"Does a lit-up Nixie tube in a forest wear out even if there's no-one to see it?"
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| Figure 1: The "Black'n'wood" Nixie Clock (blue back-light turned on) with the PIR (Passive InfraRed) sensor to the right. With no detected movement in the room, the high voltage supply turns off, reducing wear on the tubes. Click on the image for a larger version. |
This article has not so much to do with this specific model of Nixie clock, but rather adding a PIR (Passive InfraRed) sensor to turn off the display when there is no-one in the room to look at it.
They wear out!
Nixie clocks and other neon-glow displays (e.g. Panaplex), along with VFD (Vacuum Florescent), Numitron and CRTs have a "wear out" mechanism when they are operating: In other words, when they are on, they are slowly degrading.
By limiting the "on" time of such displays only to when someone is likely able to see it one can prolong its overall useful life in many cases. As many of these devices are no longer made, the supply of "new, old stock" tubes is very limited and what there is still available is becoming more expensive year upon year.
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A few years ago - at a swapmeet - I picked up a "Nixie" 1 clock - the "Black'n'wood" by Nocrotec. It was in a plastic bag with loose parts, but for only $20 I couldn't resist!
Getting it home I found the problem: One of the elements of the "10s of hours" tube was shorted to the anode and very visual close inspection revealed that two internal wires were touching each other. A bit of "percussive repair" (banging it on the table) moved the two wires away from each other and the tube was once again usable. I suspect that the problem was originally caused by the tube experiencing mechanical shock.
The current limiting resistor associated with this same element was burned, so replacing it returned the clock to full operation.
Over the next year or two the clock has continued to work fine - although the display got "glitchy" and began to dim - but the biggest clue was that the flashing colon neon lights were flickering but this was quickly traced to the failure of the main high voltage filter capacitor on the 180 volt supply: These problems went away with the replacement of that capacitor - but I digress.
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All of this brings me to the main topic of this article: Reducing the wear and tear of the neon tubes.
Sitting unused, in a box, many "vacuum" devices (I'll include neon indicators and other cold-cathode tubes in this category even though they are not strictly "vacuum" devices) have the property that laying on the shelf, they (usually) have little/no degradation over time. There are many (now) century-old devices that have been sitting around that work just as well as they did when they were made - the caveat that they haven't been compromised in some manner (e.g. broken, corrosion, failure of a seal, etc.)
Like most "vacuum bulb" devices - which include thermionic tubes/valves (with a filament) and those without a filament - like neon indicators - there is a definite lifetime related to acceptable performance when they are operating. For normal tubes/valves, the emission from the filament/cathode will inevitably drop over time - often due to gradual degradation of emissivity and the "work function" of the cathode. "Cold cathode" devices (e.g. those without a filament) like neon indicators also suffer degradation - and the causes are broadly similar: Degradation of the materials and contamination.
In the case of the neon indicators, one major cause of degradation is the inevitable "blasting" of atoms from the electrodes' surfaces (called "sputtering") where the metal gets liberated - only to redeposit elsewhere. The most obvious result of this is that the inside of the of the glass envelope darkens, reducing the brightness of the brightness of the display - but even if the glass were to remain clear, this and other effects conspire to reduce the brightness overall.
Running neon indicators such as these "Nixies" at lower than maximum current will reduce these effects - but what about not running them at all?
Nixies are meant to be seen!
The entire point of a "Nixie" clock is that it is cool to look at - but what if no-one is there to see it? If we operate a Nixie tube in a forest and no-one is there to see it, it still wears out!
The goal, therefore, is to turn off the display when no-one is in the room.
Turning off the display
First, we need to figure out how to turn off the display, presuming that the clock or other device has no obvious means of doing so (e.g. there is no "turn of the display" switch or pin). Two options came to mind:
Approach #1: Removing the power
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| Figure 2: Interface with HV converter. PNP transistor "Qa", when its base is pulled low, injects current into Pin 5 of the HV converter chip, effectively turning it off. Click on the image for a larger version. |
After I'd first repaired the clock, I applied correction factors via its menu and got it to stay within a fraction of a second per day - but I had assumed that this was done in the clock module itself (as some Dallas/Maxim devices are equipped) but this was not so: While the timekeeping module continued to run on the battery, the firmware on the clock itself - being powered down - was obviously not and the calibration that I'd applied was missing, explaining why it was keeping time so badly.
To be sure, I could have likely done something to "fix" this (e.g. trim the oscillator with a tuning capacitor, added a GPS module to auto-set the clock, etc.) but since the clock - while it was running normally - was very stable, I decided to try another approach.
