Tuesday, October 25, 2016

Craig Anderton's Multiple Identity Filter

Craig Anderton's Multiple Identity Filter™ originally appeared in a series of ariticles in Contemporary Keyboard in 1979. It is a CEM3320-based VCF with each of the four stages operating semi-independently. Each stage has...
  • Three Filter Modes: Lowpass, Highpass, Allpass
  • Two frequency ranges (10:1 ratio)
  • Series/Parallel Switching
Add to that voltage-controlled resonance, straight/filtered signal mixer w/phase inverter (for phasing, changing bandpass to notch, etc.) and more. Craig was in top form—this is one tricked-out VCF.

Here is the article: MIF.pdf

Here is a mirror on Mediafire in case the link above doesn't work for you. I've been told that some web browsers (or their extensions) warn that Mediafire is a dangerous site. It's not but the page does have a lot of extraneous links and other junk that can make it confusing to navigate. To start the actual download of the pdf file click on the big green "DOWNLOAD" button that also has the file size in smaller letters: MIF pdf

Update 2017.2.26

Note that this design is not optimal. More work has to be done to get the stages to "behave" properly. This has been discussed on the Synth DIY list on multiple occasions. There is a searchable archive here: SDIY Archive

And there's some very good news: a clone of the CEM3320 has been fabbed. It's available here and the price is very reasonable. The datasheet is here.

Wednesday, February 24, 2016

The Oscilloscope Artist

The Oscilloscope Artist originally appeared in the November, 1975 issue of Popular Electronics. It creates all sorts of fascinating moving geometric patterns on an oscilloscope screen.

All you need is a low bandwidth 'scope with horizontal and vertical (XY) inputs.

Update (2018/02/16): David Dixon spotted a really stupid typo in the original pdf file that was shared below. This has been fixed and the latest version is now 1.2.1. All other (unattributed) copies floating around the Internet are of the earlier, incorrect version.

A reconstruction of the complete article is in a pdf file. All the original parts are still available!

The original block diagram is a bit confusing so I redrew it. Oscillators A & D create a Lissajous figure baseline from triangle/square waves. Oscillators B & C are multiplied and summed with A & B.

To make things interesting, C is 90° out of phase between the multipliers. All the oscillators are sync'd to A, which is ~60Hz. All of this produces complex patterns which move and shift.

I've also used it with a laser projection system with X and Y galvos and it works although the corners aren't too sharp.

Friday, November 4, 2011

Halloween 2011

Another Halloween come and gone. I'm rather worn out from setting it all up and taking it all back down.

Click to enlarge



One bulb blown

Everyone said it looked great.

Sunday, August 21, 2011

Room temperature diamagnetism with pyrolitic graphite

Updated 2011.09.01

I just acquired some pyrolitic graphite and powerful neodymium magnets from United Nuclear. Pyrolytic graphite has the highest diamagnetism of any room temperature material [see correction below]:
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, these electrons will oppose the magnetic field changes provided by the applied field, preventing them from building up. The result is that lines of magnetic flux curve away from the material.
Here are some small pieces of pyrolitic graphite on top of four 0.5" square neodymium magnets:

Click to enlarge
This is not magnetic repulsion. If that were the case the pieces of graphite would simply fly off the magnets. The magnetic field is strongest at the edges of the magnets so the largest piece is trapped in the center; it's pushed inward to the point of least magnetism. If perturbed it will snap back to this same point and orientation. The smaller pieces are standing on edge because they are repelling the magnetic field on both sides and can't fall over. If one is pushed over it will immediately snap back into a vertical position.

In this view you can clearly see the largest piece levitating and the smaller pieces standing on edge. The medium size piece is tilted slightly. This may be because its thickness is slightly uneven, which would make the repulsion slightly stronger on the more massive side. This calls for more experimentation.

Click to enlarge
Needless to say this behavior is completely unintuitive and surprising. When pushed the pieces react in a totally unexpected way. It's as if they've fallen into a sort of bizarre magnetic well. Here's a video showing this behavior in real time.

Superconductors have an even more powerful diamagnetic effect but require liquid nitrogen to cool them sufficiently.

I'll have more on this topic in the future as I've also acquired some bismuth, which has the highest room temperature diamagnetism of any metal.

Correction (2011.08.31)

Here's the difference between bismuth and pyrolitic graphite (without the math):

"The most strongly diamagnetic material is bismuth, although pyrolytic carbon may have a [lower] susceptibility in one plane."

So there you have it. I played around with some bismuth the other day and it was quite interesting. Stay "tuned"...

Friday, August 19, 2011

Crookes tube first test

My Crookes tubes showed up today and I had time to get a couple of quick shots of magnetic deflection in action with my invisible magnet:


Thursday, August 18, 2011

Induction coil—another new toy

I have a bunch of HV stuff coming in this week, starting off with this induction coil:

Click to enlarge
I got it from Sci-Supply.com. I'm going to be powering Crookes tubes with it so I didn't pay the extra money for something bigger. Once you get over 15-20kV X-rays become a hazard. I have enough problems already. Like the fact that my Crookes tube order is sitting in a facility in Michigan and not moving. So much for Priority Mail.

Wednesday, August 17, 2011

Ball gap for the Dirod electrostatic generator

I finally have the ball gap to show off. It's not finished—the capacitors (Leyden jars in the old days) are rather crummy and will probably have to be redone but I managed to get some action shots anyway.

Construction is simple, just follow the instructions at the end of the manual. I had to guess at the tubing lengths but this seems about right. The most time-consuming part was cutting the bases off the dummy doorknobs. Sadly, my camera was occupied with time-lapse experiments and I didn't get any pictures of the process. It took a couple of hours, a moto-tool with an emery wheel, face mask and goggles. The trick is to cut the metal in narrow strips, working them down until they're small enough that they can be broken off with pliers. I slowly worked my way around until the knob came loose.

After grinding down the stumps of metal stick out of the knobs I glued them to pipe caps with metal-filled epoxy. They don't look as snazzy as I'd hoped but they do work.

With the capacitors I get a very loud snap with a .75" gap. It's the middle of summer right now; next winter will be the real test.