So my father is a cardiologist who is also a geek, so I thought I’d make him something special this year for Christmas.

An iPhone stethoscope!

So, this is actually a very simple project, but it did take me a surprisingly long time to figure out because of the way Apple maps the pins of their connectors.

Physical Design

The physical design of this thing basically consisted of a regular stethoscope and a microphone.

Apple devices like 2.2kOhm, -44dBV microphones.

Here’s the top part of the stethoscope.There was a soft rubbery outer shell filled with a slightly more rigid semi-translucent inner material.The inner material is presumably just to keep the earpieces rigidly held together as it ended right as the stethoscope hose began.Cutting the hose right at this point let me squeeze the microphone into the hose at the point where it was widening and secure it with silicone sealant:

I wanted this thing to look professional (if it had any chance of being used in an actual doctor’s office), so I gave it the special ch00ftech seal.

Alright, so it didn’t come out perfect, but I still think it’s cool.

Electrical Design

The Apple headphone jack has four connections: two for left and right audio, one for microphone, and one for ground.The connector is called a Tip-Ring-Ring-Sleeve connector (TRRS) as opposed to the typical TRS you see in a normal pair of headphones. The idea is that if you plug a normal TRS connector into a TRRS jack, the sleeve of your plug shorts the sleeve and second ring together basically turning it back into a TRS jack. I had to buy one special from Digikey.

Now, typically with TRS connectors, the tip is left audio, the ring is right audio, and the sleeve is ground.This makes sense because the sleeve will run outside of the other wires and is usually connected to some kind of mesh that shields the other conductors from unwanted electrical interference.Because of this, I assumed that the sleeve was also the ground here.I connected the tip and first ring to the left and right channels of a TRS jack (for the user’s headphones) and the second ring to ground.Ground was also connected to the ground of the microphone.The sleeve was connected to the microphone’s positive output.

When I tried this out, the microphone wasn’t working.I figured I had the mic backwards and rotated it around, and it worked!

Now here’s the really funny bit.I couldn’t find my headphones during testing, so I was just trying it out with the headphones disconnected and playing the audio back on my PC speakers.When I finally found my headphones, I noticed that regardless of whether or not the iPod was recording, I could hear audio from the microphone coming through my headphones.This is when I figured out what I was doing wrong.

So I was using the microphone as the headphone ground.With the headphone ground bouncing around, I was picking it up as audio in my headphones.Well, anywho, with all of that fixed, I was done.

Performance

The stethoscope works surprisingly well!You just have to find the right app.Apple’s voice recorder app is meant for just that and does a terrible job.I believe it adds some kind of filtering that is meant to cut out all frequencies outside of the typical speech band.

I got pretty good performance using the iStethoscope app which is really meant to use the iPhone 4′s internal microphone, but can also accept audio from external sources.Here’s a video showing it off:

www.youtube.com/watch?v=bMGmIxnyAzY&feature=youtu.be

There is an odd popping that you hear from time to time, and I’m not certain what’s causing that.I think it might have something to do with the audio processing going on in the app, but it’s still very very useable.

So there it is.I wonder how many other cool microphone-based devices you can make for iPhone…

[via: http://ch00ftech.com]

Today I decided to take on this bad boy:

Intro

If you’ve been reading my blog regularly, you’ve probably already seen this thing in my icebreaker post.The toy functions as a portable low-resolution scanner that can scan and save a number of images and then reproduce them using persistence of vision.

In the icebreaker, I ripped off how they handled measuring the oscillation-rate to produce the persistence of vision.I could do this without even opening the case.For a future project, I wanted to find out how they managed the scanning function.That’s a little more in depth.

My assumption was that they were scanning using the LED-as-a-sensor trick that I wrote about in this post.The only problem was that with my implementation, the trick requires twice as many micro-controller pins as LEDs.When you get up to 32 LEDs, this gets kind of ridiculous.I wanted to see how they did it.

