Neon lamps are fun to play with. These old-school indicators were once heavily utilized in many types of equipment for indication purposes but now seem largely relegated to mains voltage indication duties. Here’s a fun video by [Ashish Derhgawen], discussing the photoelectric effect of neon lamps with some simple demonstrations.
[Ashish] demonstrates the well-known photoelectric effect by triggering a sub-biased neon lamp with visible light from an LED. Neon bulbs work on the principle of voltage-induced ionization, creating a visible glowing plasma. If the applied voltage is high enough, around 60 to 80 V, electrons get knocked off the neutral neon atoms. The now free electrons, roaming around highly energized, will eventually come across a neon ion (missing an electron) and recombine to make it neutral again.
The results are a lower total energy state, and the difference in energy is resolved by the emission of a photon of light, which, in the case of neon, is a dull reddish-orange. Nothing unusual there. However, nothing will happen if the applied voltage bias is just below this device-specific threshold. There’s not enough energy to strip electrons.
Apply an external light source, and this threshold can be exceeded. The photons from the LED are just energetic enough to strip a small number of electrons from the surface of the electrodes, and this causes a cascade, or avalanche effect, lighting up the plasma and turning on the neon lamp. Take away the external light source, and it dies down and goes dark.
The video also shows an interesting effect due to the wavelength of applied light. The photon energy needed to release an electron depends on the atom it strikes. Neon bulbs have all manner of electrode materials. [Ashish] shows that a particular neon lamp can be excited to emit a specific wavelength corresponding to a certain energy level. With some materials science work, this can then be used to ascertain what the electrode material is. Finally, the video shows some simple astable and relaxation oscillators initiated by light, making us wonder if one neon bulb could activate some neighboring bulbs and create a neat wave propagation effect for some electrode material and bias levels? You can see in the video that when the spectrum thrown from the prism is passed over the bulb, it illuminates in the orange section. So this could work. If you know, then do let us know with some examples.
Neon light hacks are plentiful around here. Neon lamps have many other uses beyond indication, even detecting sound. Of course, they look nice, but driving them is a hassle. Why not just fake the look with modern tech?
This is basicaly a geiger “counter” :-) It will probably detect ionizing radiation. But it gets more practical if you wrap it in tin foil (or some similar material that blocks light to prevent discharge being initiated by light, leaving ionizing radiation as only option to cause discharge)
I’ve done this and it’s not all that sensitive, unfortunately, but it is cool that if you’re really careful you can tune it to trigger off a green laser but not off a red laser.
Many many years ago a friend of mine had a very powerful green laser that he used for laser shows, this was before there was a ton of regulations on them. One of his fun party tricks post the show was he could put a green party balloon in front of a red one and pop the red one through the green one, or blow up a red balloon inside a green one and pop the red one but not the green one. Kind of like a coin shrinker, a bit of a parlor trick but a darn good one.
There are and have been commercial products that use neon lamps as a detector. Most grind the glass down over some part to improve sensitivity, but none are as sensitive as a true GM tube. The dead thin window of a general purpose GM tube, and the correct gas mixture and pressure make it sensitive to radiation a neon lamp can not be. The devices using neon tube detectors don’t get much if any sensitivity to alpha.
Yes, I have tried a few (professional need for detectors,management need to minimize cost. Welcome to engineering)
Older devices with neon indicators often had ignition issues as they aged, and many went through , or persisted, in a state where the lamp would only ignite if there was sufficient light in the environment.
I had several power strip switches go there, and a neon wall light switch do it. The light switch was OK when you shut the lights off – the neon lamp ignited (and you could them find the switch in the dark) as most lights didn’t immediately go dark (Incandescent and fluorescent at the time).
But try to find the switch after a power failure….
I have an old power strip with a neon lamp in the rocker switch. It doesn’t ignite, but a flashlight will cause it to do so. At least it used to because I just tried it and it now only flickers WHILE the flashlight is being used to illuminate it.
That might explain why my old kitchen fluorescent bulb has nyctophobia. They come on right away during the daylight but at night, they often take a few minutes to work. If I shine the flashlight toward the end with the starter, it comes on.
I’ve been meaning to replace that light fixture, it is over 50 years old.
“Take away the external light source, and it dies down and goes dark.”
No, it doesn’t, as shown in the video.
Literal text from the video: “Once the lamp is on, it remains lit, because neon lamps have a much lower turnoff voltage”
Er, yes. Quite right. Would you know, I built some memory circuits out of neon lights a few years ago. You’d think i’d remember…
It’ll work that way on AC, like the neon indicator of a power strip. Well, or more likely still flicker, but tend to be mostly off in the dark and mostly on in the light.
Very much a basic latching memory bit. Always cool to see what we can do with older tech.
I have a switch in my kitchen where the neon has aged so that it doesn’t light.
But it will light if you touch the plastic filter that covers it so it’s not just the photoelectric effect, it must be picking up hum.
What, no mention of the radioactive elements included in neon lamps (and old fluorescent lamp starters)?
