The controller of the telephone line (KTL) is designed to protect telephone lines from unauthorized connections. It indicates a short circuit or disconnection of the telephone line. When connecting additional loads to the line (the resistance is not more than 100...150 kω) KTL indicates that the line is busy or bugged.
A simplified version of the KTL (Fig.1) comprises a bridge circuit forming the voltage (VD1...VD4). a threshold device control the voltage of the telephone line (relaxation pulse generator on the avalanche transistor VT1), and the alarm indicator status line formed on the analogue thyristor (VT3, VT4) and pulse generator (VT2).
The device operates as follows. When you first connect KTL to the telephone line thyristor "latch" b " is off, the pulse generator serves a full power supply voltage (60 V), the two generators are working, LEDs HL1 (green) and HL2 (red) are lit. After touching the sensor "Reset" or pressing the "Reset" button, which can be connected parallel to the chain of R5, C3, and it locks automatically triggered thyristor latch (VT3, VT4). The operation of the generator on the transistor VT2 is disabled, the device enters the normal (working) mode. Potentiometer R1 is adjusted so that when removed the handset(line voltage of about 12 V) generator transistor VT1 developed a rare flash of light. At the same time connecting in parallel with the load resistance of the order of 100..150 ohms should cause breakdown of generation. Of course, if parallel connection of high impedance load to the line was completed earlier than has been connected and configured KTL, the device will respond only to connect one parasitic load (second telephone set for high impedance headphones, tape recorder, etc.).
When installed on a phone line nonstandard devices connected in parallel or sequentially, that is connected to phone line break or short circuit for more than 3, the unlocking of the thyristor latches. On VT2 is energized, and the generator starts to generate flash red (HL2), indicating an alarm condition of a telephone line. The generation is performed until then. until you press the button (sensor) Reset.
Delay in the response of the device to short circuit or open circuit line is determined by the time constant R2 C2. The small value of the time constant of the chain increases the sensitivity of the device, but increases the probability of a false alarm device when dialing.
Author: M. Shustov, Tomsk; Publication: N. Bolshakov, rf.atnn.ru