Luldlum Model 12 Restoration

Recently I’ve been restoring a Ludlum Model 12 rate meter. What is that you might ask?

The best ratemeter on this side of Si Valley, that’s what!

I say this, because it’s one of those rare instruments built with “user serviceable” in mind, much like old tektronix oscilloscopes. Along with its bare, “don’t touch this lest you get shocked” circuit board, it contains trim potentiometers and for nearly everything one could wish to adjust; the high voltage power supply, the pulse discriminator, the integrator dividers, as well as a switch to change the integration speed and to reset the integrator entirely. But those are just big words for people who don’t know what the hell a survey meter is!


What is a rate meter / survey meter?

A survey meter is a radiation measurement device; something that provides for a measurement probe;

  • A high voltage power supply. Most radiation detectors (eg, geiger-muller tubes) require a high operating voltage, typically on the order of 700 to 2,500VDC.  Geiger-muller tubes typically require a potential of 400-700V, photomultiplier tubes a potential of 900 to 1500V, and proportional counters a potential of 1200 to 2500V.
  • A pulse discriminator. During a count events, an electrical pulse is formed across the radiation detector. On some detectors, such as neutron proportional counters, this magnitude of this pulse is co-relates to the detected particle’s energy. The discriminator is a circuit capacitvely coupled to the probe’s high voltage power supply which monitors these pulses, and triggers a count event if the pulse exceeds some magnitude. By adjusting the reference voltage, one can discriminate between say, a neutron, and a gamma ray.
  • An integrator. This is just some display mechanism that integrates the pulses, to give a “counts/minute” measurement. The accuracy of the integrator is of course, highly dependent on the length of time which it is active.
  • A speaker. Click-click. Click-click-click-click.

Naturally, being able to adjust all of these things means you can use a Ludlum-12 with any probe you wish!


My rate meter

Ludlum-12’s in working condition are notoriously hard to find, as they’re highly coveted instruments, and no one lets go of a working one easily! As such, I bought mine semi-broken and much beat-up. The meter I acquired had lost its voice at some point in its life, and was in much need of a face-lift.


In all her pride and glory

WIN_20140906_103228 (2)

Analog dreams

By “losing its voice”, I mean that my meter had no ability to make sound. This is rather unfortunate, because while it might seem like only a minor inconvenience, those clicks could be the alarm one needs to learn whether or no one is in a radiation field!

The Ludlum-12 is an entirely analog instrument. Nowhere on its board exists any form of proprietary turing machine, which is rather fortunate as this makes it quite repairable. So where was the damage? Here!


Ludlum-12 Audio Circuit, courtesy Andrew Seltzman

The audio circuit in this machine is rather simple. An astable multivibrator (CD4098) is triggered by the pulse discriminator (not labeled), which creates a few-kHz audio tone that’s fed out of Q2 into a NAND gate (CD4093). This NAND gate, being of the schmitt trigger type, squares-up the signal a bit, where it’s then further buffered by a second NAND gate and sent into a piezoelectric speaker. The audio “mute” switch is simply an SPDT toggle that holds the multivibrator’s RES2 (reset) pin low. These are the two ICs that somehow failed in this meter; likely due to some sort of electrostatic event given that they’re old and of fragile design. Interestingly, the multivibrator failed in such a way that pulses were able to make it out of Q2, but not periodic square waves. Weird. Replacing these ICs fixed the issue.


The Facelift

Whilst fixing this meter I was convinced by a fellow artist to make it snazzy.


Get Funky

The above was done with rust-oleum enamel, masking tape, and a razor blade.



Though I will eventually use this as a scintillation meter, I needed a probe for testing purposes. Not having one on hand, I decided to use a Navy surplus GM-tube that had been collecting dust for some years. Since the Geiger tube runs at a high potential (700VDC, in this case) it’s required that the tube be insulated lest I shock myself while using it. My solution, given a lack of tools was a composite tube built from paper, epoxy, and vinyl tape. Given a small coat of polyurethane for strength, it’s just about as good as one made from aluminum!


A DIY Composite Tube


Fits my Geiger tube snugly


And hosts a nice BNC connector


To make one sexy probe


The Video

No sir I don’t want spicy Mexican; I want hot. Add a dash of Radium Chloride please. ∎

The Bass Cannon

Every once in a while I build something ridiculous, and this would be one of those whiles.

Ladies and Gentlemen of the internet, I present to you what I understand to be the world’s first Bass Cannon.



What is a “Bass Cannon” you might ask?

It’s a weapon of mass destruction. A party on your shoulders. Something to frizz your hair with. Something to peeve your neighbors.

It’s when you get when you take

  • An AirZooka
  • A pair of voice coils
  • Epoxy
  • Miscellaneous analog parts
  • A class D amplifier
  • A lithium-polymer battery
  • Birch plywood
  • Threaded rod
  • A physicist with too much time on his hands

And put them all together in a room with a soldering iron, and a laser cutter.

WIN_20140904_173907 WIN_20140904_173918

WIN_20140825_232241 WIN_20140826_105252

I’m not going to try to flaunt, nor will I make a step-by-step guide on how to construct one of these contraptions. That said, if you’d like to make a portable party for yourself, the above photos, and the below schematic should be enough to get you started!