A starting question

Before getting into the details of the circuit, something has always bothered me about this thing.It has 32 LEDs on its front, and can produce images with 32 pixels of height.As you scan, you can see LEDs illuminate in order.My assumption was always that one LED would turn into a sensor and one or two neighboring LEDs would illuminate and bounce light off the scanned page into the detector.This means that the micro just has to store the values picked up from each LED and then reproduce them when the device is in playback mode.

Using my SLR, I took a 1/1250s photo exposure of the scanning mode in action, and came up with this:

So that confirms that only one LED is lighting at a time.I guess the two neighboring ones are being used as sensors.

The only problem is that it seems like not every LED is being used as a sensor.Take a look at this video.

From here on out, the LEDs being used as sensors will be called “sensors” and the LEDs used as lights will be called “lights”.This only applies in the context of scanning.During reproduction, all LEDs are “lights”.

I figured there had to be some kind of algorithm running inside the micro that would interpolate the pixels between rows and turn a 16 pixel high image into a 32 pixel high image.To test this, I came up with a little experiment.

I used a marker to cover up three of the sensors and did a quick scan.I came up with this:

You can see the three sensors are lighting up (dimly) behind their black marker mask, and the lights around them are also lighting up.This means that the firmware picked up that three of the sensors in a row were coming back black, so the lights between them must also be black.

Now, it is still possible that the LEDs swap jobs for a very very short period of time (lights become sensors, sensors become lights).The results of this experiment could be explained by the fact that the sensors (now being used as lights) are covered up so that the lights (now being used as sensors) have no light to detect.To test this theory, I ran the experiment again supplying light from an external source to the group of covered sensors.I came up with the same results.

So, there is some algorithm that determines the values for the lights using the data from the neighboring sensors.

The circuit

The circuit is split into two boards.

In this image, the bottom board deals mostly with power regulation and the oscillation sensing unit.The top board is where all the brains are.There are four ICs on the top board:

• 74LV4051 – A standard 8 port analog multiplexer.A multiplexer means that the device has no means to illuminate two LEDs simultaneously.They must be pulsed in turn very quickly.
• 24LC16 – A 16K EEPROM that is likely used to store the images after they are scanned (the device can store up to six different images and retains them through power loss).
• “1AP G0K1″ – I was unable to find out what kind of IC this was, but I think it’s safe to assume that it’s some kind of micro controller.

So.The LEDs are multiplexed.Interesting.This is a little worrying for my future endeavors because blinking LEDs that quickly requires a pretty hefty micro controller or an FPGA.The former is expensive, and the latter I’ve never worked with.

Either way, I took another photo of the back, and loaded them up in Photoshop as two separate layers.After mirroring the back and inverting its colors to add contrast, I was able to line them like this:

Playback

My first task was to figure out how images are being displayed on the LEDs.I started by using the paint brush tool in Photoshop to color code exactly which LED is connected to what.It looked like half of the LEDs were connected to the first multiplexer and half to the second.Here are the connections for the first one:

It’s hard to make out, but there are little dots on the pins of the leftmost IC which are color coded to the LEDs. And here’s the second:

As you can see, each pin from a multiplexer connects to two LED anodes.This makes sense; there are 16 ports available between the two multiplexers and 32 LEDs in total.This likely means that control for the LED cathodes is split so that only one of those two LEDs can be illuminated at a time.

Looking at the cathodes, I noticed a pattern where it seemed that every other anode was connected together.You can see that in this image stenciled in white and gray (I only bothered to do half of the LEDs, the other half follow a similar pattern but on two different lines).

The anode lines converge on two transistor circuits shown here:

I had to figure out what the transistors were.They were labeled with just “24″ and “54″ which turned up nothing on Google.Assuming that they were BJTs and used the common pin ordering for BJTs in a SOT-23 package:

I was able to re-draw this part of the circuit as follows:

Now it was just a task of figuring out which transistors were PNP and which were NPN.I noticed that during normal operation (with an LED lit), the emitter and collector of Q4 were high with the base pulled low.That would seem to imply that Q4 was a PNP transistor: Pulling current from the emitter to the base caused current to flow from the emitter to the collector.Similarly, the emitter and collector of Q5 were low as the base was high.This would imply that Q5 is an NPN.Q4 was marked with “54″ and Q5 with “24″ so I guess all the 54s are PNP and all the 24s are NPN.Adding the appropriate markings to my schematic produces this:

It’s worth noting that R1 and R5 are 90 ohm resistors which means that these LEDs are probably being run a little above spec.Assuming a Vf of 2v (typical for red LEDs), that gives you a current of 33mA!This is acceptable though considering how the LEDs are all being driven at a very low duty cycle.