Approach #2: Turning off the high voltage
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| Figure 3: A top-down view of the components added to the high voltage switching converter IC to produce the circuit depicted in Figure 2, above. Click on the image for a larger version. |
This clock uses two switching supplies - the first one converts the nominal 12 volts down to 5 volts for the logic, but the second one - near the high-voltage capacitors - is the one that produces the (approximately) 180 volts for the Nixies. Both of these use a common type of switching supply controller chip - the MC34063 - and since the implementation of these chips is spelled out in the data sheets, we have some insight as to how they work.
Fooling the voltage converter into shutting down
Like most any voltage regulator or converter, it monitors its own output voltage - typically through a pair of resistors that are chosen to divide the desired output voltage down something close to the chip's on-board reference voltage - in the case of the MC34063, 1.25 volts, which is applied to its pin #5: If the voltage on this pin is lower than 1.25 volts, the switching converter adjusts the voltage higher but if the voltage is higher, it reduces the voltage.
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| Figure 4: A side view of the high voltage switching converter IC showing the components added to it to allow the 180 volt supply feeding the Nixie tubes to be turned off. Click on the image for a larger version. |
As can be seen from the diagram in Figure 2, I tacked a PNP transistor ("Qa") - and three resistors - across several of the pins of the MC34063 high voltage converter. Using a hot soldering iron and "pre-forming" the shape of the components to match the locations of the needed IC pins, it's possible to attach this simple circuit directly to pins 5 and 6 of the high voltage switching converter IC without risk of damage to the chip or other, nearby components.
If the base of Qa, the PNP transistor (I used a 2N3906) is pulled to ground (via the 10k resistor, Rc), it will turn on - and with its emitter connected to Pin 6 of the MC34063 - its power supply pin - it will apply current, via the 3.3k resistor (Ra), to Pin 5 of the MC34063, dragging the voltage on this pin up. When this happens, the MC34063 will "think" that the voltage is too high and effectively turn off. If the base of Qa is allowed to float (nothing connected to it), this transistor is biased off by the 100k resistor (Rb) between the emitter and base and the high voltage converter will run normally (e.g. the display will be on).
Any converter will do
While this article shows the example using the MC34063, this sort of technique could be applied to about any switching-type of voltage converter. Determining a bit about the circuit itself could be done simply by referring to the data sheet of the chip that was used - as was done here - but it could also be done with a bit of reverse-engineering.
It would have also been possible to find the power supply lead feeding the voltage converter - in this case, about 12 volts from the external power supply - and interrupt it, perhaps with a relay, a PNP transistor or a P-channel FET.
If you are using an "old-school" power supply that does NOT some sort of switching converter, perhaps consisting of a high-voltage winding, rectifier and capacitor to develop the high voltage for the tubes, your best option may be to use a relay to open the supply - preferably interrupting the pre-rectified AC side, directly. At such voltages switching DC is best avoided due to the possibility of contact-damaging arcs: Switching on the AC side (or between the rectifier and the first filter capacitor) is better in that the voltage falls to zero twice per cycle of the AC waveform and any arcing that does occur will extinguish at that time.
Getting the connection outside the clock
In perusing the manual for this clock I noticed that the 6 pin mini-DIN connector - intended for connection to an external GPS or DCF77 radio receiver - not only had ground (Pin 1) and power (Pin 2 for 5 volts), but also an unused pin (#4) that I verified to be floating - and to this I connected the end of the 10k resistor (Rc) to this pin with a flying lead inside the clock. With the three needed signals (power, ground and the "disable" line) on the mini-DIN connector, I was ready to connect it to a sensor.
A PIR sensor to turn it off and on
A PIR (Passive InfraRed) sensor fits the bill for this task quite well - and they are inexpensive. These devices use pyroelectric detectors to detect heat from warm, moving objects - which includes us humans - by focusing deep infrared energy onto a pair of sensing surfaces from an array of Fresnel lenses. A moving object in the field of view will cause a difference in the pair that can reliably indicate that an object in view was in motion.
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| Figure 5: This circuit was added to the output of the PIR to present an open-collector to allow transistor Qa in Figure 2 to properly turn off when the HV was to be turned on. Click on the image for a larger version. |
- Power. This particular PIR sensor was happy to operate from between 5 and 12 volts, having an onboard 3.3 volt regulator.
- Ground. This is the negative supply and the reference to the output signal.
- Output. This output pin goes "high" (to 3.3 volts) when motion is detected.
This sensor also has two potentiometer adjustments:
- Delay - Which is the amount of time the output will go "high" when motion is detected.
- Sensitivity - As the name implies, this sets the degree to which the device reacts to movement.
