Telephone line holding device. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The proposed device performs the function of holding the telephone line ("HOLD"), which allows you to put the handset on the hook during a conversation and go to the parallel telephone.
The device does not overload the telephone line (TL) and does not interfere with it. During triggering, the caller hears a background music.
The diagram of the telephone line holding device is shown in the figure. The rectifier bridge on diodes VD1-VD4 provides the desired polarity of the power supply of the device, regardless of the polarity of its connection to the TL. The switch SF1 is connected to the lever of the telephone set (TA) and closes when the handset is picked up (ie, it blocks the SB1 button when the handset is on-hook). If during a conversation you need to switch to a parallel SLT, you must briefly press the SB1 button. In this case, relay K1 is activated (contacts K1.1 close, and contacts K1.2 open), a load equivalent is connected to the TL (circuit R1R2K1) and the TA from which the conversation was conducted is turned off. Now you can put the handset on the hook and go to parallel SLT.
The voltage drop on the load dummy is 17 V. When the tube is lifted on the parallel LT, the voltage in the TL drops to 10 V, relay K1 turns off and the load dummy is disconnected from the TL.
Transistor VT1 must have a transmission coefficient of at least 100, while the amplitude of the audio frequency AC voltage output in the TL reaches 40 mV. As a musical synthesizer (DD1), a UMC8 chip was used, in which two melodies and an alarm signal are "hardwired". Therefore, pin 6 ("melody selection") is connected to pin 5. In this case, the first melody is played once, and then the second one indefinitely.
As SF1, you can use an MP microswitch or a reed switch controlled by a magnet (the magnet must be glued to the TA lever). Button SB1 - KM1.1, LED HL1 - any of the AL307 series. Diodes VD1-VD4 - any that allow a reverse voltage of at least 200 V. Relay K1 - RES-47 (passport RF4.500.409, RF4.500.431 or RF4.500.432).
The device is hinged and does not require adjustment.
By using a resistor R1 with a resistance of 270 ... 390 Ohm, you can increase the voltage drop across the dummy load to 20 ... in this case, use a resistance of 25 ... 15 kOhm. The voltage drop across the resistor R20 must not exceed 3 V, otherwise it will be necessary to reduce the resistance of the resistor R3.
Author: Dmitry Abutkov; Publication: N. Bolshakov, rf.atnn.ru
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