It’s not quite clear what the connections through R2 and R6 do.My first guess was that that’s how the micro controller reads the voltage off the high side of the LEDs as it drops (assuming they’re using the same method I used for the LDDs article), but as I found out later, that’s not the case.They aren’t unique connections either.The line coming from R2 also connects to R4 which serves a similar purpose in one of the other low-side drivers.

So, one issue I noticed is that all four of the low side drive circuits (these two and the two driving the rest of the LEDs) are all controlled by the same line coming from the micro controller.This really bothered me for quite a while because as I said earlier, in order to have two LED anodes on the same multiplexer port, there needs to be some way to drive the cathodes independently.Well, it turns out that the other two circuits aren’t the same as the first two.They look like this:

Where one of them includes the resistor to 5v like before.So this solves that problem.When the signal line from the micro controller is low, the first two ports are off and the second two ports are on.When the signal is high, the opposite happens.This prevents the micro from being able to control two LEDs simultaneously.It interesting to note that this method isn’t perfect as there will be some crossover when both of the LED sinks are active.If you darken the room and look closely, you can often see a slight ghosting effect as the image from the top of the stick is repeated to the bottom very dimly.This isn’t noticeable though unless your image exists primarily in one half.

Scanning

Now scanning on these LEDs is a lot more complicated.The whole method depends on the touchy-feely process of running an LED in reverse bias.LEDs aren’t really meant to do this, so the LED manufacturers aren’t going to make it super easy.This requires some gentle massaging on the part of the engineer designing the circuit.Without access to their test data and data sheets, it makes it very difficult to tell exactly what is going on.

As I said before, the high side of each LED bank is an an analog multiplexer.The common pin of this mux has a PNP transistor dropping current into it and by extension into the LEDs.One of the muxes also has an additional three terminal transistor device seen here as Q9.

Also indicated here is the “PR” trace.This trace runs straight to the micro and also on to the second board.I’m not certain what the “PR” stands for, but I’m pretty sure that this is an analog line that feeds into an onboard ADC on the micro.This would explain why it’s connected to the other board; it’s used to measure the voltage off the proximity sensor that the device is using as a cheap accelerometer as I did in the icebreaker.

Here’s a simplified doodle of this part of the circuit:

During scanning operation, BJT’s base is pulled high, turning it off.

Now back to that Q9 device.It’s not marked 54 or 24 like the other BJTs and my guess is that it’s some kind of FET.This would make sense because the gate of a FET would provide a very high impedance allowing the small amount of current coming from the LEDs to charge the capacitor and raise the cap voltage.The rising voltage on the gate of the FET would turn it on or off appropriately and allow a nice buffered signal to go the the ADC of the micro along the “PR” line.Here’s a trace of the PR line during scanning operation:

This is the device scanning a number of LEDs.The FET device appears to work as an inverter (pulling down the 33k resistor) so the sloping voltage drop you’re seeing here is actually the voltage rising on the capacitor.Two things are interesting about this picture:

1. The micro appears to spend an equal amount of time measuring each LED.This is how I realized that it had to have an ADC (as opposed to measuring the varying amount of time it would take the cap to charge to a fixed voltage like I did in my LDD).
2. Some LEDs charge the cap more than others.This makes sense because for this trace I was scanning a drawing that I did in Sharpie on white paper.

I marked the Source, Drain, and Gate in the above schematic following the standard SOT-23 ordering:

Now, unlike the BJTs, searching for a SOT-23 device marked with “KN” did turn up a result.I found the 2SJ305 which is a P-type FET.This was troubling to hear.Typically for a P-FET, the source is tied to the high voltage rail.With the source tied to ground, it would be impossible get a negative Vgs to even turn the FET on!

