Monday, December 16, 2013

One Bit ADC and a (Not) Wheelbarrow Shaped Object

With the semester winding down, and final project(s) nearing completion, the coil was revamped and the project part of my final project was made.

Mmm remounted IGBTs
The goal was to have a working analog interrupter which would take an analog signal, such as the one coming out of an mp3 player, and then be able to play it through a Tesla coil.

The block diagram is the following:

Signal -> Amplification -> Triggering -> Poopy sparks

This led me to use an LM358 and 555 timer due to their availability. A 74HC14 was added to buffer the output of the 555 to preserve waveform integrity.

Both channels of the audio signal are sent through an inverting adder and then to a monostable 555. The trigger voltage is set using R5 and the gain of the opamp can be adjusted using R3. The output of the 555 is then buffered by the inverter and sent to the opto-out. The interrupter is optically coupled to the coil to avoid the potential ground loop. Unused inputs are grounded and decoupling caps are added appropriately. 

At this point, I was running out of time, so I quickly breadboarded the circuit which also made the system incredibly noise prone. Oops :P

Input and Output Waveforms Lookin' Legit.

It even did the interrupting thing!

Further testing involved hooking up a dinky little speaker to the output to test if audio came out intelligibly. 
The results:

It's important to note that because this is, for all intents and purposes, a one-bit ADC, the audio will be pretty terrible.

Nevertheless, it seemed to work on the revamped coil.

Modifications: new secondary, primary,heatsinking, and rectifier diodes.

One concern was that the bridge was blowing prematurely due to the rectifier diodes failing - perfectly valid given that they were rated for only 4A (MUR460). They were then replaced with complete overkill: some minibrick diodes Bayley bought a while ago.

Rate for 96A at 600V. Yummy. 
The assumption was that the diodes were failing short due to transients, sending X amps of 60Hz AC to V+ and V- of the inverter. Eww. Another useful feature of these diodes is that when your bridge blows, the diodes won't.

Power testing also revealed that the new bridge is now capable of doing up to 75% duty cycle.

Heatsinking was improved by bottom mounting the IGBTs and increasing thermal mass substantially.

A squashier primary was wound using the old chassis as a coil winding jig. A power drill was used to speed up the unbearable process of winding 1330 turns of 36 gauge wire.

Only took five tries...

The final secondary dimensions came out to 7" long, 3.5" OD compared to the old 10", 2.4"OD. The resonant frequency also sank from 300kHz to ~150kHz making it suitable for brick coil use. (Whether it'll stay a coil is another question).
Unfortunately, the demo involved swapping out the secondary for a smaller one to reduce coupling - in this configuration, the coil had a tendency to be quite hot and burn-y.

For more details on the driver, you can view the project proposal here (Dropbox link).

Other news:


After relentlessly avoiding HSMXpress, I finally got around to generating the G-code to mill out the fork of my electric scooter on the MITERS CNC mill. 

And while HSMXpress seems to be incapable of informing the user why it failed to generate a toolpath, it can do this:

Totally worth it. So, sit back, relax, and listen to the elevator music courtesy of youtube's audio edit function.

And IRL:

Front assembly sans mounting screws, rear plate and brake. 
I opted to use a caliper brake as it requires a single mounting point on the fork and a shorter pull in terms of brake lever travel.

No longer a sad wheelbarrow!

Sunday, November 17, 2013


I really ought to start documenting projects as I go along. 

Broken down into three parts, yo.

1. Lasers!

Once on my bucket list, this blue laser pointer was what constituted an overnight build-a-thon at MITERS. In my laser-induced, sleep-deprived stupor, I forgot to document much of the build, although, there wasn't much to do to begin with.

Diode: 445nm rated for 200mW
Power: two lithium cells (7.2V nominal)
Driver: ebay diode driver (good for stuffing into small spaces)

Given that the laser diode has a TO-18 package, I had to make and press-fit the heatsink into a brass adapter ring to fit in the lens assembly. Some patience on the lathe and a bit of sandpaper made the task at hand much easier. This was by far the hardest task.

