the mode control and reception and transmission are carried out over two unshielded wires (see figure).
In addition, the input signal is not fed to the base of the amplifying stage transistor, but to its emitter. The power amplifier does not have a diode or resistor between the bases of the complementary pair of output transistors. The presence of a transistor-relay single vibrator allows, at the end of the call, the secretary of the director to automatically switch the latter to transmission.
The device contains a base unit and a remote control connected to the unit via sockets X1.1 and X1.2. The base unit has an amplifier made on transistors VT4-VT9, a control unit based on transistors VT1-VT3 and electromagnetic relays K1-K4. The base unit can be located anywhere, but the secretary must have an SB2 pushbutton switch and a BA2 dynamic head.
The device works like this. In the initial state, the transistors VT1, VT2 are open, the contacts K1.1 of the relay are open. To send a message from the director's console, press the SB1 button. The base of the transistor VT1 is practically connected to a common wire through the button contacts and resistors R3, R1. Transistors VT1, VT2 close, relay K1 releases, contacts K1.1 close, dynamic head VA1 is connected through capacitors C1, C2 to the amplifier. The BA1 head becomes the microphone, the conversation in front of which will be heard in the BA2 head connected to the amplifier output.
To answer, the secretary presses the SB2 button and holds it. Relays K2, K3 are activated. The director's console is connected to the output of the amplifier using contacts K3.1, and contacts K2.1 and K2.2 turn the dynamic head BA2 into a microphone. At the same time, the contacts K3.2 close the terminals of the capacitor C2 and discharge it if it was previously charged.
At the end of the transmission of the message, the relays K2 and K3 are released. Contacts K3.2 supply voltage to relay K4 and capacitor C2. Relay K4 is activated and contacts K4.1 closes the circuit of the remote control with the amplifier, since contacts K1.1 are open. This ensures the transmission of a message from the remote control without pressing the SB1 button.
At the end of the single vibrator operation time (2 ... 2,5 s) on the transistor VT3 and relay K4, all switching elements of the device return to their original state.
The device uses relays K1, K4 type RES55A, passport RS4.569.611, relays K2 and KZ - RES9, passport RS4.524.202. Details of the base unit and the remote control are located in the housings of the subscriber loudspeakers. It is permissible to place a power supply with a power of 2 ... 3 W in the base unit case. Powerful output transistors must be installed on heat sinks with an area of 2...3 cm2.
Author: A.Partin, Yekaterinburg
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