原创 Oh no! The demise of my antique analogue meter

As you may recall, I have several hobby projects on the go that will feature antique analogue meters. They include my Inamorata Prognostication Engine, my Ultra-Macho Prognostication Engine, and my Vetinari Clock.

I've been gathering the requisite meters for some time. At the same time, I've been masterminding the brass control panels and the wooden cabinets, working on the software algorithms, and generally running around doing all sorts of other things. As a result, it wasn't until this past weekend that I actually started testing and characterizing the meters themselves.

First of all, we (my electronics guru chum Ivan, who works in the same building, and I) opened all the meters up to check for (and, if necessary, remove) internal series or shunt resistors. The image below shows some of the meters I'm using spread out on a table in my office building. The four meters dismantled in the foreground are intended for the Vetinari clock. The five-meter group in the background is destined for the Inamorata Prognostication Engine; these have already had their series and shunt resistors removed.

 

The next step is to determine the value of the new external series resistance to be used with each unit. Depending on the unit in question, this can be all over the place. For example, one of my meters requires an external series resistor of only 370 Ω, another requires 5.47 KΩ, and one requires a hefty 98.6 KΩ. Since I plan on using analogue meters for a lot of future projects, it makes sense for me to streamline this process, so I whipped up the handy-dandy unit shown in the black box below.

 

The Arduino appearing in the bottom lefthand side of this image (under one of our screw-block proto-shields, which is itself under an I2C-based LCD display shield) is driving a pulse-width modulated signal. For the purposes of this test, I'm driving a maximum value of 255. This is being fed via an isolating field-effect transistor into the input terminals on the lefthand side of my black box. The output terminals on the righthand side of the box are used to drive the meter.

The three potentiometers allow me to start with a very high value of series resistance and work my way down. The switch in the middle of the black box is a two-pole, two-throw, three-position device that allows me to swap the polarity of the terminals driving the meter. Again, I'll be describing this in much more detail in a future column.

Generally speaking, everything went rather well. Once I'd constructed my test unit, it didn't take long to determine the series resistors required for all my meters. The only problem occurred when I discovered that my biggest and best meter -- the Phaostron device I intended to use in my Ultra-Macho Prognostication Engine -- has twitched its last and shrugged off this mortal coil (pun intended). Much like the Norwegian Blue in Monty Python's Dead Parrot skit, my beauty is no more. It has ceased to be. It has expired and gone to meet its maker.

This is a bit of a blow and no mistake, because the brass panel for my Ultra-Macho Prognostication Engine has already been fabricated. Suffice it to say that I am not performing my happy dance. The two Phaostron meters below are also unusable (I keep them around for spare parts). The one on the left is model number 40-014036; the one on the right is model number 40-013857. I'm sure there are other models out there that would fit the bill.

 

The physical characteristics I need include the black surround with the six mounting holes. This surround has an external diameter of ~4.5". The "can" that protrudes through the brass panel has a diameter of ~3.5". With regard to the electrical characteristics -- and remembering that I eventually will replace the faceplates -- I don't care if the meter was originally intended to display voltage, current, resistance, or some other quantity, as long as it works with direct current (DC), not alternating current (AC).

This is where you come in. Do you perchance have one of these meters in your treasure chest of spare parts? Or do you know where one can be found? Or will you be attending a Hamfest or electronics flea market in the foreseeable future? Of course, a group of us will be attending Hamvention next year, but I don't want to wait that long.

If you responded "Yes" to any of the above questions, please post a comment below. I await your communication in dread anticipation.