Well, I really worked on this problem for a while.It’s possible, that because the source is tied to ground through the 2k resistor that the source could float up a bit and allow a negative Vgs, and it was that much more believable because I figured that the designers would have been pulling some serious black magic to get this gadget to work anyway.

Well, I got frustrated and decided to ask the minds on the /r/ECE subreddit.I got some pretty awesome responses.They pointed out that it’s completely plausible that the device is actually an N-type MOSFET as often times multiple components will share the same labels (and even be mis-labeled entirely!).

Just to be sure, I ran a little experiment with this circuit:

The gate was attached to a free piece of wire that I could connect to various parts of the circuit. I soldered the circuit right on to the mystery FET straight on the board.This was easier than desoldering it, and assuming power was disconnected to the board, it would likely be no different.

Here are my results:

This is it with the gate floating (I noticed the same results when I tied it to ground, so the analog mux must be grounding it).The LED is on, but only slightly.

BOOM! When I attached the gate to the positive rail, the LED got super bright.This means that my suspicions were correct.It’s not a P-FET at all, but rather an N-FET with the source tied to the low rail like it’s supposed to be.

 Putting it all together

All together, my circuit now looks like this:

And here’s a timing diagram doodle to help out:

For scanning, the LEDs are arranged in such a way that when the current sink is active to turn on the necessary light, the current sink for the adjacent sensor must also be activated.B and C appear to fight that current sink and pull the LED cathodes high but only to about .5V.I’m not too sure what the purpose of this is because there’s nothing stopping the circuit from having the adjacent lights and sensors on two different current sinks.Perhaps if they were able to pull it up to 5V, the LEDs would conduct too much current too quickly or something?

I think in the context of displaying on the LEDs, B and C just tag along with A to help pull down the LED cathodes that much more.

Remaining questions (and answers)

My biggest remaining question was exactly what purpose R9 and R10 serve. They appeared to be some kind of pull up resistors, but they seemed to be doing a lot more than that.This is a scope trace of A (yellow) and the PR line (green) (different voltage scales):

So you can see that the cap is being charged and discharged both as A is high and low.Here’s the same trace with R10 removed:

Obviously, it’s totally screwed up.Here’s the really weird part.When I removed R10, I noticed that this seemed to break the scanning functionality of the LEDs labeled as “Right” on my schematic.Also, you can see that the PR line doesn’t oscillate when A is high.That means that as C is oscillating, the PR line does nothing.

The weird part is that R10 isn’t even close to C or the “Right” LEDs!

Well, it wasn’t until I had it all drawn out that I realized exactly what was going on.I had forgotten that this gadget is multiplexing the LEDs so that while one LED is begin used as a sensor, there is another one joined at the anode:

If that 10k resistor wasn’t there, the second LED would suck all of the reverse current out of the mux and prevent the capacitor from charging!In effect, when the mux is measuring an LED, it’s also measuring its twin, but the assumption is that the twin isn’t very well illuminated, so the reverse current will be negligible.

Now the only thing I’m left wondering about is what R4 and R6 do.They lightly attach the C and B lines to the LED light sinks.My guess is that the designer just wanted to keep the cathodes of the LEDs from floating when the source was turned off, and added that in there.Removing R6 didn’t have any appreciable effect on device function.

Conclusion

So, I still don’t know exactly what’s going on inside the micro controller, but it’s pretty easy to guess that it’s taking analog readings from all of the LEDs, using their values to determine some black vs. white threshold, and storing the values in its EEPROM to be displayed on the LEDs.

There is still more to the circuit (power stages, etc), but I covered all of the real magic in this review.I’m planning on implementing a similar technique in a product I will be developing, so this definitely provides an awesome head start.

This is my first time every trying to reverse engineer a product, and I’m really impressed with myself.I certainly wasn’t expecting to get such a complete picture with so little information at my disposal.If I find another product that is similarly cool, I’ll be sure to do another one of these write-ups!