There's
also a jumper: The piece of paper that came with the PIR sensor implies that
this determines if the output is "re-triggerable" (the default setting) or not. Being
re-triggerable means that motion will reset the delay timer whenever it's
detected: If it were not re-triggerable, the delay time would be reset only after the delay had expired and the output had turned off. Clearly, we want to use the "re-triggerable" setting so that any movement simply extends the timer.
It turns out that moving the jumper on this board from its factory position stopped the unit from working at all and a quick bit of reverse engineering revealed that whoever designed this board simply connected it to the wrong place - probably due to poor reverse-engineering on the part of the "designer" of this (likely cloned) circuit board. Fortunately, the wiring of the circuit is such that it is already wired as being re-triggerable, so we can leave it alone.
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| Figure 6: Perhaps a bit messy, but this is the two-transistor circuit depicted in Figure 5, tacked to the pins of the PIR module's circuit board. The three-conductor cable that connects to the clock via the mini-DIN connector can just be seen. Click on the image for a larger version. |
To fix this we need to provide an open-collector output - but preserving the polarity of the output - which is to say that we want it to be an open collector to allow the base of "Qa" to float high when movement is detected, but go to ground and turn on "Qa" when it is not. To accomplish this, transistor "Qb" takes the "high-active" pulse from the PIR and inverts it - and then transistor "Qc" will invert it yet again, but this time with the needed open collector. "Qb" and "Qc" can be practically any NPN transistor - I used 2N3904 types in this cicruit.
In experimenting with this PIR sensor module, I noted that when set to "maximum" the "on" time from the output was about 150 seconds - about 2.5 minutes. I was able to iteratively adjust the "sensitivity" control incrementally upwards until I found a setting that reliably detected even slight motion in the room - but seemed not to randomly "false" trigger, the result being that when I was in the room - say watching TV - it would stay on most of the time, but be easily (re)triggered by even slight movements.
Figure 1 shows the clock with the PIR sensor next to it, sitting on the shelf below my TV. I purposely set the PIR sensor back from the edge of the shelf - not just to line up with the front of the clock, but to obscure part of the view of the floor to reduce the probability that a cat would trigger it: Since cats sleep most of the time, anyway, their occasionally triggering the PIR sensor isn't a big deal.
Reducing "wear-out"
For a "cold cathode" tube like a Nixie, turning the high voltage on and off is not a stress on the tube: After all, simply changing the segments to show the time is also turning on/off parts of the tube. With no voltage present, there is no electron bombardment on the elements within the tube and thus, it will not experience wear.
Powering down other devices in the absence of "viewers"
There are other types of "antique" displays that may benefit from having some sort of "human presence detector". For example, a VFD (Vacuum Fluorescent Display) has a wear-out mechanism similar to a Nixie in that electron bombardment will gradually degrade the phosphors - and the cathode (filament) may also lose emission.
Similarly, if one has a "Scope Clock" - a vector-graphics clock that uses an oscilloscope tube to show the time - it, too, will wear out over time, the emission of the from the cathode will drop over time - not to mention possible burning of the phosphor.
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| Figure 7: An example of a "Scope Clock" - a vector-graphic clock display shown on an oscilloscope using a cathode-ray tube (CRT). In this photo, the CRT in a Cushman CE-50A communications monitor is being used to demonstrate, but an old, analog oscilloscope would work as a "permanent" fixture and blanking it when no-one is looking would extend the life of increasingly-rare CRTs. Click on the image for a larger version |
For these two examples, a bit of care should be taken in that while removing the high voltage source may partially remove the wear-out mechanism (e.g. degradation of phosphors) other steps would be required to mitigate the diminution of filament emission over time. This could include turning off the filament - or at the very least, reducing its voltage, perhaps in steps, in the absence of the anode voltage. If grid voltages happen to be present, those, too, should be carefully considered to see if they should be removed when the high voltage is turned off - but since these often share the same power supply, this problem may take care of itself.
In so-doing - and depending on the nature of the display tube - other precautions may also be required (e.g. removing all other voltage prior to powering down the filament) to avoid damage - and frequent power-cycling of the filament itself may be an issue: These are potential issues that should be considered - but are beyond the purview of this article.
Footnote:
- The name "Nixie" is a trademark of Burroughs Corp. to describe certain types of neon-glow indicators. Like many trademarks, it's become "genericized" - as done in this article - and nowadays it's commonly used to denote all types of similar cold-cathode glow devices in which each digit is indicated by a separate element within the tube in the shape of the desired numeral or symbol - whether they were made by the original trademark holder or not.
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This article stolen from ka7oei.blogspot.com
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