Dekatron Analog Meter with Analog Parts

[threeneurons]

I designed my "Dekatron DoHickie" over 10 years ago. It uses a AVR Tiny24 microcontroller, to sample analog data, thru its ADC, then send a appropriate number of pulses to a dekatron, so that the tube mimics, a magic eye. Though cost effective in the modern era, like many old farts, using a microcontroller on all projects, just rubs me the wrong way.

I wondered, if I could pull it off with a simpler circuit. Here's what I came up with, that uses a "Jelly Bean" LM324, and a bunch of descrete parts:

https://www.youtube.com/watch?v=q9y0P83Tv7Y

[Ian]

Sweet

<Dekatron_A-VUs.jpg>

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<Dekatron_A-VUs.jpg>

[threeneurons]

Idler circuit added. Problem with ALL COLD CATHODE TUBES is that they're subject to cathode poisoning. This happens when metal sputtered off the active electrode (the one that's glowing) is deposited on an inactive electrode, and that electrode is never, or infrequently, activated, so it never has opportunity to shake off that crud. Eventually, when it is, its turn, it will not fully illuminate, as with nixies, or not count properly, as with dekatrons, becoming "sticky".

The old dohickie, being a microcontrolled device, goes into pendulum mode, when no audio is present, for more than a few seconds; Idle mode. The analog version, I just posted a few days ago, HAD no such feature, and the glow would just be parked at K0, spewing its crud (sputtering) onto adjacent cathodes. For short term (under a couple of hours), this isn't a problem, but it will be, if the circuit is kept on continuously. So I added an idler circuit, that passes the 800Hz, to the guides, when in "Idle", for more than ~3 seconds. Here's a video, with the idler installed:

https://youtu.be/vNUIKaWb6GQ

The schematic for the "Idler":

And where to insert it in the original circuit:

Note its installed across R25. (A) is the input. High is Idle. Low pulses indicate activity. (B) is the output. If no activity is present, for more than ~3 seconds, The idler transistor (Q101) is turned ON, which will also turn ON Q4, passing the 800Hz, from U1-7 to the guide circuit.

[Mac Doktor]

On Aug 9, 2021, at 3:44 PM, threeneurons <yumik...@gmail.com> wrote:

Idler circuit added. 

Great stuff, Mike. Keep it up.

Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com

“...the book said something astonishing, a very big thought. The stars, it said, were suns but very far away. The Sun was a star but close up.”—Carl Sagan, "The Backbone Of Night", Cosmos, 1980

[Dekatron42]

Lovely!

Would it be possible to add the same circuit twice but change the spin direction to get a simulat display to the original one where it swings both eays, I think some kind of multiplexong is needed then so that only one driving stage at the time drives the guides?

This might also make it possible to display both right and left audio signal on the same dekatron.

/Martin

[Dekatron42]

Sorry for my poor iPhone auto correct English above, it obviously didn't work so well, but I think you get the idea...... ;)

/Martin

[Alex]

This is a really nice design from a analogue electronic theory standpoint, something which I always have to work fairly hard to try and get my head around.

What would be really great would be if you could give an overview (just a fairly high level of each of the blocks / op-amp stages) into what each section does?

I can work out various bits but, for instance, the lower op-amp stage leaves me pondering...

Always love it when a service manual has a "Circuit Theory" or "Principles of operation" section adjacent to the schematic, gives you the opportunity to just sit and get your head around something your repairing. I have just finished recapping, retrimming and tidying up my old Thurlby PL series supplies and the design for that can be a bit uncouth in some areas...

Impressive stuff though, I would not know where to start (probably with a micro in todays world, sadly...)

- Alex

On Friday, 6 August 2021 at 05:36:07 UTC+1 threeneurons wrote:

I designed my "Dekatron DoHickie" over 10 years ago. It uses a AVR Tiny24 microcontroller, to sample analog data, thru its ADC, then send a appropriate number of pulses to a dekatron, so that the tube mimics, a magic eye. Though cost effective in the modern era, like many old farts, using a microcontroller on all projects, just rubs me the wrong way.

I wondered, if I could pull it off with a simpler circuit. Here's what I came up with, that uses a "Jelly Bean" LM324, and a bunch of descrete parts:

...

