Tuesday, September 24, 2019

Olde Skool Blinken Lights

How did people make blinky lights before transistors, LEDs and the venerable 555 timer?

T'was the Night Before Christmas - Only Passive Parts

If you would venture outside during a sociable hour, you should see a very bright yellow lamp in the sky - the sun - yeah, that one - amazing isn't it?

The sun is a huge nuclear powered neon lamp - not the kind of thing you can make in your radio shack, but if you go and dig deep into the scary nether regions of your junk box, you may find a couple of NE2 neon bulbs.  (Put on some gloves, you may have to dig so deep you may cut a finger - I did... yeow...)

New bulbs only cost a few pence, so if you are scared of spiders, or other unspeakable things lurking in the bottom of your junk box, next time you order toys from Digikey or Mouser, include a handful of them.  Neons can make any new project look retro-cool with a mystical yellow glow.

A neon bulb starts to glow at about 90 Volt and stops glowing at about 60 Volt.  This non-linear behaviour can be used to make a flasher, using only passive parts, but the required high voltage capacitors can be expensive.  Look for grade X or X2 self healing polymer, or self extinguishing ballast capacitors - to avoid burning your house down.  Electronics is not a cheap hobby, even for the simplest looking things, but a fire costs rather more.

The anode and cathode don't glow.  The neon gas glows a short distance above the negative element.  The glowing region is therefore bigger than than the metal element, which gives a neon globe a unique charm.  With DC, it glows on one side and with AC, on both sides - actually, it flickers.

How does the flasher work?  Same as a two transistor multivibrator, but without the transistors...  The neon bulb that happens to turn on first at 90 Volt, drops its voltage to 60 Volt, causing the capacitor to pull the 2nd neon down to 30 Volt, extinguishing the lamp and then the capacitor charges up the other way, until the 2nd neon reaches 90 Volt and fires also, causing its voltage to drop to 60 Volt, pulling the 1st neon down to 30 Volt through the capacitor, which extinguishes it.  The capacitor then charges the other way again until the 1st neon reaches 90 Volt and fires, drops to 60 Volt, pulling the voltage on the 2nd neon down to 30 Volt, extinguishing it, ad infinitum.

One can make a flasher with only a single bulb, or many in a ring - then it will rotate, or go crazy - but that is expensive because of all the capacitors.  Note that 400V capacitors should not blow up immediately on a half wave rectified circuit (I measured 346V DC at my home), but since the mains voltage is awfully 'dirty', 600 or 1000V caps would last longer, though they cost more.

Eventually I found an old glass test tube and built 7 bulbs into it - the above random flasher.   I then needed a nice wooden base to hide the little PSU in and stand it up - three coasters glued together, with some holes drilled in the middle and top ones.

Please don't put an open neon circuit on a Christmas tree - an innocent kiddy may poke a finger at it - rather plonk your wonderful art creation in a little plastic box from Daiso, since the whole circuit is live and dangerous.

Next time you look at the sun, smile and wonder how life would have coped if it blinked and we lived on a giant disco ball...

A NE2 Cosmic Ray Particle Detector
The missus and I have property in Slovakia, which is sort of next door to the Chernobyl accident area in Ukraine.  I have therefore wondered about building a radiation detector and the Ukraine mess gives me a good excuse to try to detect high energy particles in a novel way. Doing it the usual way with a scintillation tube, would be boring!

Glow tubes can detect various kinds of radiation and if you aim a radar set at a glow tube, then it will light up, but a tiny little NE2 tube is not very sensitive - and very few people have a radar set in their radio shack.  However, one can bias the tube to just below its strike voltage - when a high energy (beta or gamma) particle then comes tootling along, the bulb may strike and keep glowing.  This may take a while, since the tube is very small and the odds of a hit is therefore low, but I would expect to see a strike maybe once or twice an hour, due to background radiation.  This requires testing the tubes and selecting ones with almost the same strike voltage - so you will need a big handful of them to sort through.

I don't smoke, so due to a lack of cigarette packets, I design circuits on sticky notes, mkay? 

A sticky note makes a good size hobby circuit.  It takes about 3 hours to find the parts in my junk box(es) and 15 minutes to build it...

I build high voltage circuits on Turret Boards - Digikey and Mouser have them in various sizes.  This keeps the wires safely far apart.  The resistors should be 1/2 Watt to handle the high voltage.  Small 1/4 Watt resistors may result in cryptic smoke signals when they arc over.  Always put a fuse in - I use Polyfuses.  When they trip, I don't have to go and dig for a new fuse, I just clear the fault and wait a little bit for it to cool down.  Always over design your circuits for much higher power than you need, then they will last a long time.

Normally, it doesn't make sense to wire neon tubes in parallel.  If you do, you won't get any more light, since only one bulb will turn on - the one that happens to have the lowest strike voltage - and the others will never do anything - they'll just sit there and sulk.  However, to count particles, one needs a physically large detector, so wiring ten or more of them in parallel, will increase the odds that any one of them will strike.  It will also be cool to know that a particle struck in a specific spot - like whack a mole.

So, here is my take on a $1 Particle Detector. "Eight cosmic-ray showers touch each square metre of the Earth's surface each second" -  https://www.nature.com/news/1998/980730/full/news980730-7.html  I have no idea whether this detector will actually work, but let's find out what happens.

Yus keep yer kotton picken fingers in yer pokkets when yer turn er on...

