Making a cheap chemical fume chamber

I will be working with sulfuric acid to decapsulate chips, and this generates sulfur dioxide gas. I have an idea about bubbling the gas through hydrogen peroxide to reform sulfuric acid, but I can't be guaranteed that the reaction will use up all the sulfur dioxide. Besides, in the initial experiment, I'm not going to try to reuse the sulfur dioxide.

Anyway, sulfur dioxide is toxic, but it can removed with an activated charcoal filter. I could set up an air filter and try to suck all the fumes through it, but instead I had an idea when I saw this [abrasives blast cabinet at Harbor Freight] (US $170):

It has a circular hole in the back and one on the side, so maybe I could adapt this as a sort of contained chemical fume hood. A fume chamber.

It doesn't have to be air-tight because I'm going to suck the air from it, which means that air has to be provided from somewhere, and random openings and leaks are just as good as anything.

I didn't need that abrasives chute at the bottom, so instead I took the support, covered the hole with a thin sheet of steel (held down with VHB tape), and used that as the bottom piece. I did have to drill holes in it to match the chamber. I also applied a line of foam tape to the outer edge to seal it.

I used a 4-inch dryer hose adapter thing on the back. The hole in the back is just a bit smaller than 4 inches, so I had to cut off part of the adapter so that I could fit it flush. I sealed with silicone caulk and screwed it into place.

Also, without the funnel, the thing was wobbly so I put some 1x6 wood supports in.

Amazingly, these Veva "[Premium Carbon Activated Pre-Filter 6 Pack for Germ Guardian Air Purifier Models AC5000 Series, Replacement Pre-Filter C]" (US $10, Amazon) are exactly the right width to fit in this duct that goes to the hole in the back. They are thin, so I had to use three of the six from the back.

The light provided in the chamber kit sucked, so I bought a [strip of LEDs] and installed those (US $16, Amazon, with [right-angle connectors] US $8, Amazon). It really brightens the interior up nicely.

Next, the gloves included with the kit were for abrasives, but I wanted nitrile gloves. I found these [Showa Atlas 772 M nitrile elbow-length gloves] (US $12.50, Amazon). I cut off the elastic at the end, stretched them over the large glove holes, and secured with the provided hose clamps.

Left: Showa elbow-length nitrile gloves. Right: The gloves provided in the chamber kit.

Left: Showa elbow-length nitrile gloves. Right: The gloves provided in the chamber kit.

I attached my laser cutter's fume extractor fan to the chamber, and when I turned it on the gloves inflated. That's not great, because if I set everything up inside and then turn on the fan, the gloves would knock everything over. The solution was to put the switch for the fan inside the chamber. That way I could put my arms in the gloves and then turn on the fan.

Total was about US $250, not including the fan or the hoses. Of course, this isn't a real fume hood. I have no idea if the paint would react with things. Certainly if acid spills, it will eat the metal, but I can mitigate that by keeping a box of baking soda in the chamber. But I think this will suit my limited purpose.

Building a simple CNC microscope (part 1)

Simple != cheap. Just FYI.

Since I've been really enjoying reverse-engineering the Unisonic 21 CPU chip, I've resolved to get into decapping, photographing, reverse-engineering and archiving dies of older 1970s and 1980s chips. I went to a bunch of surplus stores here in the Bay Area to pick up boards and individual chips in the [5400 and 7400] logic family. Primarily [Excess Solutions] and to a lesser extent [Weird Stuff Warehouse], while [Halted] wasn't a very good source (and their chips are all behind the counter WTF).

These are very simple logic chips. Perhaps the most complicated is the ['181 4-bit ALU]. But, they're simple enough to level up on TTL with, and there are many variations and manufacturers of the same chip to compare.

Following in [John McMaster's footsteps], I hit the eBay to find a metallurgical microscope, a motorized XY stage, and a 20x planar apochromatic objective with long working distance.

For the metallurgical microscope, I found this awesome [Olympus BH2 microscope] (US $1600 shipped). It is bolted to an optical breadboard, came with four objectives plus an objective adapter thing, and a bunch of other measurement things that I didn't really need. But it is awesome because I can add whatever I need to the base.

The BH2 is also infinity-corrected, meaning that you can use infinity-corrected objectives with it, which are plentiful. The microscopes that aren't infinity-corrected have a fixed tube length, and you need to use the same tube-length designated objectives. That is really limiting. See [this Olympus article] about objectives for some useful information about what tube length is, and [this article] for other objective terminology.

At first, I found what seemed like the perfect XY stage. It was actually an XYZ stage made by Newport Klinger Micro-Controle. It uses variable reluctance stepper motors, which sucks because they are noisy and controllers are hard to come by. And when I got it, it turned out to be a YZ stage because the X axis was broken, and taking it apart is proving unreasonably difficult.

So I gave that up as a bad job, and searched for more motorized XY stages, and found this awesome thing, called "MMT 80x80 XY axis Motorized Stage Cross Roller Precision VEXTA PK544-NB MRS-I-10" in the listing (US $400 shipped from Korea). It was apparently pulled from equipment.

A bit of digging showed that it used [Oriental Motor PK-544] 5-phase stepper motors with some optical limit switches, and the recommended drivers were Oriental Motor Vexta DFC1507 modules which I found at a local place (US $140 each). And there's very clear [documentation] for it!

So I ordered all that stuff and mechanically, at least, the stage works fantastically. It has lots of holes in it for breadboarding. Very customizable, and the step size seems nice and small.

