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.

Lulz.

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).

Finished!

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:

IT FINALLY HAS ANOTHER WHEEL

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!

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. 

http://upload.wikimedia.org/wikipedia/commons/3/31/Opamprelaxationoscillator.svg

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:

https://docs.google.com/spreadsheet/ccc?key=0Alcl-LrRTaxadHFSZXFJSGFGanZQMTNGcXh3SGxNS3c&usp=sharing

You can also substitute your own values of C. 

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

Yay, beepy things!