In truth there’s not a whole lot of fancy engineering that went into this project. It’s a mono audio system with a pair of x-pass filters, a power amplifier and suitable drivers. It has no battery management or protection circuitry, though, that’s a simple thing to add if you do feel it to be absolutely necessary (hint; use a relay, a BJT, a zener diode, three 1% tolerance resistors and a comparator).

Two potentiometers set the channel gains for the the pair of first order filters; one high-pass for the midrange driver, and one low-pass for the woofer. A PYLE “PLPW8D” voice coil conveniently seats snugly within the case of the AirZooka, leaving just enough room for a mid-range driver to be placed in front of it with threaded rod as a support structure.

Initially I had concern as to whether or not this assembly would shake itself to bits upon use, but fortunately that was not the case.

Instead, it shakes the windows. ∎



-18dBm of cats

I moved California. More about that on another post.

Since then, I’ve (well, we’ve) had wifi problems, specifically ones emergent of what I consider to be some terrible MIMO radios. Periodically, our router [Netgear WNDR3700] would dump everyone on the 5GHz band, and disable the radio for some 20 minutes. This turned out to not be a software issue, as openWRT did not solve the problem.

This was my solution; I figured it was worth sharing.


Don’t tell the FCC, but our house now has a 1W wireless N connection. ∎


Heisenberg’s Uncertainty Principal: The actual content of quantum theoretical kinematics and mechanics


Upon reading chapter four of my assigned physics textbook [Modern Physics, Krane], I grew both tired and annoyed with the generalizations, or “leaps of faith” which author continually made. I soon found it more useful instead, to spend time reading the papers upon which these principals have been derived. Astonishingly however, I failed to find a modern, usable English translation of Werner Heisenberg’s landmark paper! More unfortunately even, the closest I did come on the hunt for such a translation was the discovery of a broken-english, NASA OCR script from 1988 hosted on the web archive. That won’t do.

Thus utilizing a day’s time, Google translate, MathJax and my personal skills at reading broken-english datasheets, I below have provided a modern translation of W. Heisenberg’s paper. For convenience of the reader, I have replaced some original variables used in the paper to more represent those found in common texts today. New notations such as euclidean norms (i.e, \(|f(x)|\)) have been instated, as well.

Dr. Heisenberg’s various justifications alone make for an interesting (and perhaps, very useful!) read, but for those short on time I have prepared also, a “too long, didn’t read” summary immediately preceding.


TLDR Summary

CaptureIf we are to derive a model that quantizes space, perhaps to cells with lengths some finite dimension \(h\), then we are left with in the space \(\mathbb{Q}^2\) for example, a 2-dimensional grid of possible positions. Objects in this grid then, may be given some arbitrarily-defined co-ordinate, \(q\).

q of course, is a function of \((x,y)\) inside \(\mathbb{Q}^2\). \(x\), and \(y\) may only be integer multiples of h, or specifically:

\(q = \left \{ \forall (x, y)*h\in\mathbb{Q}^2 \right \}\)

(don’t be scared, I’m just having fun with LaTeX!)

CaptureNow, if \(q\) is a function of yet another quantized variable, \(t\), then \(q(x,y)\) may be broken into \(q(x(t),y(t))\).

Thus if it’s fair to say “\(q\) can move as time advances integer multiples of h”, then it is possible to define some distance \(q_x\), that \(q\) has moved in that elapsed time \(\Delta t\). We may thus define a 1-dimensional “velocity” \(v_x = \frac{\Delta q_x}{\Delta t}\).

\(q\) however, is not a continuous function in this space, as it may only take on discrete values, themselves integer multiples of \(h\). Therefore it is useless to define “the velocity at a point”. More generally, \(q\)‘s average velocity for any time interval, \(\Delta t\), smaller than \(h\), is not definable.

Restated, only values of \(q_x\), or \(v_x\), can satisfy the below statement;

If time advances as \((integers) * h\), then \(\Delta q_x \geq h\) if our definition of “velocity” is to make any sense.

By extension, momentum in this direction, which is defined as \(m v_x\) must satisfy \(p_x \geq h\), if \(m\) can be no smaller than \(h\) as well.

Now consider the thought:

What if we were to look at the object \(q\), with absolute precision? That is, \(q_x\) is exactly defined, and \(\Delta q_x = 0\).

Then, if \(v_x\) is a function of \(\Delta q_x\) then as \(\Delta q_x(t \rightarrow 0)\), or “the change in \(q_x\)” approaches zero, then the function \(v_x(\Delta q_x(t \rightarrow 0))\) becomes indeterminate. This relation works on the converse as well, such that the relation:

\(\Delta q_x * m \Delta v_x \geq h\) is justified!

In our 3 dimensional world \(\mathbb{Q}^3\), this equation becomes the familiar Heisenberg uncertainty principal:

\(\Delta q_x\;\Delta p_x \geq \frac{h}{2 \pi}\)

The factor of \(2 \pi\) is a geometric normalization.

The origins of this relation’s elegance are plain to see: it is one derived from simple principals! Below, Heisenberg purports similar arguments exist for an energy-time relationship, and proves both relations are just as true for wave-functions as they are for discrete, “particle” functions. I’ll leave that lesson to be a test of your reading comprehension skills, however.


Über den inhalt der quantentheoretischen anschaulichen the kinematik und mechanik (or, the actual content of quantum theoretical kinematics and mechanics)

W. Heisenberg, a modern translation by Adam Munich

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