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31  Using Arduino / Project Guidance / Re: MAX7219 and resistors for 7-segment displays on: September 25, 2012, 03:33:35 pm
And if you wire them in parallel without current limiting resistors, there is nothing to assure that the current is divided equally between them.

Alright thanks, that seems like it might be an issue. I have a whole couple of 7219s at my disposal so I think I might as well go with two drivers. That means more complicated code, but really it shouldn't be that bad.

Page 10 of the data sheet:
Selecting RSET Resistor and
Using External Drivers

The current per segment is approximately 100 times
the current in ISET. To select RSET, see Table 11....

I was a bit confused by RSET at first, I'm not sure how I missed it. Also the graph on page 4 seems to answer my other question about getting segments down to 10 or 20mA.

So now if all the segments are being properly connected to the drivers (instead of being wired in parallel like I proposed), and I have the appropriate RSET resister between ISET and V+, I should be fine without any more current-limiting resistors, right?
32  Using Arduino / Project Guidance / MAX7219 and resistors for 7-segment displays on: September 25, 2012, 01:54:39 pm
I'm trying to figure out how many resistors I need for my project, and where in line to put them. I'm using a MAX7219 to drive sixteen 7-segment common-cathode displays for a scoreboard. Yes, I know that the 7219 only actually supports 8 digits, so let me clarify.

The scoreboard has 4 sides. Each side has 4 digits; 2 for each player's score. Since I only need 4 unique digits, the other 12 displays (3 faces x 4 digits) are wired in parallel with the appropriate digit.

Now, assuming I want to have each segment running at 10mA, that makes a maximum current draw for 1 unique digit 280mA (7 segments x 4 displays x 10mA [not including DP since I don't use it]). Total, that means if all the segments on all the displays were lit up at the same time, it would draw 1.12A.

However, something occurred to me. The digits wouldn't actually be on at the same time because of how the MAX7219 works, right? It cycles the digits on/off very fast so they appear to all be on at the same time, but they aren't. Technically only 1 unique digit (in other words, 4 displays) would be on at a time, meaning that if all the displays were on "at the same time", they would actually only draw a maximum of 280mA total. This means that I could just put my resistors on the SEG pins of the MAX7219. This also means that I only need 7 resistors for the whole lot of displays, instead of 7 resistors for each of the displays, right?

Another thing caught my eye though. In the MAX7219 datasheet, it says that the resistor between the ISET and V+ pins is for controlling the display brightness. Does this mean that with that single resistor between ISET and V+, I don't even need those 7 resistors on the SEG pins?

Also, the datasheet says that a 10K resistor will limit the current on each SEG pin to about 40mA, what value resistor would be required to get that down to 10 or 20mA?

Bonus question: In the datasheet for the MAX7219, it says the following under the absolute maximum section:

DIG 0–DIG 7 Sink Current..............................................500mA
SEG A–G, DP Source Current........................................100mA

Does that mean 500 / 100mA per pin, or in total?
33  Using Arduino / Programming Questions / Re: Can't upload code to ATmega328P after using ArduinoISP to burn bootloader+sketch on: August 01, 2012, 09:39:26 pm
If you had two arduinos as pictured above, why not just connect USB cable to the 2nd Uno & download sketch directly?

The two-arduino setup was just me troubleshooting. I initially noticed the problem when I used my ArduinoISP shield to burn the bootloader and upload a sketch to an ATmega328P. After doing that and putting the ATmega328P back into an UNO I couldn't upload any more sketches to it, so I wanted to verify that this wasn't a problem with my shield.
34  Using Arduino / Programming Questions / Re: Can't upload code to ATmega328P after using ArduinoISP to burn bootloader+sketch on: August 01, 2012, 08:49:12 pm
That makes sense. Am I right in assuming that there's no way to upload the code "normally" using the Arduino as an ISP?
35  Using Arduino / Programming Questions / Can't upload code to ATmega328P after using ArduinoISP to burn bootloader+sketch on: August 01, 2012, 08:09:20 pm
I'm wondering if anyone can explain what's going on here and possibly find a solution. It's a little hard to explain so I'll just start by explaining how to reproduce the problem.

To reproduce it, start by wiring up two Arduino UNOs as outlined in the ArduinoISP tutorial page:

Don't forget to put a 10uF capacitor between GND and RESET on the UNO that you're plugging in to your computer (the bottom one in the diagram on that page). Follow steps 1-7 and burn the bootloader.

After successfully burning the bootloader, open the Blink example sketch and select "Upload using Programmer." The little LED for pin 13 should be blinking away nicely.

Now, unplug both UNOs and set aside the one you were just using as an ISP. Plug the UNO with the newly bootloaded ATmega328P into your computer and try to upload the Blink sketch again. The upload will fail, saying that the programmer is not responding.

Does anyone know why this is? If you burn the bootloader using the ISP but don't upload any sketch using the ISP, you can upload sketches to it directly with no problem.

Sorry if any of this is too confusing, I've spent a good 3 hours troubleshooting my connections here and I'm a bit tired. If you need clarification on anything just let me know.
36  Community / Exhibition / Gallery / Re: First shield: The ArduinoISP Deluxe shield for programming ATtiny & ATmega328P on: July 31, 2012, 02:35:40 pm
Thanks. What'd be the purpose of the decoupling capacitors? The reset pin actually does have a 10K pullup resistor on it already. I can't remember what the problem was exactly but when I had the whole thing laid out on a breadboard, I was having issues (I think related to uploading and bootloading) and the resistor solved 'em.

