A (simple) WWVB loop amplifier for radio-controlled clocks

Note:

The techniques described below should work - with only minor adaptation - for any "Longwave" time signal used by these radio-controlled (non-GPS) clocks - not only WWVB, but DCF77, MSF, BPC and both JJY signals as well.

* * * * *

Last year I moved a bunch of SDRs (KiwiSDRs, RTL-SDR) and a bunch of network gear to a new shelf in my shack, but this placed them much closer to the wall on which I'd previously mounted the two "Atomic" (e.g. radio-controlled) clocks which had been there - and operating - for years.  Since then, they hadn't been able to reliably synchronize to the 60 kHz WWVB signal out of Fort Collins, Colorado.

Figure 1:
The two radio clocks surrounded by
the four-turn loop, near a number of
pieces of "noisy"equipment.
The top clock is set to UTC and
the bottom for local time.
Click for a larger version.

While annoying, I wasn't terribly surprised.  There are several switch-mode power supplies involved in the aformentioned gear and it's not uncommon for them to operate in the 30-60kHz range, offering the potential of "jamming" the receivers.  As both the location of these clocks - and the nearby gear - is convenient, I wasn't too inclined to move them again and initial efforts to "filter" the switching power supplies didn't really help - but I wasn't surprised about this, either, since it's likely direct coupling of their magnetic fields that is the culprit rather than any electrostatic field as the clocks themselves use ferrite loopstick antennas sensitive to just the H-field.

A solution

 Many years ago a friend came to me to solve a similar problem in a downtown Salt Lake office building where the WWVB clocks in a conference room never synchronized and I constructed the remote loop and amplifier/coupling system, described here:

• Getting "Atomic" (WWVB) clocks to work indoors and in weak signal areas - LINK 

In short, a rooftop loop antenna amplified the signal and it was conveyed into the room with the clocks where it was further amplified and then, using inductive loops placed in the proximity of the clocks.  This is how the WWVB signal was coupled to them.  To my knowledge, this system worked for many years (well over a decade) and for all I know, it may still be in use.

 Why revisit?

I've tackled this type of problem before - but I decided to revisit it as the circumstances are slightly different:  I already had a signal source as noted below plus I wanted to see if I could do this with more commonly-available components in a simpler manner.

While I don't have a WWVB loop on my roof, I do have a dedicated LF E-field whip antenna - a 40 year old LF Engineering LF-400B with integrated low-pass filter.  This antenna has been on the roof wherever I have lived almost continuously since I purchased it in the mid-late 1980s and with a few repairs over the years, it still works well, having been on the roof of my current house for several decades.  Its use for LF reception as described on the following page:

• A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - LINK 

The fact that I already had an LF/VLF receive antenna system meant that I had a "clean" source for WWVB, and other devices that receive signals below 500 kHz (e.g. LF receivers for 630 and 2200 meter operation and my Blitzortung "Blue" receiver) and I decided to add one more to the list.

Other types of outdoor antennas 

Note that the circuit described here should work well with other types of active antennas including E-field types such as the PA0RDT "Mini-Whip" and the DX Engineering ARAV3 - to name but two.  An amplified loop such as the Wellbrook and similar will work, provided that it is not oriented such that the desired time station's transmitter is not in its nulls.

Buffer/Amplifier

Through back-of-the-envelope calculations I figured that the already-amplified signal from the active whip needed another 15dB or so of boost and it could then be applied to a loop of wire around the WWVB clocks on my wall.  One thing that helps greatly is that the WWVB signal is extremely strong here in northern Utah - on the order of 5mV/meter or so - and connecting an oscilloscope to my LF-400B whip's signal output showed that the amplitude-modulated time code of the 60 kHz signal from WWVB was visible among the many others.

What I needed to do was to tap off the signal (e.g. "bridge" the connection) from the existing coaxial cable without affecting was was being sent to the other devices using it, amplify it. and apply it to the loop - and I did this "tap" using a BNC "Tee" connector on my antenna feed.

The circuit diagram below gives more details:

Figure 2:
The schematic of the loop buffer/amplifier/driver showing the isolation from the power supply
via L1, the buffer circuit of Q1 and the amplifier and loop driver of Q2.
Click on the image for a larger version.

Circuit description

Of high importance is L1, a common-mode choke, liberated from a failed switch-mode power supply somewhere.  This particular unit has an inductance of about 1mH per winding meaning that it has about 377 Ohms of impedance at 60 kHz and helps to prevent a ground loop and the coupling of noise from the power mains.  If you replicate this circuit I would strongly suggest that whatever you use for L1 have at least a similar amount of inductance.  On either side of L1 are electrolytic capacitors (C1, C2 - preferably of low ESR types) to offer low impedance and a degree of reinforcement of common-mode rejection through L1 while C2 and C3 provide RF bypassing for the circuit itself.

