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451  Using Arduino / General Electronics / Re: Arduino seems to pick up excessive noise on: October 28, 2011, 01:19:45 am
Could it just be an imprecise pot? What's the variance between first and second? It may just be one, which is an error of less than 0.1%. I'm not certain the A/D circuit is that real-world accurate.

452  Using Arduino / General Electronics / Re: Arduino seems to pick up excessive noise on: October 28, 2011, 01:10:19 am
No pinMode() in setup?
453  Using Arduino / Programming Questions / Re: Problem with reading two digit numbers from serial monitor on: October 27, 2011, 01:26:10 am
Serial.read() is only going to give you one digit (byte) at a time. You'd need to do something like this:

lowtemp = Serial.read() *10 + Serial.read();

Chris
454  Using Arduino / Project Guidance / Re: Best way to control 162 outputs in under 1ms ??? on: October 26, 2011, 02:33:19 pm
@AWOL: My guess is 162 is all the LEDs in each plane (9x9x2).

@TECH GEEK: 1ms = 16,000 clock cycles (@ the normal 16 Mhz). Using hardware SPI you should be able to shift out 21 bytes of data in under 1000 clock cycles (each bit = 2 cycles * 168 bits = 336 cycles, plus overhead of loops etc). So this should be doable.

Have you looked at the ShiftPWM library (http://www.elcojacobs.com/shiftpwm/ )? From what I've read it seems to be heavily optimized to do just what you want.

Good luck!
455  Using Arduino / General Electronics / Re: A couple of power supply questions on: October 26, 2011, 09:54:18 am
Thank you!
456  Using Arduino / General Electronics / A couple of power supply questions on: October 25, 2011, 02:50:14 pm
If I have a 2.3 to 2.4A total circuit load will a 2.5A wall adapter power supply be sufficient, or is that running it too close to capacity and I should use a 3A one?

Also as I google around I see some schematics with larger (100uF - 4700uF) bypass capacitors near the power socket, and some with none...I know about placing 0.1uF decoupling caps near the ICs, but how do you know if you need a bigger bypass capacitor, and what value to use? If it matters, I'm using a regulated switching supply.

Thanks as always.
457  Using Arduino / General Electronics / Re: Is this FET acceptable? on: October 25, 2011, 11:59:01 am
Thank you as always Mr. Grumpy!
458  Using Arduino / Microcontrollers / Re: View fuse settings on: October 25, 2011, 09:44:49 am
Sorry I dont know enough to answer your other questions, but you can view the fuses by running his sketch:

https://github.com/WestfW/fusebytes

Good luck!
459  Using Arduino / Programming Questions / Re: Quesiton about reading data out of EEPROM on: October 25, 2011, 09:34:20 am
Ultimately all data is bytes, my guess is you're printing it as chars instead of numbers.

Can you post the part of your code where you output the data?

Chris
460  Using Arduino / General Electronics / Is this FET acceptable? on: October 25, 2011, 01:02:05 am
Short version: can I use this MOSFET

http://www.irf.com/product-info/datasheets/data/irlml2246pbf.pdf

to drive a 2.4A load @ 5V with 0.1ms pulses from the output of a 74ac138 decoder? Do I need a resistor between the 74ac138 pin and the MOSFET gate? If so, what value?


Long version: I have a 24x16 LED matrix I'm trying to drive as bright as possible (each LED @ 100mA for 0.1ms). The columns are controlled by TLC5916 constant current drivers connected to the cathodes; the row anodes by a pair of 74ac138 3:8 decoders.

I've tried a couple of PNP transistors but the only way I can get enough current out of them is by pulling 50mA out of the 74ac138 pins, which is at the max threshold so probably not the best idea. With the latest transistors (http://www.diodes.com/datasheets/ds31149.pdf) and 150 ohm resistors (so Ib=24mA) I'm only getting about 1.6A out.

Based on dc42's excellent explanation in another thread (http://arduino.cc/forum/index.php/topic,76363.msg577560.html) I think I'm not able to drive the transistors to saturation, which would explain the lack of power to the LEDs and the heat the transistors are generating.

So now I'm going to try some logic level P-channel MOSFETs. They need to be surface mount for due to circuit board limitation size, and I'm trying to save money because I need 16 of them and these are the least expensive ones I can find.

Will this MOSFET work? If not, can you give me any pointers on what I missed?

As always, thanks to everyone for your continued assistance.
461  Using Arduino / Project Guidance / Re: Led Cube - Transistor on: October 23, 2011, 11:45:05 am
Awesome explanation. Thank you very much!
462  Using Arduino / Project Guidance / Re: Led Cube - Transistor on: October 22, 2011, 07:52:44 pm
Hi dc42,

Can you explain this a bit? A source current on the base of 20mA and 1.28A is a gain of 64.

If I search on Digikey for NPN transistors where Ic >= 1.5A and gain is at least 70, I get pages of results (for example http://search.digikey.com/us/en/products/MPS651RLRAG/MPS651RLRAGOSCT-ND/1139913 ). Why wouldn't something like this work?

I'm still trying to figure transistors out so any advice is greatly appreciated.

Thanks in advance.

Chris

463  Using Arduino / Microcontrollers / Re: How long does Serial.print(char*) take? on: October 22, 2011, 01:20:08 pm
At what serial rate?

It would probably be easiest to read millis() before and after the print, then subtract.

Chris
464  Using Arduino / Programming Questions / Re: Why most of people using "foo" a name of variable on: October 18, 2011, 10:39:54 pm
Haha. Foo is the first half of foobar, which is the polite spelling of fubar. Here's the lowdown:

http://en.wikipedia.org/wiki/FUBAR#In_computing_and_technology

http://en.wikipedia.org/wiki/Foobar
465  Using Arduino / Project Guidance / Re: 4x4x4 LED Cube Individual Layer control on: October 17, 2011, 03:03:27 pm
Hi Nishant,

I guess I don't fully understand what you want to do. Yes the Instructibles example won't let you control all 64 LEDs directly, but you use multiplexing to fake it.

Can you maybe give more explanation and I will try to help you out?

Chris
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