Diagram Plus guide

The automatic switching consists of resistors R1-R3, variable resistor VR1 as the regulator, transistor T1 and T2, zener diode D2, and diode D1. R1 and VR1 is useful as an input voltage divider from the telephone line.

Parts list :

•     Resistor R1-R2, R4 : 47k ohm
•     Resistor R3 : 100 ohm
•     Resistor R5 : 22k ohm
•     Resistor R6 : 1M ohm
•     VR1 : 100k ohm
•     Diode D1 : 1N4001
•     Zener diode D2 : 24V, 400mA
•     Capacitor C1 : o.o1 uF
•     Capacitor C2 : 330 pF
•     Trimmer C3 : 50p
•     Capacitor C4 : 5.6 pF
•     Capacitor C5 : 10 pF
•     Transistor T1-T2 : BC548
•     Transistor T3 : BF494
•     Inductor L1 : 45rotation 36SWG in resistor carbon 1M 1W
•     Inductor L2 : 3 rotation 21 SWG 12mm diameter
•     Antenna