Ionizing radiation from minute amounts of Promethium-147 and Krypton-85 (and others) included in the “good” neon lamps promote ionization of the gas to ignite the lamp, even in the absence of external light.
However, 147Pm has a half life of just 2.6 years, so old neon lamps struggle to start in the dark. 85Kr’s half life is 11 years, so is a bit better.
I always thought of replacing the filament in a vacuum tube with a uv led, maybe with some emissive phosphor grid, since uv light can move electrons
This would allow you to make a much more efficient tube, plus the “filament” it self could be controlled via logic level and, would open different kinda tube characteristics….
And no tube heat up time
You could experiment using a photomultiplier tube!
“I always thought of replacing the filament in a vacuum tube with a uv led, maybe with some emissive phosphor grid, since uv light can move electrons”
It’s a clever conceptual idea, but it’s difficult to imagine any LED light source of reasonable size and power consumption, producing UV of sufficient intensity, to liberate the same quantity of free electrons as a simple filament.
That said, I remember reading a paper a number of years ago where the author proposed to make microscopic vacuum tubes in silicon, using common IC and micro-machining techniques. In this case, the “tube’s” cathode was simply a silicon needle. Apparently, when a needle point gets down to a few atoms in width, local field intensity is such that, in a vacuum, electrons fly off at very low bias voltages.
Well you could use a uv laser
Doesn’t take much uv light to discharge a capacitor or strike a florescent bulb, a 5w 2 meter radio could set off my florescent lamps in the winter when it was a bit cold for the electric start
Electrons can flow, but the amount of uv light and emissions rate would in a sense act like a resistor then
In that case a diode tube can still act like a full transistor
Instead of grid you’d switch the filament
“That said, I remember reading a paper a number of years ago where the author proposed to make microscopic vacuum tubes in silicon, ”
IIRC, Hackaday reported on that years ago…
Thanks for a great video — quantum physics taught with inexpensive apparatus, clearly explained!
Thorium is also used in older lamps to reduce ionization potential, and in vacuum tube (valve) cathodes and filaments. Does the low level half-life ~1.4e10 years) of alpha emission contribute to lamp ignition, too?
[BTW, another low-cost demonstration of quantum physics is an Esaki (tunnel) diode oscillator. That component exhibits negative resistance because higher-potential electrons are more localized, and have more trouble crossing the semiconductor P-N barrier.]
Glad you liked the video! I have experimented with negative resistance oscillators using galvanized sheet metal. I have some videos about it on my YouTube channel. There is also a Hackaday post about it –
https://hackaday.com/2024/03/05/making-a-crystodyne-radio-with-zinc-oxide-and-cats-whiskers/
I vaguely remember this effect being used in an old computer system as memory.
Also, doesn’t a Decatron 10 digit counter tube work this way too?
IDK, I studied this stuff almost three caveman lifetimes ago, but something fell out of the fuz I call my memory while reading this.
The google brain helped me, they used a three pole neon tube for this stuff, kind of a triode.
There were telephone exchange call accounting systems, and systems like that, using neon tube memory.
“I vaguely remember this effect being used in an old computer system as memory.”
Perhaps. But I also have distinct recollection of a story of a vacuum tube computer (with neon bit indicators) which was set up to grind through some calculations overnight… and then crashed.
The next day, they did troubleshooting and diagnostics, found nothing wrong, and loaded the problem again. It ran happily until, once again, crashing at night.
Long story short, the issue was traced to the change in ignition voltage in neon bulbs when operating in a well-lit room vs a dark one.
The “solution” was to duct tape the light switch in the computer room to the “on” position.
I didn’t know a resistor was a hassle, lol. Oh wait, I forgot the implicit assumption that every project runs on five volts and wall outlets are verboten.
“I didn’t know a resistor was a hassle, lol. Oh wait, I forgot the implicit assumption that every project runs on five volts and wall outlets are verboten.”
To your point… given present sensibilities, it’s remarkable that hams once routinely constructed transmitters and amplifiers with final B+ potentials up to the thousands-of-volts range.
Neon bulbs are convenient to use on mains voltage equipment. They use less current than LEDs and can use lower wattage dropping resistors. That’s why they are still commonly used as indicators.
It’s not too hard to make a boost converter to run neon bulbs on low voltage equipment, but running them on DC only lights up one of the electrodes and reduces the lifespan of the bulb. It just makes much more sense to use LEDs for low voltage stuff.
Even better: Don’t use a resistor. Since it’s AC, you can use a capacitor to limit current to the neon bulb. No power lost to heat in the capacitor! If you’d normally use a 22k resistor you can use a 100 nF capacitor instead. (Appropriately rated for voltage, of course — don’t jam a 50 V 100 nF bypass cap in there and hope for the best).
I’m firing up my Radio Shack Goofy Light right now. A P-box Science Fair kit with 5 neon lamps that flashed sequentially or randomly. Too much fun as a kid.
This effect is used to detect fires early, for example on buildings with thatched roofs. It works a lot better than you would expect when first learning about the concept. I’ve worked with implementations that detect a lighter flame at 10m distance within a fraction of a second.