The shaft was machined out of some aluminum stock and the end cap is mounted to the body with a set screw that isn't actually a set screw for convenience's sake. The shaft was press fit into the threaded lens/diode assembly.

Setting the power output of the diode driver:

I assembled a dummy load from a blue LED and a 10 ohm resistor and then measured the voltage drop across the resistor, which gave me the current output. The diode is set to run comfortably at 40mA to give me a 40mW output. I later adjusted this down to 8mW since I found the output even at 40mA too bright to use in any useful setting (powerpoint presentations, pointing at stuff safely).

*NOTE: the TTL pin has to be tied to Vcc in order for the potentiometer to function; otherwise, the diode driver will only give output at 0mA and 500mA and nothing in between. Also, the potentiometer is continuous, so be wary of adjusting the thing once it's soldered to the diode.

Thanks to Bayley Wang for the parts/instructions.

Diagram of the construction:

One of the more interesting aspects of having a laser of such a funny wavelength is that you can cause things to fluoresce  in the visible spectrum.

suddenly: violet!
It's rather unfortunate that photos can't do justice to its wonderful luminescence.

2. A wild speaker appears!

Yet another school project =_____=. Groups were given some magnets, washers, and bolts. We were then given the directive to build a functional speaker. Promising, I know.

Major difficulties: quantitative analysis of such a rudimentary system is essentially futile. I tried anyway. Kinda.

The requisite flat frequency response was to occur from 100Hz to 20kHz, which led me to try and get a mechanical resonance at slightly 100Hz. This would allow me to add a port tuned to the appropriate frequency to extend the bass range while keeping the high frequency response that I wanted.

In addition to having a flat response, this thing had to be reasonably loud; given that the speaker would have an impedance matched to the source, this meant I had a fixed length of wire: increasing the inductance would allow for more force to be applied to the coil as indicated by the Lorentz Force Law. This meant keeping the coil reasonably short and fat to increase inductance, as well as double layering the coil.

The tradeoff was that at a higher frequencies, the reactance could get as high as 4 ohms, however, that would only result in a 3% reduction in power, which was reasonable.

The choice for enclosure was acrylic due to its high compressive yield strength and ease of manufacture (laser cutter access). It also looks pretty.

Box volume was limited by the quantity of acrylic I had at hand - in good speaker design, the volume is supposed to be matched to the volume of air that the speaker displaces, its resonant frequency, as well as its mechanical and electrical Q. Again, quantities that are hard to measure. I ended up doing the thing I shouldn't do, which is stuff the box full of cotton to increase its apparent volume and the remove it until it seemed to sound best. Miraculously, the box without any additions seemed to work.

The port was also hand tuned to aroud 90Hz, which involved using an exacto knife to slowly cut away at its length (it ended up being ~1.2" in length).

Magnet arrangement was chosen mainly to achieve symmetry, but at the same time, maintain the flux density required to give the speaker enough output.

If there were one thing I'd do over in this speaker, it would be getting stronger magnets and reducing the inductance of the voice coil to maintain good output, but keep a nice high frequency response.

Lastly, the membrane, which provides the restorative force to the moving cone (made of a manila folder, no less),  was provide by some polyurethane sheet stretched and then hot glued against the frame of the speaker. This formed some semblance of a surround. The thought behind using such a lightweight material is to reduce the apparent mass that the coil has to move, which would otherwise attenuate higher frequencies.

The end result:


3. More coil things!

As part of a final project, I was granted funding for another coil. The nuance is that I'll have to make an analog interrupter that goes with it.

This will involve a revamp of Derpy coil into a separate driver and bridge to keep board costs down.

The interrupter is just a VCO made out of a 555 timer and an op amp fed with the audio. Nothing too fancy.

