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31  Topics / Product Design / Re: How to mount circuit boards? on: September 05, 2012, 10:30:36 pm
Sorry, each module has 2 rows of 0.1" pins, 8 pins on each row, 1.8" apart.  The pins are about 1/2" long, but I was assuming I'd trim after soldering.

I know what I linked wasn't low profile but it's a lot better than the normal headers I'm used to (like the arduino uses for shields).   My part's 5mm is longer than I'd like but should be good enough, plus it's cheap and I've had good experience with dipmicro.  Your 0.095 wouldn't be a problem at all, assuming I don't shoot myself in the foot when I try to trim the leads evenly smiley
32  Topics / Product Design / Re: How to mount circuit boards? on: September 05, 2012, 10:10:48 pm
As far as low profile sockets, I did look at which might be low profile enough, the site says around 5mm, I think I can handle that, but I'm not very familiar with them as far as how tight they'll hold, and I'll definitely have to trim my leads before insertion, though I don't suppose that's really a problem, as long as I can keep my board heights consistent.
33  Topics / Product Design / Re: How to mount circuit boards? on: September 05, 2012, 09:54:04 pm
Are these too big for in-board sockets for your component so it can sit on the board as if soldered in?

It matches 68 pages of items, I think you mean the pin receptacles that are the first match.  The smaller size is still bigger than my drill hole and probably too big to get a friction hold on my component, and at 25 cents each in quantity, for the number of holes involved, it will double the price of my project.

I'm thinking of using standoffs like which I screw to the board before I solder the module, but if it loosened, I couldn't retighten it so maybe some threadlocker is a good idea (and I've always hated threadlocker).  Also he only has 13 in stock.  I already emailed him since it says restockable.
34  Topics / Product Design / Re: How to mount circuit boards? on: September 05, 2012, 06:19:45 pm
Thanks for the reply.  Searching for press-fit studs, I just found the wheel ones you mentioned and a whole bunch of body piercings.  Expanding on your idea though, I did find "self-clinching nuts", which sound good except that my holes are 3.2mm and the smallest self-clinching nuts I could find require a 4.2mm hole.

That also gives me the idea of gluing a bolt into the holes, if that glue breaks, the bolt will have nowhere to go.  Maybe a nut directly on the other side of the pcb from the bolt head?
35  Topics / Product Design / How to mount circuit boards? on: September 05, 2012, 05:21:25 pm
I designed a 2x2" PCB that will have a 2.5" square component soldered to it, and I plan to have a row of these assemblies mounted to a plexiglass backing.  What's the best way to mount them?   I put some mounting holes in the PCBs, but once assembled, the larger component will cover them.  If I socket the larger component, the whole assembly will be too thick.

I'd like to just put the screws through the mounting holes before soldering the component on, but once I have a spacer around the screw, I won't be able to stop it from turning while I try to tighten the nut.

I even thought about gluing nuts to the component side of the PCB, but if the glue breaks, I have a metal nut bouncing around my delicate electronics and no way to reattach it.

Thanks for any suggestions.
36  Community / Bar Sport / Re: So why are there no good electronics shops? on: September 02, 2012, 11:57:29 am
There's an electronics store near me I used to get most of my parts at 20 years ago, they have aisles full of components in little baggies.  Last time I was there the baggies were covered in dust, the labels were fading, and they still want $25 for a PICF84 (far better, newer pin-compatible chips are about $2 online).  I can buy a bag of 5 resistors there for the same price as 100 online with free shipping.  I'd probably be willing to shop there when I don't feel like waiting 3 weeks for a part, but they don't seem to have anything newer than about 2002.

They're always empty and I don't know how they stay in business (though they still and always have supplied parts for industrial customers too).
37  Community / Bar Sport / Re: What is the most expensive board you ever smoked? on: September 02, 2012, 11:48:32 am
Not sure this was really my fault, but I was setting up some file servers and turned on 8 machines at once, and caused a dip AC power line which was bad enough to fry every single hard drive.  These were $25,000 machines that each had 5 2-gig SCSI drives which were worth $2,000 each at the time.  Total damage was about $80,000.   The worst (best) part was when the repair tech replaced all the drives he did the same thing I'd done.  $160,000 in total damage isn't quite your $300,000 threshold, but it was real damage to production environment equipment and actual repair dollars that had to be paid.

They upgraded the power lines the next week.
38  Using Arduino / LEDs and Multiplexing / Re: Analog/Digital Clock on: September 02, 2012, 10:24:06 am
For the inside digital time readout.  Have you built the display on a breadboard yet to see how it looks?  As you have it with 3x5 LEDs, it will be extremely hard to read.   To do it that way, you'd want more LEDs in each bar.

You can't wire the LEDs as you show in the schematic.  First when you're multiplexing, the resistors have to be on the side that can have multiple LEDs on at a time, so as you show it, you're scanning the 5 cathodes and outputting the correct pattern of anodes each time.  With 100 ohm resistors you're using 20-30mA per LED (depending on colour), which means the cathodes will have to sink 160-240mA and the Arduino is limited to 40mA per IO pin.  You'll need driver transistors on the cathodes to do it that way.

Why are you having it so you can control each LED at all?  For example, won't the 5 LEDs that make up your 10's hour digit always all be on or always all be off?

A MAX7219 LED driver chip will allow you to control 64 LEDs or 8 7-Segment LEDs from 3 arduino pins, and the code to use it will be a lot easier than what you're trying to do.

