Intercom from old phones. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

Intercom from old phones. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Telephony

Comments on the article

Now, with the dominance of mobile and radio telephones, ordinary wired devices remain "out of work" and are often simply thrown away. with a little modification, it can be successfully used for simple intercoms. One of these intercoms (for two subscribers), accessible even to a novice radio amateur, is discussed in this article.

Functional scheme wired telephone is shown in Fig.1 [1].

Intercom from old phones

With the handset down, the telephone line is connected to the ringing device (VU) through the switch SA1 and the isolation capacitor C1. which passes only the variable component of the ringing signal. When the tube is removed. SA1 moves to the top position (as shown in Fig. 1), connecting the line with the spoken node (RU). The dialer (HH) is connected to the line through switch SA2. At the time of dialing, this switch disables the speaking node. Since a dialer is not needed for two subscribers, it is advisable to exclude it.

When designing two-wire intercoms, power is most often carried out as shown in Fig. 2 (the source is connected in series with telephones, and capacitor C2 shunts it for a conversational signal). In the case of a stabilized power supply, the role of C2 is performed by the output capacitor of the filter. With this scheme, the constant voltage in the line does not change. But the caller must have a ring tone generator that turns off when the second party goes off-hook. The second inconvenience: if the power supply is on the called side and is not turned on, then communication is not possible.

Intercom from old phones

In telephone networks (city exchanges), the line is powered in parallel (simplistically shown in Fig. 3). Line voltage Ul is defined as Up=UistUr. It is equal to approximately 12 V when the handset is removed, which ensures the normal operation of the electronics (in electronic devices).

Intercom from old phones

The advantage of such a power system is that that it can be connected in parallel from any side (shown by the dotted line in Fig. 3). In these telephone networks, the ringing signal is generated at the exchange and sent to the line. In this case, the constant voltage in the line remains at the level Uist. When the called party picks up the handset (a conversational node is connected to the line), the voltage in the Ul line decreases (below 20 V), which serves as the PBX command to turn off the ringing signal. As you can see, the parallel circuit, eliminating the power supply problem, leaves the issue with the call generator unresolved.

In the proposed device, the call signal is generated on the called side. To do this, a generator is provided in the call device on the receiving side, which responds to a decrease in the supply voltage. This solution not only greatly simplifies the circuit design, but also makes it possible to turn off the call generator for sure. Replacing the device in the device (Fig. 1) with a call generator, we get that when the tube is lifted, SA1 will turn off the generator, and its signal will not get into the line. The control signal for triggering the call generator is to reduce the voltage in the line to 20 ... 15 V, which is provided by simply lifting the handset on the calling side. Alteration of the phone is reduced to the replacement of the ringing device, leaving the conversation node unchanged, regardless of the type of telephone.

The diagram of the intercom is shown in Fig.4. It works in the following way. MocrVDI simplifies the connection of the phone to the line: no polarity has to be observed. The line voltage is fed through the divider R3-R4 to the input of the DD1 chip. The divider is selected from the condition that when the voltage in the line and IC1, the level at the input DD1.1 corresponds to the logical "G. and when the voltage drops to 20 V -" 0 ". In this case, the divider must have the maximum possible resistance so as not to shunt the line. Logic" 0" at the input of the element DD1.1 leads to the appearance of "1" at its output and at the output of DD1.3 (DD1.2 and DD1.3 invert the signal twice), and at the output of DD1.4 - "0". Low level with output DD1.4 closes the transistor VT3, and high, from the output DD1.3, opens the transistor VT1 and, accordingly, VT2.

Intercom from old phones

Through the public key on VT2, the voltage is supplied to the ringing signal generator on the DD2 chip. The generator is two-tone. On the first two elements (DD2.1 and DD2.2) a low-frequency generator is assembled, on the second two (DD2.3 and DD2.4) - high. The load of the generator is a key on the transistor VT4, at the output of which the piezo emitter HA1 is turned on. The power supply of the IC with a private key VT2 is provided by the R7-VD2-C1 chain. and when open (since the current consumption increases due to the operation of the generator DD2) - VD3-R9-VD2-C1.

The described refinement refers to the simplest phones without electronics. When altering such devices, instead of a separating capacitor and a bell, the proposed device is switched on. The piezo emitter, like the new board, is located in any convenient place in the phone case. For devices with electronic calling devices (for example, on the KR1008VZh4 chip), it is enough to make only a part of the proposed circuit (circled in Fig. 4 by a dotted line). The generator itself and the sound emitter are used by those that are already in the converted apparatus.