A buffer amplifier consisting of Q1 - with a high-impedance input, but no actual gain - couples the signal from the existing antenna:  Having several k-Ohm of input impedance, it is unlikely to appreciably load the existing antenna system.  On the feed from the E-field whip, I simply installed a coaxial "T" connector to allow me to bridge across the signal feed rather than split the signal, which would have been complicated owing to the fact that the DC power for the whip was also being carried on that same cable.  The connection to the amplifier in Figure 3 was made using a very short piece of coaxial cable (about 2 feet long - less than a meter) and since this whip is not used for reception above about 500 kHz, neither its presence or that of the added amplifier had any discernible effect on the other received signals.

Coupling from the existing antenna are series components L2 and C4, selected to resonate at about 60 kHz:  The resonance is extremely broad, so finding a capacitor combination precisely equal to the "ideal" value of C4 - according to the formula below actually calculating as 0.007uF (7000 pF) - is unimportant.  This series resonant circuit is probably not essential and a simple coupling capacitor of 0.01uF (10000 pF) could be used (omitting L2 entirely) but I chose built it with L2 to broadly filter off-frequency signals - something that might be important if your E-field whip antenna doesn't have a low-pass filter to remove AM (Mediumwave) signals as mine does as well as to block any stray coupling of HF signals when I transmit.

Figure 3:
The circuit of Figure 2 built on a piece of prototyping board
in the case.  Bifilar choke L1 is on the right with the BNC
connector (J1, input) and output to the loop (J2) on the left.
Click on the image for a larger version.

The buffered signal from Q1 is then passed to amplifier Q2 which is configured to have "about" 15dB of signal gain.  This circuit is, perhaps, slightly more complicated than it needs to be, but with its feedback, it is very stable and tolerant of large signals.  The use of electrolytic capacitors for C5 and C6 is, perhaps, overkill (0.1uF ceramic would probably suffice) but I used them as they were handy.

As the signal from WWVB is quite strong at this location, there is only one stage of amplification shown in Figure 2, but if I lived more distant, greater overall system gain might be required.  Replicating the circuit involving Q2 (R4-R8, C5-C6) and cascading it with the existing amplifier would add yet another block of gain to boost the absolute signal level - but this would presume that whatever active antenna you were using outdoors to pick up the WWVB signal was working well, providing a "clean" signal and that the deficit was just in signal strength at the clocks rather than than signal-noise ratio.

Due to the smallness of the project box that I chose I couldn't mount the bifilar choke "through" the prototype board so it was mounted on the edge to minimize height.  To hold it in place I used UV cured resin along the edge to prevent it from breaking the pin connections mechanically:  UV cured resin is very handy as it's about a strong as epoxy, but it is cured almost instantly meaning that it's able to be handled immediately.  For the BNC connector, the one that I found in my parts bin didn't have its matching mounting nut, but more UV-cured epoxy did the job for that, too!  As can be seen in Figure 3, I didn't bother "mounting" the board in the box, letting it hang about on its own wires.

Indoor Coupling loop

The "coupling loop" - visible in Figure 1 and shown on the schematic - is just a loop of wire - and it is used to inductively couple the signals from the outside antenna to the clocks.  In my case, I measured a rectangle that would encompass both of the wall clocks and found a cardboard box with similar dimensions and on it I wound four turns of 22AWG hookup wire.  Connecting this loop to the amplifier, I used some shielded microphone cable:  Coaxial cable would have been fine as would just some single-pair speaker wire as this frequency is not all that much higher than audio!

Neatly forming the individual conductors, I used small "zip" ties to hold them together and with four screws, attached it to the wall, placing the clocks inside the loop of wire.  Within the loop, signals from the amplifier would be strongly coupled into the ferrite loopsticks in the clocks themselves - but being very small in terms of the 60kHz wavelength, this loop is unlikely to radiate more than a few feet/meter outside it.

To improve efficiency of the coupling loop I wanted to series-resonate it at around 60 kHz as this would increase the amount of energy transferred to the loop from the amplifier somewhat, effectively providing "free" signal gain.  Measuring the inductance of the loop I found that it happened to be about 22uH and using this simple formula, I calculated the value of C7 - the resonating capacitor in Figure 3:

LC = 25330/(F_MHz)²

Where:

LC is the product of the inductance and capacitance (e.g. Capacitance in pF * Inductance in uH)

F_MHz is the desired resonant frequency in MHz (e.g. kHz/1000)

Knowing that we have 22uH of inductance in the coupling loop and a frequency of 60 kHz (0.06MHz) we end up with "LC" being equal to 7036111.  Dividing this value by the known inductance of our coupling loop (22uH) we get  the capacitance, as in (7036111/22) = 319823pF, or 0.319uF.

Figure 4:
The finished amplifier in its box, hanging
out below the loop - connected, and in
service.  (It's just visible in the bottom
of Figure 1)
Click on the image for a larger version.

As 0.33uF (330000 pF) is the closest common capacitor value, I used that for C7.  Again, as with C4 and L2, the resonance is very broad and precision isn't too important.  The article linked near the top of this page goes into more detail on how one would construct and resonate a coupling loop.  Based on this formula, if I wanted to resonate the same loop for use with DCF77 at 77.5 kHz I would have picked a 0.18 or 0.2uF (180000 or 200000 pf) capacitor, instead.  Similar changes could be made to accommodate longwave time signals on other frequencies (e.g. 40 kHz, 50 kHz, 68 kHz).

