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DTF104B Clock Project  
Written by AnubisTTP on 2011-01-21  

Editors note: For more information on filament displays and Numitrons, please view our Filament Displays page.


DTF104B clock, completed object.

This is a clock built using an uncommon part, a vintage display tube. The DTF104B, made by RCA, is an unusual electron tube commonly referred to as a 'filament display'. Also identified by the trade name 'Numitron', filament displays are in effect complicated lightbulbs which use heated wires to display a numeral. The DTF104B is considered an 'end view' or 'top view' tube, because the numeral is viewed through the top of the envelope; the overwhelming majority of filament displays have side-viewed numerals. Many builders loathe using end view tubes over side view displays because end view tubes typically lack a classic, iconic envelope shape, but I actually prefer top view displays for my projects.


DTF104B in normal operation.

Envelope concerns aside, this model of numitron has a lot of things going for it from a project standpoint; it provides a large clear display, it will run on TTL-level voltages without any shifting, and unlike many display tubes it uses a common 9 pin 'magnoval' socket that can be bought readily from various tube suppliers. The economic reality of buying DTF104B tubes is subtly complex, the DTF104B is actually a rather rare and hard to find tube in the wild, but a seller on eBay has been selling huge bulk quantities of this tube for over five years, keeping the price at a reasonable level in the process.

Numitrons, being a heated filament device, will 'burn out' over time like a lightbulb, but typically have much longer lifespans than your average 60W Walmart-issue photon sprayer. Most RCA filament displays are rated at over 50,000 hours, which works out to about five years of continuous use as a clock display. My sense of historical preservation cringes at the thought of burning up a multiple fistful of a rare tube in a multi-digit clock, so I constructed a single digit clock that displays the time using only a single tube.


DTF104B clock schematic.

Single digit clocks are hardly a new concept, single digit PIC code written by David Tait for the 16F84 microcontroller has formed the nucleus of squadrillions of people's single digit clock projects since the time before the Interweb. The David Tait code lacks interrupt based timekeeping or error checking however and 16F84 chips are rather expensive by modern microcontroller standards, so I opted to write my own code for the cheaper 16F628 instead. The basic concept is well known, time is displayed sequentially, starting with the 10s hours digit and ending with the minutes digit, with a row of four LED's to indicate digit position. The code also includes a 'demo' mode which cycles sequential digits and displays various animations when the setting button is held down.


DTF104B clock, internals.

Space in the case was surprisingly tight, given the size of the enclosure relative to the diameter of the tube. The front panel is a laser-cut piece of plastic from Ponoko.com, a cutting service that will take a CAD vector file and cut it out of metal or plastic. The rest of the case is made from a Darice wooden box blank that has been sanded and stained to hide it's humble craft store origins. The PCB is a board recycled from another project, the two large screws on either side of the case hold the PCB in place when it is installed in the enclosure. The socket is a Chinese manufactured ceramic Magnoval type... readily available from many tube suppliers for a few dollars.


DTF104B, rear fixtures.

I am a firm believer that every good project ends with a good fastener, and dug through the various piles of scrap at the local salvage store until I found some exceptionally beefy tapered machine screws to finish out the case. It would have been nice to find an equally good pushbutton for the time setting control but, alas, none of the local suppliers had anything that met my requirements and I ended up using some cheap red Chinese pushbuttons that I bought online. These pushbuttons turned out to be of universally horrible quality by the way, every single one I have used broke within only a few hundred pushes. Far be it for me to assume that a pushbutton could withstand being pushed like a button, I actually had to bury the entire button in a block of resin to keep the back from falling off from repeated use. If all products were constructed to the same level of quality, soda cans would rupture in direct sunlight and toilet seats would dissolve on contact with human waste.


Under-tube lighting.

Many purists recoil at the idea of using LEDs for under-tube lighting, believing that a tube clock should only use vintage display technology. I have no such predilection against the use of LEDs in a tube project, and installed 505nm 'Ocean Green' LEDs under the tube and for the digit position indicators. Long the staple of hamfests and preposterously-named eBay auctions, 505nm LEDs produce a more bluish output than conventional and readily available 545nm green LEDs; I felt the time was long overdue to use some in a project. A word of warning, many LEDs claiming to be "Ocean Green" or "Aqua" nowadays are actually 515nm instead of 505nm, as manufacturers have had to raise the wavelength of their products to meet the demands of solid state traffic light manufacturers. The LED's used for digit position indication have been ground down flat to diffuse the light and make them flush to the front of the case.

Overall, the finished clock turned out looking rather nice. Is it easy or useful to read time in the fashion displayed by this clock? Well, not really. But, as a blinking steampunkish diversion that makes people coming into my office forget the reason they came there, it works rather well!


Clock in operation.



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