Telephone line busy indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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If several telephone sets are connected to the same telephone line in different rooms in an apartment, then each of them can be equipped with a simple telephone line busy indicator, which will allow you not to "break into" someone else's conversation on one of the parallel phones. In addition, the indicator is able to signal unauthorized use of the telephone line.
The scheme of the device is shown in Fig.1.
The indicator is connected in parallel with the telephone line in accordance with its polarity. Diode VD1 is designed to protect the device from incorrect connection. When all telephones, modems, faxes are "hung up", the voltage in the telephone line is about 60 V (30 V for some types of PBXs). High voltage through the current-limiting resistors R40, R1 and bipolar transistors VT2 ... VT1 operating in the micropower zener diode mode is fed to the gate of the field-effect transistor VT3. Since the gate-source voltage VT5 is greater than its opening threshold voltage. VT5 is open, and the other transistor (VT5) is closed, and the super-bright blue LED HL7 is off. In this mode, the device consumes a current of about 1 μA, which is much less than the devices from [8-1].
If one of the parallel phones is off-hook, the voltage in the telephone line drops to 8...20 V, VT5 closes, VT7 opens, and the HL1 LED lights up, signaling that the telephone line is busy. In this mode, one device consumes a current of no more than 280 μA from the telephone line. The use of a super-bright LED allows you to get a fairly bright glow at low current. Transistors VT4, VT6 limit the maximum gate-source voltage of field-effect transistors at 7 ... 10 V, preventing their breakdown. Capacitor C1 reduces the sensitivity of the device to random short-term voltage changes in the telephone line.
The device is mounted on a printed circuit board with dimensions of 35x35 mm (Fig. 2). A printed circuit board of this size fits easily into a standard Euro socket for two telephones.
Bipolar transistors KT315G can be replaced by any of the KT312 series. KT315. KT342, SS9014. The field effect transistor KP504A can be replaced with KP502A. having the same pinout, or on BSS88. KP501A, KP501B. ZVN2120. Transistor VT7 (KP502A). which can withstand a drain-source voltage of 400 V, can be replaced by an imported analogue BSS124 or (with a decrease in the reliability of the device) by KP504A. KP501A. given the differences in pinouts. Super bright blue RL50-CB744D LED with approx. DB6000-20AGCA (5 mcd. green) and others similar. The KD448Zh diode can be replaced by any of the KD3000N5...448N3500 series. Capacitor - any small-sized ceramic, resistors - C5-435 or imported analogues. You can use SMD components.
The finished device does not require adjustment. Up to 8 such indicators can be installed on one telephone line. When placing the device inside the telephone, it is desirable to connect it as follows. so that when you pick up the handset on this machine, the power of the indicator is disconnected from the telephone line.
Literature
- A. Grishin. Telephone line light analyzer. - Radio. 1993, No. 5. P.36.
- Yu.Prokoltsev. Indicators for a paired phone. - Radio 1998. No. 4. P.55.
- And Butov. Telephone line status indicator. - Radio designer, 2001, No. 11, C 20.
- And Butov. Indicators for parallel telephone. - Radiomir, 2002, No. 7, p.11.
- A. Butov. Light and sound informer of the state of the telephone line. - Circuit design. 2002, No. 10, p.38.
Author: A.Butov, p. Kurba, Yaroslavl region
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