Step down voltage to power a single LED

If you are connecting to a standard telephone line, the ring voltage is about 100 volts. A NE-2 neon tube in series with a resistor string and inverse-parallel LED could work. When the phone is answered, the voltage drops to maybe 10 volts, and the NE-2 deionizes (so long as the series resistance is adequate to prevent an arc) and the LED load vanishes.

Joe Gwinn

"Andy K"

But you must feed the circuit via a bridge rectifier first as the ring tone is AC.

... Phil

Once phone is off hook, the voltage is so low that a neon bulb CANNOT ionize. The qualifier is meaningless.

Everybody is making it too complicated I think. A 250 V or 600V 0.47 uF ca pacitor would about do it. (100V would probably work for a while, but why tempt fate?) To protect your LED from back voltage, put a reverse-biased d iode across it cheap 1N4148 will do (reverse voltage rating is not importan t here) - or get a bicolor LED that handles AC fine, or just put 2 LEDs bac k to back!

The series capacitor will limit current to about 5 mA, max, and will > I have a single LED that I would like to use as a visual indicator that m

y phone is ringing.

A batteries.

own to about 10 volts if that helps. And the bulb still works, but phone ge ts static. :-)

two and some resistors.

Don't forget the corner case - the on-hook voltage is 48 volts, and a hook flash can get you there. So, it's best to ensure that the NE-2 will always extinguish on 48 volts DC.

Compared to a zener, the advantage of of a NE-2 is that there is zero leakage, so you don't trigger the central-office tests of leaky lines requiring service attention.

Joe Gwinn

I think you understand.

Someone said that the voltage is 100 volts when it's ringing.

Is that the voltage it takes to ring the bell in the phone ?

???

If so I may hook up my coffee maker when I don't need my phone. Just kidding.

I worked repairing lab instruments for 21 years, so I think I can diagnose and study problems.

Yes. It's about 100 volts at 20 or 25 Hz riding on top of the 48-volt on-hook voltage.

Well, if the ring were continuously on, and not sourced through a thousand ohms, it could work. But a ringer takes far less power.

How about hooking an oscilloscope up to the phone line and looking?

Note that one wire will be near ground, and the other wire rests at -48 volts. This is for DC; for AC, it's a balanced twisted-pair transmission line.

Joe Gwinn

The issue is if the neon is already ionized, say by the ring current, or by a line impulse, and transitions to an arc (from a glow). If the circuit isn't designed to be self-extinguishing, this will happen all the time.

Joe Gwinn

I found, while making my spam-call killer, that most modern phones with piezo ringers, will ring aggressively on 24V peak-to-peak. ...Jim Thompson

Use the Chem Rubber Handbook of Chemistry and Physics; the FIRST ionization potential of neon is about 63V, equal to what one would measure at nominal NE-2 currents.

• Yes, approximately.

But it's an AC signal, so won't the arc tend to self-extinguish?

"Joe Gwinn"

So 0.5mA for each LED.

... Phil

Why not something as simple as this...

Newsgroups: alt.binaries.schematics.electronic Subject: Step down voltage to power a single LED (from S.E.D) - Ring_Detector.png Date: Tue, 27 Aug 2013 10:47:18 -0700 Message-ID: ...Jim Thompson

That's the long-term continuous-duty limit. A NE-2 will pass far more, to the point of melting, if one allows it. This is true of all gas tubes.

Joe Gwinn