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1  Using Arduino / General Electronics / Re: Determining how many ohms resistor is needed on: May 26, 2013, 01:21:36 pm
after a few times of calculating them... you get to the stage where you jusy slap in a rough estimate...

resistance = volts / current..



This is the magic formula.  I just wanted to add something that wasn't obvious to me at first.  Suppose you're trying to find a resistor to go with an LED, and that LED has a forward voltage of 2.4V according to the datasheet.  That means that the voltage is going to decrease by 2.4V as it passes through the LED.  So to find the resistor value, you subtract the forward voltage of 2.4V from the power supply voltage (5V usually with the arduino) and you get 2.6V remaining.  That's the number that you want to use for finding the resistor, not the supply voltage.  If you wanted to supply the LED with 15ma (0.015 Amps) to keep it a little below it's max rating, that would be R = 2.6V / 0.015A.  So R = 173.  That's not a standard resistor value, so you'd use the next step up - 180 Ohms.  If you did the same thing without taking the forward voltage into consideration you'd get a resistor value of 333 ohms, and would probably be pretty disappointed by how dim the LED was.
2  Using Arduino / General Electronics / Re: Best way to solder/make connections between holes on PCB on: May 24, 2013, 12:09:06 am
If you don't have any really thin wire and you've already used all of the legs that you've clipped off of resistors, two or three strands from a stranded wire twisted together can be tacked down and soldered.  Doing it without wire works, but it's a pain and usually takes a lot longer.  Anything that solder will stick to makes it easier.
3  Using Arduino / General Electronics / Re: manufacturer's part list request on: May 23, 2013, 09:42:52 am
You might have better luck in the "Gigs and collaborations" section of the forums.  This section is more for general electronics questions.  That being said, the schematic and eagle files for the due are available here on the arduino web site.  Can't you just figure out which components you need from those?
4  Using Arduino / General Electronics / Re: High side NPN on: May 19, 2013, 03:25:41 pm
Thanks for explaining all that, guys.  I had a feeling that it was possible to make it work, but I think I'll save high side npn switching for breadboard experiments.  It works, but as Pito said, it's not the right way to do things...

While looking around at LED matrix designs and how they solved this problem, I noticed the MIC2981 in a few schematics.  It's a little pricey at $2.83 each from digi-key, but looks like it will get the job done with just 8 ICs and a handful of resistors.  It's a one-off design built for fun, so I'll gladly pay a few bucks extra to avoid adding all those PNPs.

http://www.digikey.com/product-detail/en/MIC2981%2F82YWM/576-1159-ND/771628
5  Using Arduino / General Electronics / Re: High side NPN on: May 19, 2013, 03:53:52 am
See the pictures below - even with a "saturated" 2222 transistor the max current you can press through the 2 LEDs (Vf=2.1+2.1=4.2V) is about 5mA.
That is because you need MUCH higher driving voltage than 5V.
On the third picture we use 6V driving voltage in the same setup - the current through the LEDs is 50mA.
On the 4th picture you may see the circuit with IRF530 - nchannel FET. As you can see in order to get 50mA current through the LEDs you need 8.5V input driving voltage.
We discussed this topic already, don't we? smiley


