Busy telephone line indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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If the apartment has several telephone sets (SLT) in different rooms connected to the same telephone line, then you simply need this device. When a phone call comes in, all SLTs often go off-hook, which causes complications at the beginning of a call. If you need to call from one room, and there is a telephone conversation in another at that time, then you have to repeatedly pick up the phone to find out if the line is free. There are also cases when one of the telephones is badly hung up and, not knowing about it, you can not wait for the desired call.
To get rid of all these inconveniences will help the electrical circuit shown in Fig. 2.1. It allows you to have a light indication when the handset is off-hook on any of the SLTs.
Fig. 2.1
The circuit is connected to the telephone line in parallel with the SLT in any convenient place, for example, you can install it inside the case of each SLT, fixing the HL1 LED in a visible place.
The electrical circuit can be simplified by excluding the VD1 diode bridge, but at the same time, the device must be connected to the telephone line observing the polarity indicated in the diagram.
The principle of operation of the device is based on the use of voltage changes in the telephone line. So, if the telephone line is not busy, then the voltage on it is about 60 volts, and when you pick up the handset on any of the SLT, it drops to 6 ... 15 V (depending on the internal resistance of the SLT).
The circuit consists of a line voltage level detector on a transistor VT1 and a current amplifier on VT2, VT3. Transistor VT1 operates in microcurrent mode, which provides it with the maximum gain. For this reason, it will be in one of two states: locked or open, depending on the supply voltage.
When setting the indicator operation voltage, it will be necessary to select the value of the resistor R3, and in the manufacture of the circuit it is permissible to use R1, R2 - from 3 MΩ to 5,1 MΩ and R4 - from 750 kΩ to 1 MΩ.
The HL1 LED can be replaced with AL310A or any of the KIP series, and the VD1 diode bridge with four diodes of the KD102A, B type.
Rice. 2.2. PCB topology
Fig. 2.3
It is better to check the operation of the circuit by applying a constant voltage of 6 ... 15 V from a regulated source (in this case, the HL1 indicator should be lit). Gradually increasing the voltage to 30 V, you should make sure that at a voltage of more than 20 V the LED goes out.
The above scheme has proved to be reliable in operation and does not affect the operation of the telephone line.
The current consumption of the circuit from the telephone line with the handset down does not exceed 0,01 mA.
The topology of a single-sided printed circuit board and the arrangement of elements on it (without VD1) is shown in fig. 2.2. The dimensions of the board were chosen taking into account the possibility of installing it in a standard telephone jack (Fig. 2.3).
Publication: cxem.net
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