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Telephone LED status indicator

This will monitor telephone status without loading the telephone line, this way if you have two phones in parallel you will know if one of them is busy. Connect the two ends of circuit in parallel to phone lines.

Another circuit i designed with only diodes, these are used as gates. This was also used in some printer port switches i made.

Simple High speed data switch

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 breaksdown, 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.

Telephone LED
                        status indicator

The Source file is here

I used to use this in the DUN or Dial Up Networking days. A 386 computer and a Gateway 2000 Plugin Card Modem. Maybe 14400 bps transfer speed. It was the Netscape Browser like 3.6 light to 4.7 for better machines.

Here is another circuit with just diodes - Edison Bulb Life Extender. Then now LED Lighting is everywhere, but the warmth of the Edison Bulb will always be remembered. Even my Cat used to bump his head lovingly on the Lathe Lamps which i used as Electronic Workbench Lamps.

Design Notes - Components Selection - 05 

Selecting the parts for your product design, choosing the components intelligently is a part of Product Design Engineering.