The formula above can also be used to calculate the value of C4 with the 1mH (1000uH) L2 inductor:  If your interest was for another frequency, such as DCF77 at 77.5 kHz, C4 would be 0.0047uF (4700 pf), instead.

It need not be said that this loop should not be placed very close to whatever outdoor receive antenna you are using - but more than about 10-15 feet (3-5 meters) should suffice:  If they are too close to each other, feedback (oscillation) could occur - but as this loop is only around 0.01% of a wavelength in circumference it does not radiate efficiently at all - and since it's inductive, its signals won't efficiently couple to an E-field antenna, anyway.

In the diagram, C7, the resonating capacitor for the coupling loop, is shown at the amplifier - but it could have been placed at the loop itself.

Power supply  

First off, do not use a switching power supply for this device!

As noted, common-mode choke L1 was used to "decouple" the power supply from the amplifier - and also from the coaxial cable of the LF antenna.  To power this loop amplifier I would strongly recommend using ONLY a transformer-type DC power supply and NOT any type of switching power supply for the simple reason that the switching power supply will be comparatively noisy, and it - its harmonic - will likely operate at/near the frequency of WWVB or whatever time signal you are trying to receive.

This power supply does not need to be regulated:  Simple capacitor filtering with low-ish ripple (a few hundred millivolts) will suffice and any voltage between about 11 and 16 volts will work which means that about any old "wall wart" in that voltage range - regulated or not - would be fine.

Conclusion

Having had the parts on hand it took only a bit more than an hour to piece this together and almost as long to put it in the box seen in Figure 4.

When I forced both clocks to re-acquire WWVB's signal for syncing they immediately set themselves to the correct time and date - and since it had been the start of daylight saving time the night before but had not been able to synchronize prior to this - they "knew" the new, correct time, too!

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Using a PIR to reduce wear and tear on a Nixie clock

"Does a lit-up Nixie tube in a forest wear out even if there's no-one to see it?"

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 millenia-old riddle has a simple answer:  Yes.  Yes, it does.

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.

* * *

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 internally 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.

"Could I use one of those microwave motion sensor boards instead of a PIR module?"   Motion sensor boards that use low-level microwave energy are cheap and available.  These work in a manner similar to the old "Proximity Fuse" - an electronic sensor used to detonate bombs a certain height above ground.  By detecting interference to its own oscillator by the disturbance of standing waves in a room caused by movement, they can also be used to turn on lights, open doors, etc. While the use of radio waves instead of (infrared) light means that they can sense movement through walls or behind many non-conductive materials like plastic or wood - albeit with some diminution in sensitivity - this property may make them less desirable in this application:  If you want to turn off the clock when no-one is there to see it, you likely don't want it to turn on when it detects movement at a farther distant - even in the next room. One advantage of a PIR sensor is that it may be placed to limit its range of sensitivity to reduce false triggering - including from pets:  If you can't see it, it probably can't see you!

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.

* * *

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 subsequent 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 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?  As the conceit at the top of the page states, 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

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.

As the "Black'n'Wood" clock has a battery back-up, I first tried the most obvious thing:  Interrupt the power to the clock if no motion was detected.  This worked - in theory - but I soon noted that the clock was losing almost a minute every day.

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

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.

The clock itself runs on 12 volts DC and to get the 150-180 volts needed to drive the neon displays, there is an onboard voltage converter.  While no schematic seems to be publicly available for this particular clock, the various sections of its circuitry are easily identified by visual inspection:  A large-ish inductor adjacent to a high voltage capacitor flagged the location of the voltage converter and the chip next to those components.  

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 and a schematic isn't necessary to complete our task.

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 ("Rdiv" is one of them, depicted in Figure 2 - the other, not shown, would go between the chip and ground) 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.

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.  "Pre-forming" the shape of the components to match the locations of the needed IC pins along with using a hot soldering iron to pre-tin the leads of these components and the IC itself 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 have 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 is in motion.

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.

The PIR sensor that I chose was found on Amazon - three of them for under US$10 - and it has exactly three connections:

• 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.

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.

One problem with the 3.3 volt output is that it is a logic output that is limited to 3.3 volts because it has both pull-down and pull-up transistors, internally.  As we discussed in the previous section, we need to ground the base of transistor "Qa" through the resistor to disable the high voltage and let it float to an unknown voltage to allow it to turn on.  

Because the output is not an open collector or open drain, it cannot be pulled higher than approximately one diode drop above the 3.3 volt supply on the PIR chip:  This voltage is lower than the emitter voltage of the transistor that we added ("Qa") which means that Qa will always be turned on, always disabling the high voltage converter!

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 even when I was in the room and not moving much - 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 and the display remains off most of the tim.

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.

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:

1. 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, 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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