There's also pre-assembled 4-digit modules you can use available from sure electronics, adafruit, and lot of other places with the driver built in so you just need 3 IO pins to control them.

You can also consider a bar graph LED driver IC such as the LM3914 for your seconds circle.

Your idea of using 2 UNO's is horrible.  If you were short on IO there are far better solutions; you can use an Arduino Mega which gives you 70 IO pins.   A port expander IC such as the MAX7313 will give you 16 IO pins and you can connect up to 64 of them to your Arduino using only the 2 I2C pins.

In this case though you're not short on IO, just use the correct driver chips and you shouldn't need more than 6 IO for all the LEDs you've described.
39  Using Arduino / LEDs and Multiplexing / Re: MAX7219 and LED matrix on: September 01, 2012, 02:22:15 am
The wiring is very basic, and what you have should work fine, you just need to pass your 3 pins to the LedControl constructors.   I did wire it as in but without the 10uF capacitor, and 18k for the ISET resistor.  I've got din, clock, and load going to 12, 11, and 10 on my arduino, and the LED pins based on my matrix which I worked out by testing.

The main thing you'll need is the instructions for the library which are here:    and it includes a test sketch to make sure everything is working.
40  Using Arduino / LEDs and Multiplexing / Re: Powering Sure 3208 from Arduino? on: September 01, 2012, 01:38:58 am
It helps if you link the device you're talking about when you ask questions.  If this is the board you mean, the spec says it takes 220mA with all LEDs on, in which case you can power it from the Arduino.
41  Using Arduino / LEDs and Multiplexing / Re: MAX7219 and LED matrix on: September 01, 2012, 01:29:27 am
Looking at the code linked, the font is storing "byte"s in program memory, you can't do that and when you read it back you'll get a garbled result pretty much exactly as you describe.  Read the documentation here:

I would also suggest you look at the LedControl library for the MAX7219.  

I have mine set up on a breadboard while I'm waiting for PCBs to arrive, and it's working perfectly.  The MAX7219 only scans the display at 800Hz, so I can't imagine any sort of "high frequency" problems.

42  Using Arduino / LEDs and Multiplexing / Re: Scrolling matrix display on: August 30, 2012, 03:57:16 pm
I have a scroller and I do basically what you do but there is no problem decoding the text via the font on the fly with no need to store the bitmap anywhere except in the driver/shift registers.  The Arduino is fast enough for that, at least it was for me with 6 8x8 displays.

Very nice looking display.  That's what I'm working on, currently I have one 8x8 matrix on breadboard wired as you described and I just ordered some PCBs for 7219 backpacks to build a row of modules, exactly what you have but I haven't decided how many modules I'll use yet.

Is your actual display as choppy as the video?  I thought about decoding the font on the fly but figured it would make the timing more difficult and just feel kludgy in code.  Are you using a blocking delay or using millis() to make sure the time interval between updating the display is constant?  Being more careful with the time between updates could make your scroll more smoothly.

Right now, my code is incredibly simple, I don't even need to read all the symbols from the font.  For a time display when I want a colon, I just set 2 columns to 0x24, and to make the colon blink seamlessly as it scrolls, I can blank and reset those 2 columns every second.  Replicating that functionality without the buffered bitmap would be difficult.  Same goes for any scrolling animations.

Anyway, thanks for your reply, I guess you've given me the right answer, which is to work out the more complicated code.  Either that or upgrade my mega168 to a 328 and limit my messages to 150 characters or so smiley.

Edit:  I didn't notice you posted code at first, I just read through it, it's not quite as difficult as I thought, so I really should spend some time trying to do it that way (at least if I don't want animations).   I'm a bit confused by your triple loop, is it because your font is storing columns but you're updating the displays as rows so you're twisting around bits?

Is the delay(10) after the for i loop the only time keeping?  If so, is it scrolling as fast as it's able right now?
43  Using Arduino / LEDs and Multiplexing / Re: Scrolling matrix display on: August 30, 2012, 12:00:57 pm
They keep the message data in the flash memory area, rather than in SRAM. See

I'm using program memory to store font and symbols, but for the actual message, I'm dynamically building it at run time.
44  Using Arduino / General Electronics / Re: Why use 2 decoupling capacitors? on: August 30, 2012, 10:06:20 am
In theory any cap bypass cap has less and less capacitance reactance (AC resistance) to ground (which is good , acts like a high pass filter to ground) as the frequency of the noise or spike increases.

There's a lot of information in there, thank you.  So first off, if there are several chips, I would just put the single 0.1uF near each chip's power pins as I normally do now, and not worry about the 10uF near each chip, that's something to think of as part of the power supply circuit as needed.  For a commercial switching power supply, would the electrolytic near it make a difference ?

Second it's not simply the values that are important but the chemistry, so am I right that if I were to use a 10uF ceramic SMD cap right next to the 0.1uF ceramic cap near the power pin, it would have no more benefit beyond the 0.1uF cap alone?
45  Using Arduino / LEDs and Multiplexing / Scrolling matrix display on: August 30, 2012, 09:38:40 am
I'm working on making a scrolling LED matrix display.  I'm storing a 1-bit bitmap of the entire display content that's being scrolled in ram and then using a moving pointer to update the display with a portion of the ram area.  It my own idea, but I don't know if I'm reinventing the wheel or there's a better way to do it.  It's working beautifully; the problem is that I'm burning way too much ram with this technique.  A 100-character message will use 600 bytes of ram with my 5x7 font, which seems excessive when the MCU only has 1 or 2k.

What do people normally do to implement a scrolling display?

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