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1  Using Arduino / General Electronics / Designing current sources for driving LEDs. on: April 17, 2014, 01:08:13 am
For high power LEDs, it is very inefficient to use resistors to limit current.  The resistor can end up dissipating more energy as heat than the LED does.  So for a lot of the power LEDs out there, there are power supplies which are AC-to-DC power supplies whcih also limit current.  My question is how does one go about designing a DC-to-DC (no voltage change) current source like this rather than buying off the shelf?  For example, a white power LED might have a forward current of 30V and take up to 1.5A.  Let's say you already have a power supply that can provide 30V and 5A.  How do you design a circuit so that it limits current to 1.5A?  At that point I would assume you could control brightness with a logic level MOSFET and the PWM capability of the Arduino.  Does anyone have a good reference concerning designing such a current limiting circuit?  Thank you.
2  Using Arduino / General Electronics / Re: Shift registers - only good for LEDs? on: April 15, 2014, 10:46:50 pm
I have used a shift register to control a character LCD with only three Arduino pins:
http://42bots.com/tutorials/arduino-controlled-lcd-using-a-shift-register-and-the-spi-library/

So, definitely good for more than LEDs...

Specialized ones have additional applications, though I think the thread is mostly talking about the general purpose and cheapo shift registers.  For example, most VFD drivers are simply high voltage shift registers.  Here are two I have used with no problems:

MAX6921 (20 output, 76 volts)
PT6306 (64 outputs, 70 volts)

The second one is both a lot cheaper than the Maxim part and also has three times the outputs.  It works great for displays with a large number of grids and segments.

Don't lick the project board when running it at 70 volts.  smiley-mr-green
3  Community / Bar Sport / Re: Has Atmel significantly raised 8-bit MCU prices recently or is this Digikey? on: April 14, 2014, 04:33:28 pm
Good catch on the prices.  Maybe Digikey is trying to take advantage that they are the only ones who have much stock right now.  Hopefully it will all normalize.  Possibly, Atmel got behind on production.
4  Using Arduino / LEDs and Multiplexing / Re: Maxim 7219 - Real and Fake compared on: April 14, 2014, 02:10:52 am
Yep, mine are the fakers for sure.  Except for the ones I got directly from Maxim, those are just like the "real" photos.  Though, and I must stress this, the "fakes" do work for me.

I also have some SOP "MAX7219s".  Any information in determining if these are real or not?  Typical auction:

http://www.ebay.com/itm/50PCS-MAXIM-MAX7219CWG-MAX7219-SOP-24-LED-Display-Driver-IC-8DIG-24SOIC-/310723512133

Almost certainly fake, I would think.

Thanks for the photos, that was great.

5  Community / Bar Sport / Re: Has Atmel significantly raised 8-bit MCU prices recently or is this Digikey? on: April 13, 2014, 11:45:36 pm
This has forced me to go count what I have left to make sure Atmel doesn't leave me high and dry.

14 ATMega328P-PUs out of the initial buys of 60 (10 came from Jameco with bootloaders which I don't use).
11 ATMega328-PUs out of the buy of 25.
16 ATMega32P-AU out of the buy of 25.

Lots of ATTinys still - 25 ATTINY88-PU, 48 ATTiny85-20PU, 22 ATTiny85-20SHR.  All a lot cheaper when I ordered them.

Various other chips - Mega, XMega, UC3 via sampling.

I should be set for a bit.  Waiting for the price to come back down...
6  Using Arduino / LEDs and Multiplexing / Re: Cost of the Atmega328p-pu going up? on: April 13, 2014, 10:43:18 pm
I posted a message concerning this relatively recently.  I agree with you.  I have a few points of data of my buys over the past couple of years and prices now are quite a bit higher.

http://forum.arduino.cc/index.php?topic=228782.0
7  Using Arduino / LEDs and Multiplexing / Re: Maxim 7219 Eagle PCB & iTead files for Adafruit 0.56" 7 segment clock display's on: April 13, 2014, 06:51:03 pm
Sorry joeN, I was forgetting non-members can't see pictures on picaxe forum. But edr1924 is correct, by my understanding, those are real maxim chips. The pin 1 dimple is the simplest tell-tale.

Hey, no problem at all.  I will wait patiently until they finally OK me.  I'd like to see the images.  Though, I do have the cheap 50 cent Chinese sourced chips (almost 200) and ten ones I know are real as they came directly from Maxim (thank you, Maxim, for your generosity).  I think I will simply pull them out and look at them under the microscope.
8  Using Arduino / LEDs and Multiplexing / Re: Maxim 7219 Eagle PCB & iTead files for Adafruit 0.56" 7 segment clock display's on: April 13, 2014, 01:58:34 pm
Well, the people who run the PIXAxe forum are a bunch of (redacted) and you can't even view the attachments without registering and there is no automatic registration, every registration must be OKed.

I bet I have fake chips, then.  Other than the 10 or so I got directly from Maxim as samples.  Funny thing, they work fine and I have used as many as 20 of them in a single project.  I don't get noise out of them.  In fact, I wonder how you could get noise out of them other than as EMF which manifests itself somewhere else, like on your computer or radio as sound.  These chips do so little I wonder how Maxim can price them like they do.  On the other hand, I wonder how the Chinese company cloning them can afford to sell them for 50 cents.  There can't be that much demand for fakes, right, other than the hobbyist market which we know is tiny.