Although the PBX system is taken as the basis for the intercom, it makes no sense to set the voltage at 60 V. For such a device, 30 V is quite enough. For phones with "electronic filling", it is important to ensure that the line loaded on one phone gives a voltage within 14. ..18 V, and for two - 10 ... 14 V. This ensures normal (without distortion) operation of the conversational node. If you want to reduce power consumption, you can connect a second power supply (as shown in Figure 3). In this case, the caller turns on his power source, but there is one inconvenience: when connecting the line, polarity must be observed so that if both sources are accidentally turned on at the same time, they will not be connected back to back. To do this, LEDs can be switched on on both sides of the line. If you use modern super-bright ones, then they have enough current of 2 ... 3 mA, which will not affect the operation of the circuit.

The device is assembled on a printed circuit board made of one-sided foil fiberglass with a thickness of 1,5 ... 2 mm and dimensions of 100x40 mm. The drawing of the board is shown in Fig.5.

Intercom from old phones

The power supply circuit is shown in Fig.6. It must provide the required voltage (in my version - 30 V) and current consumption of at least 100 mA. The second requirement is the minimum ripple at the output, since the background of 100 Hz is very well heard in telephone sets. Instead of a ballast resistor, a miniature light bulb (26 V, 0,12 A) is used. This is convenient because when both devices are turned off, the light does not light up at all, when one (during a call) is dim, when talking it is bright.

Intercom from old phones

Details. The transformer is typical, TA-1 or TA-2, but any one that provides a voltage on the secondary winding of 35.. .40 V and a current of at least 100 mA will do. Fixed resistors in the device - MLT-0.25, variables - SPZ-22. Electrolytic capacitors - type K50-35 or their foreign analogues, constant - KM, KD or their analogues. KT3102B transistors can be replaced by any other low-power p-p-p structures with a permissible collector-emitter voltage of at least 45 V, KT940A is replaced by KT801, KT603, etc.

Diode bridge - any. You can also assemble a bridge from individual diodes KD521 or KD522. Zener diode VD2 is selected with a low stabilization current - KS191E. Piezo emitter ЗП22 can be replaced by ЗП1 or ЗП5. Instead of DD1 K561LA7, you can take K561LA5 (without any modifications) or K561LN2, but R3 should be excluded and the change in pinout should be taken into account. DD2 can be replaced by K561LA5. You can also use their analogues of the 176 series. If the device is supposed to be used around the clock, the KR142EN12 stabilizer IC should be installed on a small radiator

Structurally, the device is made as a separate module and is located in the telephone in any convenient place. In older phones (with an electromagnetic bell), the sound emitter can be placed under the handset holder or on the back wall, and the board can be placed at the place of the call. In phones with electronics, the emitter is already installed, the board itself is small (there is no generator), so it easily fits even in handsets. The power supply is made in the form of a separate unit, it can be placed either at one of the devices or anywhere along the telephone line.

Before adjustment device, the regulator R4 of the power supply sets the output voltage (UMCT). The load is connected. The resistance R3 is selected (Fig. 4) so as to provide "1 * at pin 11 DD1 when the tube is off (the line voltage is below 20 V) and "0" - when it is put (in the line - UMCI). Capacitance C3 is selected to provide the desired timbre and loudness.The length of the line, experimentally tested by me, exceeded 300 m.

Literature

A.I. Kizlyuk. Reference book on the device and repair of telephone sets of foreign and domestic production. - M. Antelkom, 1998.
Akimov N.N. and others - Resistors, capacitors, chokes, switching devices for REA: A Handbook. - Minsk: Belarus. 1994.

Author: G.Sauridi, Ryazan

See other articles Section Telephony.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive

90 nm and even 65 nm from SAMSUNG ELECTRONICS 22.03.2003

At the international technology symposium held in Kyoto (Japan) on June 10-14, SAMSUNG ELECTRONICS Corporation presented new technological solutions that allow moving to a topological step of 90 nm and even 65 nm.

These solutions will be used to build memory chips with a large amount of memory.

Other interesting news:

▪ Visual stimuli amplify sound

▪ Manometer in the eye

▪ Metabolic weight gain switch

▪ Free Micro Bit Computers to Learn Programming

▪ Laptop for $100

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Medicine. Selection of articles

▪ article We strangely met and strangely disperse. Popular expression

▪ article When did you first start eating crabs? Detailed answer

▪ article Rights of trade unions in the field of labor protection