Current iteration of the driver
OCD has been added in the schematic, but has yet to be laid out. Just think of it as a UD, but without the totem pole driver.

More to come...

Saturday, October 12, 2013

Beepy things...because school.

As part of the course ES-93-5, "Music and the Art of Engineering", I am required to build a lot of things on breadboards.

I. Hate. Breadboards.

Or rather, I hate the fact that whatever I build on them usually requires n times more effort than what I'd put into say, etching a board, and that the breadboarded project ends up being a waste of time because at the end of the day, I have a breadboard.

That isn't to say breadboards aren't good for anything. It's just that they're terrible. 

The current assignment requires the construction of the following circuit (or something similar; the bare minimum does not require so many oscillators):

Broken down, this is a a bunch of NAND gate (with hysteresis) oscillators feeding into a power amp, specifically, an LM386, which then feeds audio to a speaker. 

Now, I find it quite quaint that my latest "project" is essentially a repetition of my first "legit" EE project, namely, a 555 timer organ I built in the summer of 2012. As someone who had a lot of trouble finding a good explanation as to how these worked, I see it fitting that I write one here. 

The concept behind signal production remains the same in both the NAND gate oscillator and the 555 timer organ of yonder: the frequency of the signal is determined by the time constant of each RC circuit formed from the output resistor and the accompanying capacitor. 

Let's look at this closely.

JP1 consists of a switch between one of the NAND gate inputs and "high" (9V), and R12 is a pulldown resistor (sets input "2" to 0V when there is no voltage applied through the switch).

Potentiometer R2, R11, and C4 consist of an RC circuit that determines the frequency of the oscillations. R26 is the input resistor that eventually feeds into the summing op-amp used to drive the speaker.

When the switch is open, input 2 is OFF, making input 3 ON. This is regardless of the value of input 1. When input 2 is ON, the state of input 1 can then alter the output of the NAND gate.

While the output is ON and there is no induced oscillation, the RC circuit charges up, but does not discharge, and it assumes steady state behavior. 

However, when input 2 is ON, the RC circuit will charge up, causing the capacitor's voltage to appear at input 1. This causes the NAND gate to change its output to OFF. It's during this off period that the RC circuit then discharges (all the while, input 2 is still ON, and input 1 appears ON for now). Once the voltage across the cap goes below the threshold voltage, the output will then go to an OFF state.
One half of each cycle consists of the charging and discharge of this RC circuit (illustrated by the red waveform above). 

Thanks to the magic of hysteresis, the trigger voltage at which the NAND gate decides the voltage is ON or OFF is roughly that after 1 time constant. 

This allows us to specify the frequency further as now, the period can be defined as two time constants.

with f = 1/T, f = 1/(2RC)

This then allows us to "tune" the frequency of each oscillator with values of R and C to acquire a specific frequency, thus explaining why a potentiometer is added to the resistance of the RC circuit. 

Here is a spreadsheet of the approximate R values for a 'C' scale:

You can also substitute your own values of C. 

The (semi) finished product (note the lack of battery connectors)

Yay, beepy things!

Tuesday, September 24, 2013

Slightly less imaginary scooter

Riding my gallant steed

U-channel: check.
Motor: check.
Wheels: check.
Aluminum plate: check.

Thanks to the magic of oxyclean caffeine, most of bluescooter (yes, it has a name now), was done in the lead up to Maker Faire New York. Sadly, it wasn't completed in time for the faire due to a lack of functional waterjets, but the progress so far deserves a post of its own.

Among the first tasks completed was the fitting of the motor hardware. Since I was using a motor with a 6mm OD shaft and had an 8mm ID sprocket, I bought a bronze bushing to fill the gap.

But, as with the best laid plans of mice and men, it required some love on the lathe.

Something's telling me that I should've used a smaller chuck.
The motor shaft was then milled to accommodate the set screws, and the sprocket was slipped over the bushing to drill the hole that would let the set screw hit the mating surface of the motor. 