I hope you enjoyed it!

[via: http://ch00ftech.com]

I’ve been working on a way to push data into a microcontroller using a computer monitor (or smart phone) which flashes black and white. I’ve done some preliminary tests using one photoresistor read by an ATmega168 analog comparator circuit. The results have been mixed.

Analog Inputs

Here are the two circuits responsible for voltages being measured by the analog comparator. The circuit on the left should produce 2.5 volts on the AIN0 pin. Strictly speaking the 330 Ohm resistors (R1 and R3) are unnecessary, but I’ve included them to protect the microcontroller from unrestricted current if the trimpot is turned all the way to one side.

On the right is the circuit with the photoresistor. When it is held up to a white screen, the voltage is around 1.5 volts. A black screen produces about 3.5 volts. This is distinctly different from the 2.5V reference and easy for the analog comparator to differentiate.

The Results

So far the results are not entirely good. I have no problem detecting edges with the comparator running in toggle mode. But when I have the microcontroller count those edges I find it’s registering multiple (sometimes dozens) of edges for each transition. I have experimented with little success to filter these out in software by checking a running timer before registering the next reading. I also tried only detecting rising edges, or switching between rising and falling detection after each interrupt. I’m not entirely certain if timing is part of the problem. I’m testing with a JavaScript application that flashes a sync signal, preamble, and data packet using Manchester encoding with 100ms bit frames. It could be that the timing for that is very unreliable, and outside of the +/- 10ms window I’m allowing for in each half-bit frame. (Give it a try if you wish)

It seems the best I can do with this method is to almost reliably count rising edges (within maybe 10%). However, I know that the ATmega168 has input capture capabilities built into Timer1 which include a hardware noise filter. I’m still studying the datasheet on how this works, but you can bank on seeing a post about that functionality soon.

This would be a lot easier to resolve if I owned an oscilloscope. I would be able to toggle a pin on the uC with each interrupt, using that output to measure the timing with the scope. Oh well… I don’t think I’ll have that tool in hand any time soon, if ever.

[via: http://jumptuck.com/]

After I posted the menorah yesterday where I said in passing that you’re not supposed to have circuit traces bend at right angles, I got a few questions from readers asking exactly why this is.The explanation is kind of complicated and involves some of Maxwell’s equations as well as multivariable calculus, but I’ll do my best to explain it in a way that doesn’t require any special knowledge of either of these topics.This is going to be a doozy, so read on if you dare.

Terminology

Flux

Flux (Φ) is a term that is used when dealing with vector fields.In this article’s context, it sort of describes the amount of electric field that is passing through a surface.Think of it this way: let’s say you have a river flowing by you and you dip a tennis racket into it.If you were to multiply the area of the tennis racket by the rate of water flow, that would be the flux of the water through the surface described by the rim of the racket.Increasing the size of the racket would increase the flux as would increasing the flow rate of the water.

If you were to turn the racket sideways so that the water runs along either side without passing through, the flux would be zero.Basically, flux is only concerned with the component of the water’s motion that is perpendicular to the area of the racket.If the water runs parallel to its surface, there is no flux.

Electric Field

Electric fields are vector fields that originate in positive charges and end in negative charges (we’re using conventional charge here by the way).A positively charged particle, when placed in an electric field, will feel a force in the direction of the field proportional the the strength of the field and the charge of the particle.

This is why electric fields must be zero inside conductors.If there was ever a non-zero electric field inside a conductor, the charges would be able to move along that field.Given enough time, all of the charges would find their final resting places and there would be no electric fields remaining.This all happens at roughly the speed of light, so it’s pretty safe to say that there is never an electric field inside a conductor.

Electric fields are typically represented as curved lines with arrows pointing towards negative charges.The spacing between the lines represents field strength with closer lines representing stronger fields.

Electric fields can take on a number of shapes given the geometry of the charges.A single charge’s electric field might look like this:

When placed near a negative charge, it might look like this:

One configuration that is actually quite common (though it might not seem like it should be) is an infinitely large charged conducting plane.The following is a cross-section of such an arrangement.Notice that the charges line up on the surface.This is because they’re trying to get as far away from each other as possible (like charges repel).

