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# Telephone line vigilance principle

2016-02-02 05:25
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Next, alert the telephone line principle. The most effective way is to grasp the principle of combining schematic view text, combined with text to understand the principle. See Figure: whether the telephone line polarity of the input circuit, proper DC polarity is maintained at the bridge rectifier diodes D1 C1 to D4 components. DC voltage is developed in the capacitor C1 is used to check the telephone line conditions according to Table i this circuit draws negligible current from the telephone line; Therefore, when it is connected to the telephone line, the normal phone operation is not affected. This circuit can be divided into two parts. The first part includes a Zener D9, transistors and diodes D5 T1 T4. It is used to verify that the phone line is good or not a continuous loop. The second part includes a zener D10 and transistor T5, T10 is used to check whether the telephone line in use (or abuse) or not. Zener diode D9 (3.3 v) conduct when the telephone line loop is intact and not torn. D9 set control Zener voltage transistors T1, T2, T3 and T4 is cut. Therefore, the green LED lights but can not hear the sound of the  buzzer . The telephone line circuit is not continuous, no voltage is available through capacitor C1. Therefore, the zener D9 and transistors T1, T2 and T3 and T4 be nonconductive. Now the green LED turns off and the continuous sound heard from the buzzer. When the telephone line is good, but not in use, the Zener voltage of the capacitor C1 as the D10 is quite high. This results in conduction of transistors T5 and T6, and cut off the transistor T7 (collector of transistor T6 is close to the ground potential). Therefore actively 9 v rail is not extended to the following multivibrator circuit based on transistors T8 and T9. Therefore, the red LED is not lit, the buzzer does not sound. When the telephone line is in use, do not apply zener D10. Thus, the transistors T5 and T6 are not conducting and the transistor T7 for. 9 v is now extended to the multivibrator circuit. This design of this multivibrator collector transistor T9 higher refresh every 5 seconds, forward bias transistor T10 and it carried. Therefore, in an interval of every 5 seconds to hear the buzzer beeps. Beep interval can increase or decrease the value of the capacitors C3 Although the volume can be adjusted with the help of VR3 default. The principle is simple but effective. You can read several times to grasp the principle.

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Telephone Line Vigilant

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