It turned out something like this:
Not too shabby...
Following no specific order thus far, I thought it'd be a good idea to get some work done on the chassis.
Then I realized I had to mill diagonals. Kids, don't mill diagonals. 

The process involved clamping a reference-specifically a nicely water-jetted octagon someone had left in the stock pile- against the bed of the mill and then resting the u-channel against it. The u-channel was then held in place with the magic of step clamps. 


Le result.
A few hours later, I ended up with a nearly done chassis. The only parts missing were the mounting holes for the motor, rear, caddy and fork.

Those horizontal dropouts <3
Motor mounting involved machining a set of standoffs, which were then drilled and tapped. The outer surface of the chassis was also countersunk for that extra hardcore effect.

Note: I had to mill off the top and bottom of the motor mount, leaving the motor secured by only two points. I have no idea how well this will fare in vehicular duty. There's still room for an extra standoff running from the opposite wall of the chassis, however, if need be.

In lieu of time, the aluminum rear caddy was ditched for some blue acrylic lying around at miters.

Notice the sad tip of one of the panels: this is in no way a permanent solution.

I finished up mounting all the holes and ended up with the shiny version of melonscooter's ass.

Horizontal dropout tensioners were added to prevent the chain from sagging too much and falling off. The fact that there's about 2mm of clearance between the frame and chain make this a vital addition.

Bored with the fact that I hadn't made much obvious progress, I took it upon myself to mount the fork, which made bluescooter look more like a scooter than a sad wheelbarrow.

I ended up using the same mounting hardware that came off of the razor A4 in order to avoid tapping the metric screws and machining a new mounting plate. I might end up having to mill off the sides of the plate that stick into the body cavity for space reasons, but for the meantime, it makes for a simple solution.

There was also the task of mounting the bottom cover, which would eventually play some part in keeping the batteries and motor controller from falling out.

18 holes yet to be countersunk

Anyway, it turned out pretty nice.

Next up:

Battery pack, motor controller, and fork assembly!

Monday, August 26, 2013

Weekend Update

The pile of parts on my shelf is starting to resemble a scooter.

This weekend went off to a slow start, but, among other things, progress was made on the scooter.

Dimensions for the rear caddy platform and fork were finalized through the magic of Draftsight (solidworks for noobs).

Totally legit.
As you can see, I've been very thorough.

All that's left in terms of parts is getting the fork and rear platform sent out for water jetting. The only concern I have is the motor mount, which, considering I don't have any reference material, has been left out of this sketch. Not a problem as some creative milling can probably fix any clearance issues.

Trolling will be courtesy of a jasontroller, which will eventually be abused  modded for the purposes of increasing output wattage via decreased shunt resistance.

An in-depth how-to can be found here:

Power, originally coming from some derpy li-polys off of hobbyking, will be replaced by some A123 LiFePO4 cells that I'll have to weld together and arrange log style. I should be able to barely fit in a 10S2P pack (~36V and 5Ah).

Hub machining was a great success; I managed to mill down the ends of the rear hub to accommodate for the 4" aluminum u-channel.


To be continued.

Monday, August 19, 2013

Derpy photo shoot

Now that DOHLAC (Derpy/-oneTesla/Hot Long/Ass Coil) lives at home, I finally decided to take pictures.
Bad lighting is bad >__<.

Half bridge of 60N65s kept well within spec to avoid damn-I-blew-the-bridge-again syndrome (up to 75% duty cycle at 340VDC)

Gate drive provided by a pair of UCCs and feedback courtesy of question-mark antenna.


2.5" x 10" PVC, 32AWG wire

fRES @ 316kHz

I can probably push the coil to give more spark, but due to my unwillingness to replace the bridge, it'll be kept at a lofty 7.5". 