[Dekatron42]

In the video "threeneurons" explains some of the information you are asking for, and there are small texts beside the different stages like AUDIO, RAMP, 800HZ, /RST, RTN, /G1 & /G2 that corresponds to the video explanation.

/Martin

[threeneurons]

Having it mimick the dohickie, would require a toggle flip flop, to switch directions on alternate counts. It could be either a descrete 2 transistor stage, with associate resistors and caps, or a chip such as a 4013. Making respond to stereo would also mean ping-pong-ing between two audio channels. definitely turning into a "Rube Goldberg" contraption. If I ever decide to go that route, using a uC would be the simple way out. Just add another audio input stage to the current doHickie. It already has analog switching internal to a Tiny24 uC. The "fun" way, would be to use only tubes. Descrete circuit, only using tubes.

Maybe in the future ...

[threeneurons]

Lets grab that schematic:

The first stage (pins  1,2,3) is just a microphone amp, with a gain ~200. The output is level shifted by way of C12 & D8, such that the negative going peak, sits ~0V (Gnd). Its fed into an envelope detector (aka AM demodulator; D7 C13, R15, R16), that feeds the peak audio level to the second stage (pins 12,13, 14). This is a comparator stage, where its compared to a ramp voltage generated by stage (pins 9, 10, 11), at a rough frequency of ~40Hz.

This generates the "RESET" pulse. For quiet audio signals this is mostly high, and for loud audio, its mostly low. The "RESET" signal has two functions. (1) is to force reset the dekatron to cathode K0, when it transitions low to high, by way of Q1, and its associated circuitry. The result is K0 dropping ~170V negative compared to any other cathode on the dekatron, forcing the glow to that electrode. (2) The RESET pulse, also only, lets pulses from the 800Hz squarewave generator (pins 5,6,7 of U1) pass onto the Guide pulse generator, by gating Q4 on. The width of the RESET signal (both high and low) is determined by the audio level. The resulting signal CNT, has fewer pulse when quiet, and more pulses when louder. This signal directly steps Q2 (for G1/) and slight delayed Q3 (for G2/) so that the dekatron is stepped accordingly. The glow will stop at the appropriate cathode, per the number of pulses. Then the reset, RST/ is issued sending the glow back to K0.

Rinse and repeat for the next sample.

The Idler circuit is not included on this explanation, as this is only for its most basic function. 

[Mac Doktor]

On Aug 11, 2021, at 6:45 AM, Alex <ajlg...@gmail.com> wrote:

I can work out various bits but, for instance, the lower op-amp stage leaves me pondering...

Always love it when a service manual has a "Circuit Theory" or "Principles of operation" section adjacent to the schematic

What I like are schematics that I can "read". Despite being visually compromised I've always been able to "acquire" visual information more rapidly than most people with 20/20 eyesight. In High School I could read 1000 words per minute. These days not so much.

Mike's schematics are GREAT and his use of color is a rare thing. They're rather busy due to the use of so many discrete components so this really helps. I use color all the time for outlines, spreadsheets and databases. It helps me immensely.

The worst thing about Eagle is the schematic line art. Too small with too much white space around everything. European schematics that use rectangles instead of the resistor symbol are a problem for me. My visual system can just deal with it as long as the values are in the boxes. I've schematics that have them outside the box and that simply doesn't work for me.

The service manual for the RadioShack MG-1 analog synthesizer is a work of art (the whole manual, not just the schematics and block diagrams). It's work looking at even if you don't know what I'm talking about. A model for us all.

Terry Bowman, KA4HJH
"The Mac Doctor"

"If only you could see what I've seen with your eyes."—Roy Batty, Blade Runner

[Alex]

Hi Mike,

Many thanks for taking the time to walk around the circuit, that is most appreciated and perfectly annotates your timing diagram...

WRT service manuals, I bought a SM for early bruel & kjaer 1618 Band Pass Filter and that includes some lovely colourful block diagrams as well as schematics... The PDF does not really do them justice though....

Thanks again and all the best,

- Alex

[Bill Notfaded]

I also have a bunch of your boards Mike.  I really like your documentation too.  At one point while I think you were maybe on a scuba trip I was afraid maybe you weren't ever coming back.  I didn't know at the time and sent you many ebay messages.  Glad to see you're doing more dekatron stuff!

Bill

[Bill Notfaded]

Also I have a huge collection of Dekatron tubes... I hope to be able to eventually do more with them.

Bill