I already had a [BeagleBone Black] which I wanted to use because it has [MachineKit] available for it, basically a CNC controller, and that uses the BBB's PRUs (Programmable Real-time Units) meaning that it's far less likely to stutter since the control signals are offloaded to a dedicated processor. All I really needed was pulse output (and later, limit switch input), so I chose the simplest configuration I could find,  CRAMPS. This refers to a hardware driver, but I didn't need one because I already had a driver, the Vexta units.

The only problem was that the Vexta units take a CW/CCW pulse while the PRU configurations in MachineKit can only output STEP/DIR pulses. The CW/CCW or UP/DOWN config is [not implemented], but there's [a placeholder] for it, as if I don't have enough projects :( .

So I doodled for a little while and rooted around in my surplus chip box and came up with a 74139 (dual 2-4 decoder with inverted outputs) which could be configured to convert two STEP/DIR pairs into two UP/DOWN pairs.

Finally, I needed the objective. Apparently Mitutoyo is the best, and you really want an objective that is not likely to screw you over. Something plano (so no distortion around the edges) and apochromatic (so no to little chromatic distortion) and long working depth (because I'll want to image dies that are still in their packages, so at least 2mm). Also infinity-corrected. The Mitutoyo MPLAN APO 20x is perfect, but you [really don't want to buy one new] if you don't have to (eBay, US $800 shipped from Korea). Get one with a money-back return policy.

It's pretty huge, and luckily I had that objective adapter thing. That's why infinity-correction is important. I could whack that adapter in there and it wouldn't matter much how long it was, since the rays inside are parallel.

Next, I need to put everything together and put a camera on it. But first, a test image. This was not taken with the 20x objective. It is a 74245 die. The reticle in the photo is from the eyepiece.

Total so far: US $3080. As I said, it's not cheap.

Unisonic 21: Another driver circuit, more simulations in LTSpice

Tracing out the circuit section after the clock generator, I found some really large capacitors:

Next to each capacitor I've put their size in squares. C3 is familiar, being part of a bootstrap load. But what are huge capacitors C1 and C2 for? And why is there an extra capacitor C4 just hanging out across Q15?

Searching through the Rockwell patents, I found patent [US3480796], "MOS Transistor Driver Using a Control Signal". The reference at the beginning of the patent to a filing called "MOS Driver Using Capacitive Feedback" is, in fact, patent [US3506851] which I mentioned back in [part 2], and explored in the [bootstrap load simulation post].

In that simulation, we saw that the output voltage decayed, and it was worse with a really small capacitor in the bootstrap load. This patent is supposed to overcome that decay problem.

Figure 1 from patent US3480796.

Figure 1 from patent US3480796.

The input stage shown is an inverter. The original signal (8 in the diagram) drives the high-side transistor (5) while the inverted signal (4) drives the low-side transistor (1). There's an extra capacitor (C) and signal (17) which we need to explore. C1 in the diagram represents "the parasitic capacitance of the device", which for now I'll take to mean the gate capacitance of transistor 1.

I've redrawn this circuit, without the extra capacitor, in LTSpice. I've also flipped it so that the supply voltage is positive. Finally, the W/L of the transistors in the inverter are set as in the previous post, and the output transistors' W/L are set to 16/1 since they are supposed to be driver transistors.

Looking at G- (the red trace), we see that it goes from 1.5v to 14v as expected for a circuit without a bootstrap load. The output trace (blue) goes from 4.8v to 14.6v. That weak zero-level (4.8v) is due to the weak zero-level of the non-bootstrap load (1.5v). The weak zero-level is also what would happen if M9 were replaced with a bootstrap load with a very small capacitor, although there would be a brief period with a "good" output before the output decayed.

The patent describes adding a large capacitor at G-, fed by a signal Φ (VPH in the below schematic).

When the low-side transistor M1 is turned on (by G- going low, meaning IN is high), VPH goes high. This charges up the capacitor to VPLUS minus the threshold voltage (14.6-1.5, or 13.1v). Once the capacitor is charged up, VPH goes low, and because capacitors resist changes in voltage, this drives G- very negative, to -13.1v. And, like in the bootstrap load, this turns M1 on very strongly. The larger C1 is, the longer it will take for the output to decay.

In the simulation trace below, V(n001) is the PH signal. I've set it to be on for 0.1 msec.

We can see that after 0.1 msec, G- is driven very negative, which has the effect of turning M1 on strongly, which will let the output be driven strongly to 0v. We can also see that G- is decaying.

This circuit has the same effect as a bootstrap load, except that the capacitor is controlled by an external signal rather than the input signal. Why is this important?

For one, these output transistors are large and meant to drive heavy loads. Replacing the 100k output resistor with 10k barely changes the output voltage, but the bootstrap load cannot handle this output.

At 10k load, the bootstrap load can only drive the output down to about 5.4v.

At 10k load, the bootstrap load can only drive the output down to about 5.4v.

When the output transistors M1 and M3 are large, the increased drive can bring the output to about 0.6v on the low side, but only to around 11v on the high side.

When the output transistors M1 and M3 are large, the increased drive can bring the output to about 0.6v on the low side, but only to around 11v on the high side.

Also, large transistors come with large gate capacitances, which effectively adds more load.

So, that's the purpose of those large capacitors in the Unisonic 21 circuit. We can see that the signal marked PH is the external signal for capacitor C1 which helps drive Q2 low. Q9 and Q10 form a NAND gate. The /A signal is one input to that gate, while the other signal is an inverted delayed version of /A, courtesy Q14 and C4. The result is a pulse, which is just what is needed to drive the large capacitor.

Equivalent circuit.

Equivalent circuit.

What is also important is that for logical simulation purposes, this circuit has no effect. It is purely there to improve the drive characteristics of the output, and if the output is idealized, this circuit disappears.