Another circuit i designed with only diodes, these are used
as gates. This was also used in some printer port switches i
D1 to D4 make a bridge so that LED's are powered in correct
polarity. LED1 indicates line ok no broken line, LED3 can
light only when a 12V Zener breaks down, this shows if line is
busy or free, so that you dont go online when someone is
talking on the phone in another room.
LED2 lights on an incoming call ringing signal. This is a
higher voltage at least in older exchanges, so the cap
protects the LED, and LED Lights when a AC ringing signal
occurs. D6 protects LED2 from reverse polarity.
20th May 2020
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