One of the disadvantages of the use of paired phone - waiting for the moment when the neighbor will release the line. So every minute not to grab the phone in the hope to hear the whistle of a free channel, it is useful to have an indicator that clearly notified about the occurrence of such moment.
Mandatory requirements to be satisfied by the status indicator, not to create a load on the line above the norm and to sustain a surges that accompany the call signaling. Since the device must to work indefinitely, it is advisable to use auxiliary the power source. Along with the task, this indicator will help notice damage to the line.
The simplest indicator of the pressure in the line can serve as a voltmeter, consisting of resistor, and a needle of the microammeter M476/1, used in tape recorders (PU1 in Fig. 1). His current full deflection - 0,11 mA - how best to comply with the minimum requirement of the line load (current consumption from line expectations should be not more than 0.5 mA).
When the handset is on the phone, the line voltage of about 60 V and through head PU1 flows a current of about 0.1 mA, the meter pointer is on the right the edges of the scale. When the tube is removed, the voltage drops to 5...15 V and arrow moves to the left. The line is a neighbor or a malfunction of the arrow is the extreme left position. Diode VD1 protects microammeter from positive pulse ringing. Protection from negative impulses not call required because the blocker paired phones emit signals of one the polarity.
Considered the indicator is very simple, however, to observe his testimony can only in the immediate vicinity. More convenient with light indicator indication, operating on the principle of relaxation oscillator cumulative capacity (Fig. 2). Its drawback is that too a long pause (about 4 C) between the led flashes HL1. During pauses charging of the capacitor C1 to the voltage at which triggered a dinistor VS1. The capacitor C1 is discharged through HL1 led and resistor R2.
The led stops flashing when the off-hook at the main or paired the handset. The moment of liberation lines delineated by the appearance of flashes the led.
To increase the frequency of alternating flashes twice, while maintaining the duration themselves outbreaks of the order of 0.1 s, allows the indicator is performed by the circuit of Fig. 3. After activation of diacs the capacitors are discharged: C1 through the diode VD1, and C2 - through diode VD3. Resistors can be applied such as MLT-0,125, capacitors - K52 - 9, or any other. A dinistor should have a voltage of about 30 V.
Author: Yu Prokopcov, Moscow