As I mentioned in the first post, I've seen lots of examples of why a 5v signal can't be used to control a 12v source using an npn in a high-side switching configuration.  But following your examples, what happens when there's 5v on the base and 4v on the collector?  I guess what I'm getting at is...  How much higher does the driving voltage have to be in order to control 4V?  How about 3V?  Where's the cutoff point where an NPN will work?
6  Using Arduino / General Electronics / Re: Substitute Arduino UNO Rev3 Atmega328p-pu with Atmega328-pu on: May 19, 2013, 01:27:20 am
Yes, you can do that.  As long as it has the arduino bootloader on it, a 328 is a 328.  Some models have a different minimum voltage, but that won't be an issue on the arduino board.
7  Using Arduino / General Electronics / Re: I made a mistake with transistors. on: May 19, 2013, 12:50:35 am
You could always solder wires to the legs and attach them to the proper pads.  Or, if it fits the footprint, you might be able to turn it at an angle and get C and E connected to the proper pads with B connected via jumper wire.  With only 3 pins, it shouldn't be hard to make it work.
8  Using Arduino / General Electronics / High side NPN on: May 19, 2013, 12:00:47 am
So I was working on redesigning the controller board for my 8x8x8 LED cube (common cathode) to make it brighter when I noticed something odd.  I could find lots of examples on the net of why npn transistors can't be used for high side switching which illustrate the point using a schematic with a 5V signal controlling a 9V or 12V load.  Something I couldn't find was an example of using an npn for the high-side switching of a lower voltage source.  If I dropped the voltage supplied to the LEDs to something like 4V, couldn't an NPN be used in that case?  I ask because although adding pnp transistors would allow me to make the LEDs brighter with a simple software change to invert the logic, soldering 8 darlington arrays is a LOT more appealing than 64 sot23 or to-92 packages.  Of course this is assuming that the forward voltages don't add up to more than 4V, in which case the PNPs are the only way to go...
9  Using Arduino / General Electronics / Re: CY7C68013A-56 EZ-USB Logic Analyzer...? on: May 06, 2013, 10:41:00 am
I don't think there is any firmware, or anything to reverse engineer.  I've seen quite a few products sold as saleae logic analyzers that could double as a usbee logic analyzer.  Switching between the two was supposedly just a matter of changing the device id on an eeprom chip on the back of the board via usb, and most were being sold for $5 - $10.  For that price, they probably didn't put much effort into it.

Has anyone who has one of these tried using it with open source logic analyzer software like the OLS client?  If it can be used legally, it might be a pretty useful little device!
10  Using Arduino / General Electronics / Re: Resistor Jumpers on: May 03, 2013, 09:50:20 am
It's an interesting idea and might be useful for some components, but resistors are so cheap...  In order to protect a part worth $0.005, you're connecting parts worth far more.  The wire alone is probably worth as much as 10 resistors.  One similar thing I've done in the past that can be quite helpful, though, is to cut off one of the legs on an LED and solder in a resistor in it's place.  Then attach wires to the ends like you were planning to do with the resistor.  Besides being used for simple LED projects, it doubles as a silent continuity tester.  Cathode goes to ground and anode gets poked around to see where the circuit is broken.  Great for those late night sessions when everyone is in bed and the multimeter's continuity test is beeping too loudly.
11  Using Arduino / General Electronics / Re: 5V regulated power on: April 30, 2013, 12:42:08 pm
Fair enough.  I was just hoping that you knew why the warning was there.  smiley
12  Using Arduino / General Electronics / Re: 5V regulated power on: April 30, 2013, 12:08:06 pm

And about Arduino's warning on their product pages?


Have you ever damaged an arduino by connecting a regulated 5v supply on the 5v pin?
13  Using Arduino / General Electronics / Re: where to get info about ledseedunio ? on: April 30, 2013, 11:57:15 am
Although I don't have any info about the board, they're sold by ledsee.com.  You might be able to contact the manufacturer for more details about it.  BTW, what bridge are you referring to?
14  Using Arduino / General Electronics / Re: Eagle net names and parts connected in series on: April 24, 2013, 12:32:23 pm
Great, thanks!  I had a feeling that was how it should be done.  I've just begun redoing my first few attempts at PCB designs, and I'm trying to label everything this time.  I'll follow your advice and start using more descriptive names, with different names on different segments.
15  Using Arduino / General Electronics / Eagle net names and parts connected in series on: April 24, 2013, 12:17:22 pm
I'm having kind of a silly problem when trying to move designs from the arduino and breadboard to homemade PCBs.  Any time a part is connected in series such as a resistor on a signal, eagle wants to route a trace through both pads of the part.  I'm assuming that the problem comes from having the same net name on both sides.  Currently I just work around it by leaving that bit unrouted, but I was wondering if there's another way to handle it that I've overlooked.  Should I be using a different name for VCC on either side of a ferrite bead?  At some point I may actually design something that's worth showing off, and I'd like to be sure my schematics make sense to others.
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