Indicators for a paired phone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Indicators for a paired phone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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One of the inconveniences of using a paired phone is waiting for the neighbor to release the line. In order not to grab the handset every minute in the hope of hearing a beep of a free channel, it is useful to acquire an indicator that would visually inform about the onset of such a moment.

Mandatory requirements that the status indicator must satisfy are not to create a load on the line above the norm and to remain operational during power surges accompanying call signals. Since the device must operate indefinitely, it is advisable not to use an auxiliary power supply. Along with the task, such an indicator will help to notice line damage.

The simplest indicator of line voltage can be a voltmeter consisting of a resistor and an M476/1 pointer microammeter used in tape recorders (PU1 in Fig. 1). Its full deflection current - 0,11 mA - perfectly meets the requirement of the minimum load on the line (the current consumption from the line in standby mode should not exceed 0,5 mA).

Indicators for a paired phone

When the handset is on the device, the voltage in the line is about 60 V and a current of about 1 mA flows through the PU0,1 head, the arrow of the device is at the right edge of the scale. When the tube is removed, the voltage drops to 5 ... 15 V and the arrow deviates to the left. When the line is occupied by a neighbor or a malfunction, the arrow occupies the extreme left position. Diode VD1 protects the microammeter from positive pulses of the ringing signal. Negative call pulse protection is not required because the paired phone blocker outputs single polarity signals.

The considered indicator is extremely simple, but it is possible to observe its readings only in close proximity. More convenient is an indicator with a light indication, which works on the principle of a relaxation generator with a storage capacity (Fig. 2). Its drawback is too long pauses (about 4 s) between flashes of the HL1 LED. During pauses, the capacitor C1 is charged to a voltage at which the dinistor VS1 fires. Capacitor C1 is discharged through LED HL1 and resistor R2.

Indicators for a paired phone

The LED stops flashing when the handset is picked up on the main or paired set. The moment of release of the line is clearly visible due to the appearance of LED flashes.

To double the frequency of alternating flashes, while maintaining the duration of the flashes themselves on the order of 0,1 s, allows the indicator, made according to the scheme in Fig. 3. After the dinistor is triggered, the capacitors are discharged: C1 - through the VD1 diode, and C2 - through the VD3 diode. Resistors can be used type MLT-0,125, capacitors - K52 - 9 or any others. The dinistor should have a response voltage of about 30 V.

Indicators for a paired phone

Author: Yu.Prokoptsev, Moscow

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