When you think of neon, you might think of neon signs or the tenth element, a noble gas. But there was a time when neon bulbs like the venerable NE-2 were the 555 of their day, with a seemingly endless number of clever circuits. What made this little device so versatile? And why do we see so few of them today?

Neon’s brilliant glow was noted when William Ramsay and Morris Travers discovered it in 1898. It would be 1910 before a practical lighting device using neon appeared. It was 1915 when the developer, Georges Claude, of Air Liquide fame, received a patent on the unique electrodes suitable for lighting and, thus, had a monopoly on the technology he sold through his company Claude Neon Lights.

However, Daniel Moore in 1917 developed a different kind of neon bulb while working for General Electric. These bulbs used coronal discharge to produce a red glow or, with argon, a blue glow. This was different enough to earn another patent, and neon bulbs found use primarily as indicator lamps before the advent of the LED. However, it would also find many other uses.

How It Worked

Despite the name, a neon bulb typically has only 99.5% neon, and the rest is usually argon, which tunes the voltage where the gas breaks down. This breakdown voltage is the key to the bulb’s properties. The gas is at a very low pressure. Other gases and impurities can also change the color of the bulb, but the most common ones were neon and argon.

There are two electrodes within, an anode and a cathode. When a DC voltage excites the bulb sufficiently, a glow forms around one electrode. AC makes both electrodes glow alternately. The striking voltage changes based on ambient light or radioactive exposure, as well as the bulb’s gas mix and pressure.

Until the strike voltage occurs, the bulb is effectively an open circuit. When it does strike, however, the resistance goes down and will sustain even at a lower voltage. Like an LED, current limiting is essential, or the bulb will burn out. The NE-2, arguably the most common neon bulb, triggers at 90 V, nominally, and will conduct until the voltage drops to about 60 V.

So It Lights Up?

The lighting up is good, but you do need a lot of voltage to get it going. The bulb will easily light up from 120 V line voltage, for example.  But the really interesting property is that the bulbs, when glowing, exhibit negative resistance. That is, as current increases, voltage decreases.

You can also make the bulbs operate in a bistable mode, where they can work in logic circuits. They weren’t common, but some bulbs had special features for logic use. These bulbs were not made to glow necessarily, and sometimes had a third wire used as a control electrode.

Since the gas inside the tube can ionize, neon bulbs can also detect things like light, microwaves, or heavy electrostatic fields. They can even pick up audio.

What Could You Do?

Of course, the normal application was to use the devices as a lamp, like you’d use an LED today. Power pilot lights were common. Special neon lights looking like digits form the basis of nixie tubes.

Another neat display trick was the “blown fuse” indicator. Fuse holders often had neon bulbs in them that connected across the fuse terminals. In normal operation, the voltage across the fuse was practically zero, so the bulb stayed dark. But if the fuse blew, you’d have 120 V across the bulb, which would then light up. A high-value resistor prevented any significant current from flowing.

By far the most common non-lighting use was as a part of a relaxation oscillator. Consider a circuit with a resistor and a capacitor, but the capacitor has a neon bulb across it. The capacitor will charge until it hits the neon bulb’s trigger voltage. The bulb will light and discharge the capacitor until it drops below the holding voltage for the bulb. Then the process starts over. You could use neons to make a clock.

Long History

[E. Norbert Smith] wrote about the “1001” uses for the NE-2 — probably not an exaggeration, but [Smith] didn’t get that many in the article — in a 1965 Elementary Electronics magazine article.

The circuits he shows include a 50 V regulated power supply. (Regulators weren’t held to the same standard in those days as we would expect now.) Need 150 V? Use three of them. Or put them in parallel to improve regulation performance.

Some of the circuits are probably not useful if you aren’t building with tubes. And, of course, if you aren’t building with tubes, you are less likely to have the high voltages you need, so there is that.

He also covers the classic self-indicating fuse and the relaxation oscillator. Of course, if you can make one neon bulb blink, you can also make two blink alternately. Blink it fast enough and you can make a code practice oscillator with just a few parts and a 90 V battery.

If you wondered how neon bulbs could handle logic, that same article will answer that question, too. Just be aware that a logic 1 is 10 V — not a problem — but a logic 0 is -10 V. The nice thing about demonstrating logic circuits with neon bulbs is that you don’t need a logic probe or scope to see the state of the machine.

There were many other ways to use these bulbs. Since the trigger voltage was stable, you could use it as a voltage indicator if you coupled it with a voltage divider. In fact, many cheap AC socket testers still work this way. A typical circuit for a capacitor checker could be found in “36 Time Tested Circuits,” a book from Popular Electronics.

The capacitor is hooked up to the AC line via some 470 kΩ resistors. If you connect a capacitor to it, the neon bulb should light up. If not, it is open. When you push the button, you switch to DC, and you should be able to see one side of the neon bulb dim. If it doesn’t dim or doesn’t go all the way off, the capacitor is shorted or leaky. Supposedly, you could get a feel for the value of the capacitor by how long it took half of the bulb to go out. Makes you appreciate your digital capacitance meter, right?

Why Gone?

Why do you so rarely see neon bulbs today? They are still around, but the number of circuits you have where you have the requisite 100 V or so to drive them is not what it used to be. On top of that, as an indicator, an LED is usually a far better choice.

If you want negative resistance, your choices are less obvious. Some special diodes present a negative resistance in certain operating regimes, and you can coax the behavior from some transistors. However, as a matter of practicality, today, you’d probably just use an active switch and be done with it, especially for an oscillator circuit. Then again, if you really want an oscillator, as we are always reminded, you can do it with a 555, among other methods.

We have no doubt that [Smith] was right. There are probably at least 1,001 different uses, but you get the idea. Did you use an NE-2 for something interesting? Let us know about it in the comments. Still want more neon bulb circuits? We’ve seen plenty.