Intercom Director-Secretary. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The proposed intercom (Fig. 1) has the following features:
- control and reception-transmission are carried out via two unshielded wires;
- at the end of the call by the secretary of the director, the latter automatically switches to transmission.
Fig.1. Schematic diagram of the intercom (click to enlarge)
The device contains a base unit and a remote control (director). The base unit has an amplifier (VT4, VT5, DA1), a control unit (VT1, VT2, K1), a single vibrator (VT3, K4), a VA2 speaker and switching relays K2, short circuit.
The device works as follows. In the initial state, transistors VT1, VT2 are open, relay K1 is on, contacts K1.1 are open.
To send a message from the director's console, press the SB1 button. Through the communication channel, the button and the resistor R1 in the remote control, the base of the transistor VT1 closes to ground. Transistors VT1, VT2 close, relay K1 releases, contacts K1.1 close, speaker BA1 is connected to the amplifier and turns into a microphone.
To answer, the secretary presses the SB2 button. Relays K2 and short circuit are activated. The remote control is connected to the amplifier output using relay contacts K3.2, and the BA2 speaker becomes a microphone using contacts K2.1, K2.2. At the same time, contacts K3.1 close the capacitance C3 and discharge it if it was previously charged.
At the end of the message transmission, when SB2 is opened, relays K2 and short circuit are released. Relay contacts K3.1 supply power to relay K4 and capacitor C3. Relay K4 is activated and closes the circuit of the remote control with the amplifier (contacts K4.1 are connected in parallel with open contacts K1.1). This ensures the transmission of a signal from the remote control without pressing the SB1 button.
At the end of the one-shot operation (2 ... 2,5 s), all switching elements of the device return to their original state.
Structurally, the base unit and the remote control are located in the housings of the subscriber loudspeakers. Power supply - remote, power 2 ... 3 watts. Chip DA1 should be installed on a heat sink with an area of 3 ... 4 cm2. Relay K1, K4 - RES55A (0601) or RES49 (280), relay K2 and KZ - RES9 (0001).
Setting up the amplifier does not have any difficulties, however, to eliminate possible self-excitation, the conductors going to the "ground" from the BA2 and the remote control should be soldered to one point on the amplifier board. Installation can be done with unshielded wire, for example, MGTF-0,12.
Author: A.Partin, Yekaterinburg; Publication: radioradar.net
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