Circuit schematic diagram of two same-line telephone switching devices

Same-line telephone switching device (1)

On many occasions, one telephone outside line is connected to multiple telephones. Although the utilization rate is high, the confidentiality is too poor. This device can effectively solve this problem with the minimum number of components. When using the phone, the one who picks up the phone first will have priority, and the one who picks up the phone second will be silent. When answering the phone, both phones A and B will ring at the same time. The one who picks up the phone first will have priority, and the one who picks up the phone second will be silent. During the call, machine A and machine B keep each other confidential.

working principle

The circuit principle is shown in Figure 1 (click here to download the schematic diagram). Two telephones A and B are connected to an outside line L1 and L2 through relays J1 and J2. In the figure, status relays J1 and J2 are both in the released state. Therefore, both telephones A and B are connected to the outside line. When a call comes in, the ring current voltage causes both phones to ring through the coupling capacitors C1 and C2. Assuming that phone A picks up the phone first, relay J1 provides a DC path and its contacts are closed (that is, the movable contact switches from upper to lower), thus cutting off the path to phone B. If phone B picks up the phone first, similarly, the access to phone A is cut off, thereby completing the mutual confidentiality function. When using the phone, whoever picks up the phone first will occupy the outside line first, and then there will be no sound when picking up the phone.

The functions of the capacitors C1 and C2 in the component selection and production
circuit are to provide an exchange path for the ring current and voice; and to prevent the relay from jumping during dialing. Generally, 1000u electrolytic capacitors are used. The relay is single pole double throw (5 pins) and the coil working voltage is 9V. The entire device must distinguish the positive and negative external lines when connecting.

Same-line telephone switching device (2)

The same-line telephone switching device introduced in this article uses thyristor. In addition to having the same functions as the same-line telephone switching device (1), it also has its own characteristics: ① The number of extensions connected in parallel is large; ② The circuit volume is larger It is more compact; ③ There is no need to distinguish positive and negative when connecting the device to external lines; ④ Installation and wiring are simpler; ⑤ Less impact on external lines during operation.
working principle

The circuit principle is shown in Figure 1 (click here to download the schematic diagram). Diodes VD1-VD4 form a polarity conversion circuit, so that there is no need to distinguish between positive and negative when connecting to external lines. In the static state, each extension is hung up, and the line is equivalent to a DC circuit break, so the thyristor VS is cut off. The figure only shows the circuit of one subunit. In actual use, multiple subunits can be connected in parallel. When a call comes in, the AC ring current voltage breaks down the VD5 Zener diode and triggers the thyristor VS through R2 to turn on, causing the phone to ring. Assume that phone A picks up the phone first, then phone A forms the DC path of VS, and VS remains in the conductive state, and the light-emitting diode LED lights up to indicate the off-hook state. After phone A picks up the phone, the voltage between the external lines L1 and L2 drops from 60V to about 10V. The thyristors of other extensions remain in the cut-off state because the Zener diodes cannot break down, so picking up the phone has no effect. When each extension phone calls, its working principle is similar. The one who picks up the phone first VS connects first, occupying the line first, and leaving other extension phones in a blocked state (silent).

Component selection and production
　　

See the table below for the component list.
 

It should be noted that the voltage stabilization value of the Zener diode is 12V in the table. During specific debugging, it may be slightly adjusted for different switches.