2019-08-08 LED Powered By Telephone Line

From FB group Bell Telephone of…

2019-08-08

So you want to light up your phone. You don’t need a transformer to power the LED, you can use the current of the phone line to power the LED when the phone is off hook or when it is ringing. Most of the time you want the light to see the dial or to tell that the phone is ringing. You can use red LEDs for this. A red LED can handle up to 30 milliamps, and a phone line can have up to about 55 mA, so two red LEDs in parallel will each share half the current. But in order to work with either polarity and to handle the AC ringer current, the LEDs are connected back to back. So it takes four red LEDs. It doesn’t matter which lead you put them in because they are not polarized – two will light up when it goes off hook, and the other two won’t light. When the phone rings all four will flicker. The voltage drop across any LED will be about two volts, which is less than the voltage drop between the CO and your phone. One issue may be that the LEDs are too bright. If so, you can put some nail polish or paint or even some paper or plastic to dim the light. Or you could put a resistor across the LEDs to shunt some of the current. If you use a resistor you need only two LEDs back to back. The resistor could be between 22 and 100 ohms depending on the loop current and the desired brightness.
Here’s a wiring diagram.

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