OK, I built it and it probably works, but little neon tubes are clearly not the world's most sensitive particle detector.  If it detects something, then you should probably run away!

That 330k resistor is best made of a 220k + 47k + 47k potentiometer.  Adjust the pot until the circuit oscillates, then back off till it juuust stops.  (One of the lamps in the detector array will be more sensitive than the others, no matter how well you select them).  Then, you need to find a source of very energetic beta and gamma particles - fly to Fukoshima, get a chunk of uranium ore, a gas lamp sock, a smoke detector, or your great grampaw's pocket watch...

This circuit is similar to the above astable circuit.  The first neon acts as an autoreset, so that it can detect the next particle, without one having to press a reset button. This reset method works remarkably well.  Now if I can just get it to actually detect something.  Well, OK, I guess NOT detecting anything is actually good.

I think what I need instead of the miniature tubes, is a big neon strobotron tube.  These were used as a car ignition timing lights for tuning prehistoric cars - I had one to tune my VW Combi camper van:

Since this kind of neon tube is much larger, I should have better luck with it and I should be able to misuse it in a similar way as a particle detector, since it is after all just a fancy neon tube, but where to get one today - I'm sure I don't have one in my junk box.  I would also need to make a very stable power supply, to be able to bias the tube very close to the trigger level, without getting false triggers due to noise.  Ah, the joys of analogue signals and noise...

Maybe there is something in my favourite high voltage toy store: https://tubes-store.com/product_info.php?products_id=1059&osCsid=6l9laavqcpubvhi05qqquodq15

PS - I was playing with Neons again and then thought that I should wire three bulbs in series - if any one strikes due to a particle, they should then all light up.  I haven't tried it yet...

Note that if you want to use an oscilloscope to debug a high voltage circuit, make sure that the 'scope is floating with the earth wire NOT connected and set the probe to x10, then use one hand only.

Nixie Night Light
A Nixie tube is a fancy neon bulb with multiple, shaped cathodes.  Same as the neon bulbs above, one can power a Nixie with a very simple, direct mains power supply using only a diode, capacitor and 100k anode resistor, provided that there is no human or computer interface that can be zapped by mains spikes.

Nixie - Seven of Nine

Ol Albert Einstein remarked:
A thing should be as simple as possible, but no simpler.

For a Nixie night light, you don't need a switching PSU, or a microcomputer.  You don't even need a circuit board.  The little NE2 tube I have in there is only to show me that the thing is plugged into the mains while I am debugging it.  Your night light should have either a NE2 or a Nixie - not both.

If you are happy with a single glowing digit, then the above can make a simple, mystical night light.  If you wire a Cadmiumsulphide photo cell in series with a 100k trimmer across the Nixie anode and cathode, then you will be able to find a point (about 65k) where it will turn off during the day and on at night.  Tweak it with an insulated screw driver, or you may get a surprise!

Nixie - Neon Night Light

If you build multiples, then you can wire each one with a different number, or symbol.  This is also a good way to breathe new life into a worn tube with one or more bad digits.

Nixie Shift Register
I would like to make a simple circuit that will step a Nixie through the ten numbers, without having to use a computer, or a double handful of transistors - just to make it either simple with a minimal number of parts, or a bit weird!

NE2 Ring Counter

A Nixie tube looks like multiple neon bulbs stuffed into a single bottle, which means that once one cathode struck, there is plasma swirling around the bottle, which makes it easier for the other cathodes to also strike and one can get into a ghosting situation where multiple cathodes light up at the same time.

Nixie - A Neon Bulb With Ten Cathodes

While it is sort of possible to make a Nixie flash in a relaxation oscillator using one cathode, it doesn't seem possible to make it work like a ring counter all on its own, stepping through all ten numbers, since there is just too much plasma in the tube and the cathodes are too big with uneven distances to the anode.

To make a relaxation oscillator, you got to bias the bulb such that it is barely off, with a large resistor on the cathode. Then when you put a capacitor across the cathode resistor, it will cause the bulb to light up and start flashing.  A Nixie however, has very large cathodes and they don't extinguish suddenly over the whole cathode.  A seven may keep glowing on its corner for example, then with a capacitor added, the 7 will breathe - go brighter and darker if you look closely, but one cannot call that flashing.  With an IN8-2 tube, I could only get the 1 and the dot to flash, but that is not really worth the bother.

It also doesn't seem possible to directly drive a Nixie tube with a NE2 ring counter, since little NE2 bulbs don't draw enough current to handle a Nixie as a load (trigger tubes will work, as in the figure below).  One could make opto-couplers by strapping Cadmiumsulphide photocells (About $1 each - bah humbug!) to the NE2 tubes and using those on the cathodes to fire the Nixie digits, but that is a bit of a hassle, though simpler than the trigger tube design.  I don't see any other practical transistorless way though.

There is just no simple way to make a Nixie count its toes and how about using a Nixie as a particle detector? Nope - doesn't work either!

A Few References
GE Glow Lamp Manual, 1965:

Counters and Counting Circuits, 1955:

Glow Transfer Counting Tubes:

Signalite, Ring Counters:

Hivac Cold Cathode Tubes, 1958:

Burroughs Nixie Indicator Tubes:

Many moons ago, people actually built calculators with these glow bulbs, long before anyone thought about the effects of cosmic rays on computer circuits and the clock speed of a super computer could be measured with a stop watch.

Have fun!


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