Note that the fields are parallel to each other.This can only really be true in an infinite sheet.Think about it this way: if the plane goes on forever in every direction, what reason would the field lines have to arbitrarily go in any particular direction?That’s a pretty weak proof, but it’s easy, and it happens to lead you to the right conclusion.

In reality of course, there are no infinite planes of charge, but on a small enough scale, you can think of any plane as an infinite plane and get fairly accurate results.

Electric potential

Electric potential describes the energy that charges store when they are placed into an electric field.Remember what I said earlier about charges moving along an electric field?The movement of charge in such a manner is the conversion of potential energy to kinetic energy.Potential is usually thought of as a difference in potential between two points and is measured as the energy that would be released if a charge were allowed to travel between these two points:

Note that it is only concerned with the distance travelled parallel to the electric field.If you were standing on an infinite plane of charge and moved a free charge to the left a foot, you would not be changing its potential at all because its motion was not in the direction of electric field.

An easy way to think of this is to draw “equipotential” lines which represent….well…points that are at equal potential.When we moved that hypothetical charge to the left a foot, we were moving it along an equipotential line.

If you want to measure the potential between two points, you just need to count how many equipotential lines you cross when going from point A to point B.

Measuring potential would be rather confusing if you had to constantly specify where your starting point was every time.Because of this, engineers typically use “ground” as a reference point.Although the ground potential in my wristwatch’s circuit is not the same as the ground point in your laptop, for all the components in a single circuit, the ground is the same.

So that’s enough for vocabulary.On to the nitty gritty.

Gauss’s Law

Gauss’s Law is one of Maxwell’s equations.It describes the relationship between the electric flux through the surfaces of a 3d shape and the charges contained within that shape.All you need to know is that the more flux you have, the more charge you have.Here’s an example:

This is a single charge inside an imaginary sphere that we will consider out “Gaussian Surface”.To measure the flux, think of the surface of that sphere as being made up of tons of tiny little tennis rackets from our metaphor, then measure the electric field passing through each of them.

Note that A-perpendicular is just A in this case because the geometry dictates that the electric field lines will always be perpendicular to the sphere’s surface.This is why we don’t use a cube in this example.

The sum of all of these little fluxes will equal the total flux generated by this charge and will be proportional to the charge.

You might be wondering how this can be.What prevents us from changing the size of the sphere without altering the charge at all.After all, it’s an imaginary sphere, we can do whatever we want to it without touching the charge. Wouldn’t changing the Gaussian surface change the flux?

The answer is: No.

Notice that the electric field lines spread out as they get farther away from the charge.As I said before, the density of the electric field lines represents the field’s strength.As it turns out, the farther away you get from a charge, the weaker the electric field.

So, let’s say you use tennis rackets of the same size to make a larger Gaussian surface.You would probably need a lot more tennis rackets.Each of these rackets would have a weaker electric field passing through it, and after adding them all together, you would end up with exactly the same amount of total electric flux and therefore the same amount of charge.

Gauss’s Law on an Infinite Conducting Plane

Let’s return to our concept of an infinite conducting plane.Does Gauss’s law work here? Yes!

Here is a drawing of a Gaussian cube measuring the charge on the surface of an infinite plane:

That’s a little confusing to deal with, so let’s drop down to a two-dimensional cross section.Once you understand it in 2d, it should be pretty easy to jump back to 3d.

Applying Gauss’s law is quite easy.All of the cube’s surface that exists inside the conductor can be ignored because the electric field inside a conductor is zero.Likewise, all of the sides of the cube can be ignored because their tennis rackets run parallel to the electric field.That means that the area of the top of the cube is proportional to the charge inside it.

This makes sense when you think about it.The only way to increase the flux is to increase the area of the top.Making the cube taller or shorter will have no bearing on the flux.The perpendicular area and electric field strength are unchanged.

So how does this all relate to bent traces?

