Specifically Nixie tubes. I have this counter that has a connector that provides me with all the active-low transistor drivers for each digit. What are the gotchas for using these lines with a uC input? I'm guessing that it's all very high impedance when the transistor is off, but what happens when digits switch? Isn't there a plasma in there and can there be high voltage spikes on the output during transitions?
Before you say "measure it", this is just conjectural as the counter is not here at the moment.
The voltage on the anodes of the tubes is in the 150-200V range, typically. But the 74141 can't do that so it has zeners built in. The outputs only go up to around 60V, that is enough to cause the Nixie element to extinguish.
When it is lit and the 74141 pulls low it depends on how big the Nixies are and thus how many mA each element draws. There can be 1-2V residual.
In a nutshell, no, this is not suitable to drive a uC. You'd need at least a resistive divider and tolerate hitting the substrate diodes once in a while. Any connection to these nodes would have to be really hi-Z or you might get a "sheen" on the Nixies.
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If discrete drivers are used, the high level output can go beyond 150V but the low level may be below 1V. Nevertheless you cannot rely on a voltage divider only but you'll have to take measures to protect the micros inputs. The high voltage to power the nixies is not always regulated and may vary due to load or mains variations.
Suppose the nixies are driven by the usual SN7441/SN74141 decoders, the general answer is still NO. The high-level output is about 60V, still much too high for a micro. The low level output can be as high as 2.5V too high for a reliable low level micro input. Of course you can build an interface circuit to overcome these problems but at a cost.
If you have the driver already, then it's probably cascoded -- hence active low. That's how I'd do it, cascode an MPSA45 or whatever, running from a ~2.5V reference. Perfect match to TTL, and regular 5V CMOS will drive it with insane speed.
Tim
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On a sunny day (Thu, 3 Nov 2011 03:00:21 +0100) it happened "petrus bitbyter" wrote in :
How about an opto in series with the Nixie segments? Optos work OK at low currents. Things wont change fast, so you can use a huge resistor in the output:
OK, maybe I wasn't 100% clear. The circuit is the counter inside a Tektronix 6R1. This is the digital display unit for a early 1960s sampling oscilloscope. Big. Heavy. Clunky. Bandwidth in the GHz range.
The 6R1 has 4 decimal Nixie tubes, powered by 300V through 82k. Each tube is driven by 10 open collector discrete transistors, themselves driven by discrete counters. Not an IC in sight!
There is a large military circular connector that gives me access to the open collectors. I believe this was for use with a printer, so maybe it made all decimals light up through the coils of whatever was inside the electromechanical printer?
If I use the same type of electronics as back then, I believe I can use a diode as a switch. If I select a low reverse leakage diode with also a high enough Vbr, that should isolate the 6R1 from my circuit and detect the low logic levels.
Would a humble 1N4007 do the trick? Or would it leak too much and light up the digits?
I'd connect the cathode to the 6R1 side, and the anode to my uC with a biasing circuit. VCE sat of about .2v in the counter, the diode conducts and I sense a "0", well, a 0.9 - 1.0V "low". Transistor goes open, the diode is reverse biased and disconnects the uC from the high voltage, with leakage current in the uA range. "high"
Make sense? Maybe toss in some protection circuitry on the uC side?
trr is frightfully long on those. Better candidate:
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Check where the guaranteed threshold for "low" is for your uC. Sometimes that's under 1V, a bit close for comfort.
That gets involved. You'd need diodes to VCC and GND, plus another series resistor from there to the uC. All that times 40 I guess. Plus possibly a shunt on the VCC rail if it's a very low power uC (or a load and some big caps). I'd see if it can't be handled by the substrate diodes.
I need 50 of them, 40 digits, 4 decimal points, 5 symbols (V,S,n,u,m) and a "print" command that means data valid. I think the "print" isn't hooked up to a tube though. Then I need 4 10:4 priority encoders to cut down on inputs to the uC.
I need 50 of them, 40 digits, 4 decimal points, 5 symbols (V,S,n,u,m) and a "print" command that means data valid. I think the "print" isn't hooked up to a tube though. Then I need 4 10:4 priority encoders to cut down on inputs to the uC.
I come to a pretty extensive circuit to cover the (all?) posibillities.
- The DA2JF81 as discussed.
- The Schottky to prevent the voltage to rise above Vcc
- R1 to make sure the voltage will not stay too low when it should be high. (I consider leakage currents too unreliable)
- The voltage divider to make sure the voltage to become low enough when it sould be low. (Only for Vcc | | | '-' | | ___ to | +--|___|--+--encoder | | | | .-. | | | | --- | | | --- | '-' | | | | -----+-------+---------+------GND created by Andy´s ASCII-Circuit v1.24.140803 Beta
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If you use a 147 type priority encoder you do not need the 0-outputs of the nixies. When the nine other outputs are inactive, the output of the encoder automatically decodes to zero. This way you also prevent the need for extra measures to detect the suppression of leading zeros.
Super. I came up with similar stuff. In one case, I simply lifted the ground of the 10:4 encoder by .7V with a diode. Another approach is to create a D/A with resistors after the diodes and use the uC DAC to read the result. I'm also wondering what can be done with transistors instead of diodes, high voltage parts.
I'm still chewing over various designs. Since I need so many inputs, I want to minimize parts count. I'm getting ready for my winter project.
Of course, there's always the modern approach. Point a webcam at each tube, and put an embedded Linux ARM system on each camera to process the image, send everything to the cloud, and receive an email on your phone with the number.
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