For PWM chips I mean chips like the TI TLC5940 and Worldsemi WS2803.  These chips you give each channel a value for its output (10 bit for TLC5940 and 8 bit for WS2803) and it sends that duty cycle to the LED.  So it works to dim the LED and allows you to mix different levels of red, green, and blue for RGB displays to get "full color".  But you have to do the multiplexing yourself.  That is what the MAX7219 will do for you that simplifies your code quite a bit.
9  Using Arduino / General Electronics / Re: Pull-up Resistor on: April 13, 2014, 04:37:16 am
Off subject, but I can't help but notice that two of the posters in this thread have personal icons sourced from different but equally brilliant Kubrick films.

OTOH, I have an icon sourced from a really piss poor comedy written and directed by talentless hacks.

smiley-sad
10  Using Arduino / LEDs and Multiplexing / Re: free Maxim 7219 PCB for Adafruit 0.56" 7 segment clock display's on: April 13, 2014, 04:24:22 am
Can't you just zip them up and attach them here?

I like those chips too, though chips with PWM functionality are really a lot more capable.  7219's are available on eBay at 'fell off the truck' prices of well less than $1 USD each delivered.

www.ebay.com/itm/25PCS-IC-MAX7219CNG-MAX7219-DIP-24-DRIVER-LED-DISPLAY-8DGT-NEW-GOOD-QUALITY-/290746404262

11  Using Arduino / General Electronics / Re: Shift registers - only good for LEDs? on: April 13, 2014, 12:48:15 am
Serial-to-parallel shift registers like the 74HC164 or 74HC595 are good for driving a lot of outputs with a few microcontroller pins.  Any time you have that situation, consider a 74HC164 or 74HC595 shift register.  LEDs are obvious, but if you really wanted to write, for example, a parallel memory's address pins or the digital input of a parallel DAC, you could use a 74HC164 or 74HC595 shift register to be able to send signals to them without eating all your pins.

Parallel-to-serial shift registers like the 74HC165 are good for reading a lot of inputs with a few microcontroller pins.  Any time you have that situation, consider a 74HC165 shift register.  A large set of switches are obvious, but if you really wanted to read, for example, a parallel memory's data pins or the output of a parallel ADC, you could use a 74HC165 shift register to be able to retrieve signals from them without eating all your pins.

In both cases, you trade pins for speed.  With the Arduino and its very slow default port access speeds, you might lower the number of times you can touch an external device to a few thousands of times per second if you are using a shift register to control a large number of inputs or outputs.  Still, this is usually fast enough for most applications.

This document says the 74HC164 can source 25mA per pin or sink 20mA per pin and dissipate 750mW per package.  I would say it is perfectly fine to drive normal LEDs on such an IC.  Limit current to 20mA with resistors.

http://easyelectronics.ru/datasheet/74HC164.pdf
12  Using Arduino / Microcontrollers / Re: Add a SIMM to your Arduino :) on: April 10, 2014, 06:14:40 pm
joe, you dont seem to recognize the difference between ram and flash. by my calculation only .05% of cpu time to refresh and 256kbyte not 4kbyte. using mega on that same simm would give 256kbyte. i have i huge box of those am tempted to give that a try.

Without redoing the calculations myself, I was relying on what the authors of this project said themselves:

https://github.com/zrafa/30pin-simm-ram-arduino/blob/master/README.md

Quote
WARNING: using this kind of RAM will put your Arduino CPU to refresh the SIMM RAM data most of the time. It means that your sketches (programs) could run slow. Also, we need to use most of the Arduino pins to be soldered with SIMM RAM pins so there will be just few available extra pins for your project.

Quote
SIMM RAM has 12 bit address for rows and columns. Max RAM size here is 16MB. We are using just 6 pins for rows and columns, so we are limited to 4KB of RAM space.

Addresses space will be of 64rows x 64columns = 4096 addresses 1 byte each = 4KB of space.

This addendum:

Quote
If you need to use more RAM you can solder the 3 analog pins remaining to simm ram module pins A6, A7 and A8. It will give you 256 KB of RAM space.

Yep, you can get 256KB if you use every single pin.  Enjoy.  Or 8MB using only four pins with a cheap serial flash memory (and add more SPI devices at the cost of one pin each),  Yes, I understand the differences, the most obvious one is that Microchip's current serial flash only allows for 100,000 write cycles.  If that is a problem, several manufacturers make inexpensive or free (samples) serial SRAM ICs and your project won't look like an entry into a steampunk contest.
13  Community / Bar Sport / Re: The perfect arduino murder! on: April 10, 2014, 03:27:11 am
I think you are going to need a bigger microcontroller.  Maybe a Freescale Qorivva.   smiley-mr-green

As far as your story goes, you know you made yourself the bad guy, right?  Time to rework that into third-person.
14  Using Arduino / Microcontrollers / Re: Add a SIMM to your Arduino :) on: April 10, 2014, 03:08:16 am
 smiley-razz

Give us all your pins and almost all your CPU time and you will get 4KB extra memory.  Fair trade!  

Or ask Microchip for some 64Mb serial Flash memories and they will send them to you for free.  Your choice...

This would be a more interesting project on the Mega.  You could implement the entire SIMM and not kill all your GPIOs.  Still, serial memory is more compelling these days.
15  Using Arduino / LEDs and Multiplexing / Re: Control 8 x 7 Segment display. on: April 07, 2014, 05:34:40 pm
i know, I'm still experimenting with the display. My dictionary only knows Beer, boob, yeah and hello

Well, at least it allows you to display the important words...   smiley-grin
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