On a small enough scale, you can think of a trace in your circuit as its own little infinite plane (it’s a stretch, I know, but it works, trust me).

Let’s try applying this Gauss’s law technique to a slightly bent infinite plane:

For the Gaussian shapes on the right and left, nothing has changed, but for the one in the middle, a lot has.Remember how we could make the cube as tall or short as we wanted before without changing anything?With the geometry of the bent edge, making it taller will increase the area of the top surface.Because the charge inside the surface isn’t changing, this means that the electric field must be getting weaker so that the flux remains the same.

This is kind of like our sphere problem from before.More tennis rackets means weaker electric field.You could also have figured this out by just looking at how the electric field lines in the drawing get farther apart as you move away from the plane, but who wants to trust a doodle?

So what’s the big deal?Why can’t you let your electric fields get weaker?The real issue is not them getting weaker, but them getting stronger.With the case of the infinite plane, you could get infinitely close to the conducting plane without altering the electric field at all.Now, as you get closer, your relevant Gaussian surface shrinks in size.Because the surface is shrinking, the electric field must be growing the compensate.Take a look at what this does to your equipotential lines.

As you can see, the equipotential lines are packed closer together because the electric field strength is higher, closer to the plane.

Now eventually, with this configuration, you’ll reach some kind of maximum potential when your Gaussian surface crashes into the surface of your conducting plane.What about in the case of an infinitely pointy plane?

No matter how close you zoom in, you can still draw the same gaussian surface that encompasses the same charge.That means that as your tennis racket gets infinitely small, your electric fields must be getting infinitely large!Of course, this doesn’t happen in real life, but it gives you an idea of the trend you can observe when your conductor gets pointier and pointier.

Okay, so we’ve got a lot of equipotential lines crammed together and a lot of electric field. Who cares?

If you remember from before, the strength of the electric field dictates the amount of force experienced by charge placed within that field.If you have a sharply pointed charged conductor, you will have a strong electric field around that conductor. So smooth it out a bit.45 degrees is as sharp as you should get.

Basically, when you boil it down, having a pointy trace will increase the likelihood of your circuit acting strangely.For example, high frequency signals passing over these traces will generate large, rapidly fluctuating electric fields which could cause your circuit to give off some unwanted EM radiation.

Also, pointy traces can also be prime targets for electrostatic discharge as the large electric field will suck charges out of charged bodies that get near them.In fact, a lot of circuit builders will often intentionally place a pointy ground trace and a pointy signal trace very close to each other to draw any ESD on that signal trace (say, if it goes to an external port that the user might touch) safely to ground and away from any sensitive components.

In fact, a lot of people have been using this technique on a larger scale.Lightning rods are usually made to be very pointy so that will literally suck charge out of the sky and direct it safely to the ground out of the way of any sensitive structures or people.

Conclusion

I hope that sums it up for you.I actually had to do a little review from an old textbook to write all of this, so it was definitely a worthwhile refresher.I hope you enjoyed reading it as much as I enjoyed writing!

[via: http://ch00ftech.com]

/*

AVR ATmega168 Analog Comparator Demonstration

by Mike Szczys

I’m using a voltage divider with a photoresistor on

PC6 and a voltage divider with equal values on PC7

to yield a 2.5V reference signal.

The analog comparator is set to throw an interrupt

whenever there is a zero crossing. I then check the

ACO bit to see if I should turn on LEDs on Port B or

not.

jumptuck.com

*/

#include <avr/io.h>

#include <avr/interrupt.h>

voidinitComparator(void){

cli();

ACSR|=(1<<ACIE);//Enable analog comparator interrupt

sei();

}

voidinitIO(void){

DDRB=0xFF;//PortB as Outputs

PORTB=0xFF;//All outputs high

}

intmain(void){

initIO();//Initialize I/O

initComparator();//Initialize the analog comparator

while(1){}

}

ISR(ANALOG_COMP_vect){

//Check for rising or falling edge

if(ACSR&(1<<ACO))PORTB=0×00;

elsePORTB=0xFF;

}

[via: http://jumptuck.com/]