Home Telephone Ring Light for Hard of Hearing Folks

A friend of mine asked me to make a light that would blink when the phone rang as his father is very hard of hearing. So this is a home telephone ring light that meets this need.

Supplies

Things you'll need are:

1 - 1N007 Diode

1 - 1K resistor 1/4W

2- 20 V Zener diodes 1N5250 or similar

2 - 4.7uf non-polarized capacitors 50V minimum

1 - small piece of prototype board

4 to 6 - LEDs

1- Home Telephone cable

Also:

Cable ties

Hot glue

Heat shrink tubing

#24 AWG wire

3D Printer PLA White and Black

Solder

Step 1: Circuit Diagram

The landline telephone system uses a 90V AC 20Hz sinewave ring signal. It also has -48VDC on the line. We want to detect the 90VAC ring signal without disturbing the -48VDC signal or audio on the line. The attached circuit will do that.

The capacitors block DC voltage and will each have an impedance (reactance) of about 1.7K ohms at 20 HZ for a combined impedance of 3.4K ohms. This combined with the 1K ohm resistor in series will give us a total impedance of 4.4K ohms. The Zener diodes each drop 20V. The 1N4007 diode takes the strain off the LEDs when the ring signal sinewave is opposite polarity to the LEDs.

A typical LED voltage drop (forward voltage), varies significantly by color, ranging from about 1.2V for Infrared (IR) LEDs to 3.0-3.6V for Blue, UV, and White LEDs, with Red LEDs around 1.8-2.0V and Green/Yellow in the 2.0-3.0V range, all at a standard 20mA operating current. We are going to use a little less than 20mA to preserve the LED life.

So when there is a positive cycle of the ring sinewave applied to the positive terminal of the LED string, we will have a 20V drop across one of the Zener diodes, the other Zener will drop about 0.7V. I used two red and two white LEDs, so we can expect a voltage drop of approximately 3.0 + 3.0 + 1.8 + 1.8 Volts across four LEDs for a total of 9.6 Volts. So we drop 20 + 0.7 + 9.6 Volts or 30.3 Volts across these, that leaves the voltage drop across the capacitors and resistor at 90V - 30.3V = 59.7V. Recall the combined impedance of the capacitors and resistor is about 4.4K ohms. Current I = V/R so in this case I = 59.7/4.4K = 13.6 mA which is about right for an LED.

Step 2: 3D Print the Light Shroud and Plinth

I designed the light case in Free cad, then 3D Printed the light shroud (Cover) in white PLA and the pedestal in black PLA. The shroud fits on the plinth as a pressure fit, it is a very tight fit by design, so you will have to push hard to get it on or even sand it down slightly. I'll provide the gcode files. This is kind of an Art Deco design.

Step 3: Putting It All Together

The phone line is in my case an old piece of phone line with the phone jack on it recycled from and old phone, or you can still buy a new one on the internet. Thread the phone line through the outside hole in the plinth into the inside and tie a cable tie around it as a strain relief. Solder the parts to a small piece of protoboard that will fit inside the plinth. Solder the LEDs in series, paying attention to the polarity using the wire. Use enough wire so that you can place some LEDs at the top of the support post and some in the middle. Cable tie and hot glue the LEDs to the support post in a position that suits you. Solder the wire leading to the LEDs to the protoboard and also solder the phone wires to the board. Phone line polarity should not matter at all for this circuit as the capacitors and Zener diodes take care of that. Just make sure the 1N4007 diode and the LED string have opposite polarity though. Tuck the board into the bottom of the plinth and push the shroud on. You are done.