Long pulse widths for fire-y streamer goodness.
Longest spark on this run from a nice straight streamer. Top-mounted breakout point seems to distribute the E-field in a nice symmetric way that promotes longer sparks. 
Special thanks to Bayley and Kramnik for helping me troubleshoot this thing at one point or another.

Thursday, August 15, 2013

Project Dump

Working from nine to five doesn't seem to be helping my projects.

Part 1: The still imaginary scooter 

After learning how2mill, I set out on machining one of the few components that didn't require the motor or battery (which I have yet to order): the fork!

I should learn how2CAD at some point too.

After rough sketches and some trigy math in the comp book, I sadly discovered that If I were to set the axle in line with the handlebar shaft, I'd end up with a scooter nose-up. Not to worry though, as forward offset fork is here to the rescue! Combined with some derpy angle finagling by cutting away part of the  rubber 'shock absorber,' I'd achieve a scooter that is both aesthetically and structurally sound. Well, aesthetically sound at least. 

The resulting hunk of 1/4" thick aluminum resembled something slightly too angular for my taste, but as the first thing I've ever milled, I think it turned out ok. 

Part 2: Derpy Coil lives!

Also known as "Long Hot," Derpy coil has finally come to life after scraping the secondary base feedback scheme. 

Here's how the troubleshooting adventure went:

1. Coil is completed, run off a variac at low power. Nothing

2. Scoped across primary: looks legit. The primary waveform only goes to shit when it's running off of its own feedback. 

3. Add turns to secondary base feedback CT and hope it doesn't reach saturation. Nothing.

4. Remove high-pass filter. Poopy sparks appear. 


5. Feed the coil a 316kHz offset sine wave. Success! Kind of out of tune 2" long sparks appear.

6. Abort secondary base feedback, go to antenna: nothing.

7. Give up on this driver board and repopulate another one (without secondary base feedback junk attached).

8. ???

9. Success!

Here it is playing Solfegietto by CPE Bach. Spark performance isn't quite impressive as duty cycle was turned down to maintain note clarity. Modified oneTesla interrupter pulse widths were overlapping each other, which you'll notice when the really low notes start playing.

After Bayley reflashed the interrupter for true continuous wave output, several things happened:

Hot Long finally got hot and long.

Then the bridge blew after getting to ~140VDC. 

The number of primary turns then grew to 27.

Then the bridge died again, but at ~200VDC.

Failure is probably due to transients - further testing will resume after TVS is added and primary current is scoped.

UPDATE: 8/18/2013

Long Hot will no longer endure CW abuse and will continue to live life happy at 75% duty cycle, occasionally pushing out the odd midi file. 

Thursday, August 1, 2013

-oneTesla Lives!

After weeks hours of deliberation, I sent out -oneTesla to OSH Park, hoping to receive a functioning board.

But, oh, I got so much more.

Dat gold plating.
It might have been the fact that this was my first fabhouse board, but, damn. That looks sexy. The board features silkscreen, gold through-hole plating, and purple solder mask, making it quite the specimen.

A fully populated "Derpy Coil" with secondary base feedback tentacle.
Look! The inverter is inverting...!

It even does that inverting thing.

Power testing to come soon...

Monday, July 29, 2013

Maker Faire Detroit!

My first Maker Faire, and in the Motor City, no less.

July 26th marked the first day of Maker Faire Detroit and all of its insanity: lots of fire, homemade jet engines, (one)Tesla coils, giant dragon-sculpture-things, and of course, the wonderful community of makers to name a few.

I was there on oneTesla business, showing off demo coils and whatnot, but for me at least, the excitement lie in the other exhibitions.

(side note: oneTesla won editor's choice!)
The oneTesla team.
 I also happened across Paul Kidwell from the Geekgroup: their youtube series on SGTCs is part of the reason I'm coiling today, and as you can imagine, I was screaming like a soldier at a KPOP concert.
My expression does not do justice to the fangirl trapped inside omgomgomgomgogmomg
Enough words. Photodump!