I designed it in Fritzing. I'd opened up Eagle and played around a little, but I found the interface a bit intimidating as a beginner. I had designed one PCB in Fritzing before (a fairly simple one that "syncs" an analog clock using an RTC) so I had a decent knowledge of how to use it. The only real challenge in Fritzing for this project was that I had to create the part for the ZIF socket. I'll definitely be looking into using Eagle for future projects though. It's definitely a more widely-used application.

The PCBs were made by iTeadStudio. After I submitted my other clock PCB to OSHPark I kept looking around for places to get PCBs made out of curiosity. iTeadStudio popped up and their prices were amazing compared to everyone else. 10 PCBs up to 5x5cm for only $12? Awesome. I ended up needing more than 5cm, so I went with the 'up to 10x10cm' option. Because I knew that the ZIF and switches would be red, I decided to pay the $6 premium and get red PCBs. Overall it was only $35 + shipping for 10 boards (they randomly gave me 2 extras actually) with 100% e-test. Not a bad price, and the quality was awesome, save for the silkscreen missing for one of my resistors (but that was only on one of the 12 boards).
37  Community / Exhibition / Gallery / ArduinoISP Deluxe shield for programming ATtiny & ATmega328P on: July 23, 2012, 05:49:18 pm
UPDATE: I've got a more recent version out now that does ATtiny2313 and ATtiny4313 chips as well.

I've gotten into the habit of using the bare ATmega328P and ATtiny chips in my projects. The problem is that programming them / burning the bootloader to new chips can be annoying. You have to either get a programmer, or wire up an Arduino every time you need to program them.

I decided to make a shield where all of the chips I use can be programmed from one single socket. All you have to do is load the ArduinoISP sketch onto your Arduino, pop this shield on, and you can program an ATtiny 24/25/44/45/84/85 or an ATmega328P. For the ATMega328P, there's an external 16MHz crystal so you don't have to rely on the internal one. There's also a test LED so you can verify that the chips are working properly.

Questions and critiques welcome. This is the first real PCB I ever designed, I'm still learning! I may also do a second revision of the board so suggestions for that would also be welcome.

I also decided to sell my extra ones (with components)!




- Extra long, stackable headers
- Support for: ATtiny 24/25/44/45/84/85 and ATMega328P **
- Blink LED for testing purposes
- 16MHz crystal for programming an ATmega328P
- ZIF socket: simple to insert and remove chips for programming
- One socket for programming all different chips!

It's super easy to use. Just load up your Arduino with the ArduinoISP sketch, pop the shield on, and you're ready to program your chips and / or burn the Arduino bootloader to them.
38  Topics / Product Design / Re: Extra long headers for shields? on: July 19, 2012, 12:42:10 pm
Those don't say how long they are, and they're also male headers. I found these ones from Adafruit but they're too short (10.5mm, like in my first photo). I wouldn't be totally opposed to using male headers, just as long as they're long enough.
39  Using Arduino / Project Guidance / Re: 2 digit 7 segment score keeping on: July 19, 2012, 11:55:07 am
I did something similar, a completely automated foosball scoreboard:

It uses a trip laser in the goal posts and an RF transmitter / receiver to communicate. You can see more pics / a video here:
40  Topics / Product Design / Extra long headers for shields? on: July 19, 2012, 11:40:48 am
EDIT: Found them. 6-pin version and 8-pin version.

Hey all, I need some advice here.

I just got my first Arduino shield back from the fab and I'm pretty excited about it. I had grabbed some headers that were advertised as "long" and "stackable arduino headers" but I've found that they're too short (10.5mm). When I put my shield on top of an Arduino Uno the board presses against the top of the USB port. This is a problem because it can cause a short on my shield. Here's a photo:

The only other shield I own is an ethernet shield and its headers are really long (14mm). When I put it on my Uno, it has no problems touching the USB jack:

I found some extra-long headers at a local shop but they were so expensive - $1.10 to $1.20 for ONE 6 or 8-pin header. Adding these to the shields will add about $5 to the final cost, which sucks.

Is this a regular problem for people who design shields? Are there any places online I can get these "extra-long" headers? I tried looking around but found nothing conclusive. Should I include these extra-long headers when I sell them, should I include the "long" ones knowing they'll be too short for people, or should I let people buy their own headers completely?
41  Forum 2005-2010 (read only) / Interfacing / Module for recognizing a tone or sound? on: May 21, 2010, 09:38:45 pm
I'm thinking about my first arduino project. Right now I'm just in the research phase, so I'm hoping I can get a little help here.

My project is to a visual notifier for text messages. The best way (I believe) to do this is by taking a bluetooth headset and hooking the earpiece to arduino. When I get a text, the bluetooth headset will play a sound (or rather just a tone, this might be easier for the arduino to recognize), and the arduino will recognize it and activate an LED until I press a button to clear it.

Does anyone see a flaw in this? Would the Arduino be appropriate for this? Thanks in advance.  smiley
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