The scrolling LED strip(1)

This post describes working out how a 72 x 7 pixel LED strip works, and then connecting it up to an Arduino to display custom text under program control.

The intention was to use the strip to display custom data (for example, the temperature) rather than having to manually (and somewhat laboriously) type the text in through the inbuilt keypad.


The strip





This was purchased a while ago for around $50 from memory. It has 72 pixels horizontally and 7 deep, giving a total of 504 pixels. Also there is a 55-key keypad visible in the photo which lets you enter text:




The internals



The device consists of 5 circuit boards. Two hold the LEDs themselves:

Part number 80-947C 2002-09-12:



Part number 80-947D 2002-09-12:



Visible also in the above photos is the board containing the keyboard.

Once opened you can fold out the other two boards with the control logic on them:

Part number 80-947A-2 2004-04-15:



Part number 80-947B 2002-09-12:




Various parts are labelled following the investigation described below.


Reverse engineering



Some work with a magnifying glass revealed that the chips were 74HC595 8-bit shift registers.



More details about them here:

http://alicebrain.blogspot.com/2013/11/using-74hc595-output-shift-register-as.html
These are standard output shift registers with a latch, which means you can shift out to multiple chips and then "latch" the data (copy from a temporary register to the output pins) in one operation, providing flicker-free updating.

With 72 columns of LEDs, and the 595 chips providing 8 bits each, it was reasonable to deduce that 9 of the chips were dedicated to driving the columns. Underneath each of those chips were 8 x 150 ohm resistors, which would be for current-limiting of the LEDs. Measurements indicate that there is around 4 mA per LED going through the resistors (a 600 mV voltage drop). This is a total drain of 32 mA for each 595 chip (if all LEDs are lit) which is within the spec for that chip.

The column drivers sink current (so to light an LED the corresponding output has to be zero).

The tenth 595 chip labelled "row driver" on the photo was clearly intended to source current for the rows, via the 7 x 8550 PNP transistors on the right, driven from that chip via 7 x 4.7k base resistors.

Since the row drivers are driven via a transistor which inverts the output, the row driver must also have an output as zero, in order for the transistor to source current.

In other words, if all 595 chips are outputting zero, then all LEDs are lit.

Using a 74HC595 output shift register as a port-expander

A cheap and simple way of expanding your processor's output capability is by using a "shift register" like the 74HC595 described here.
Example of it in use, displaying 8 LEDs:




Pin outs

The pin-outs for the 74HC595 are:

You can find it on HQEW datasheet.



You can "daisy-chain" them to connect multiple ones together, thus giving you 8, 16, 24, 32 or more extra output ports, by simply connecting the "overflow" bit of one register to the "data in" of the next.


Schematic



The 10K pull-down resistor on the SS (Slave Select) is designed to keep the registers from clocking in bits while the main processor is booting, and the SS line might be "floating" and in an indeterminate state.

Master reset

I have tied /MR (master reset) high, so the chips are never in a reset state. If you needed to reset them from time to time you could parallel up those pins and connect them to a processor pin.

Output enable

I have tied /OE (output enable) low, so the chips are always in output mode. If you needed to have them high impedance (neither high nor low) you could parallel up those pins and connect them to a processor pin (eg, D9).




Example of 4 shift register chips breadboarded with 32 LEDs:



There are about 8 x 0.1 uF decoupling capacitors there between +5V and Gnd, to keep it all stable.

There are 1K resistors on the board in series with the LEDs.

I made it as a picture

Please find below a brief description of the thickness of surface finish for a printed circuit board:

The above mentioned are FYI, thank you.

Parts in China

In general, what is the thickness of surface finish for a printed circuit board?

Please find below a brief description of the thickness of surface finish for a printed circuit board:

Surface Finish	HQEW PCB Capabilities(FYI)
	General Thickness	Max. Thickness(please contact our sales representative)	Notes
HASL HASL pb free	0.7mil (17.5μm)	1.2mil (30μm)
ENTEK/ OSP	0.4μm
Immersion Tin (ImSn)	30~40μ"
Immersion sliver (ImAg)	6~8μ"
Immersion gold (soft gold)	3~5μ"	12μ"	The general thickness for immersion gold is 100~120μ". If your specification is beyond this range, please contact our sales representative. For your information, our capabilities for such service can be reached to 350μ".
Gold Plating	3~5μ"	50μ"
ENEPIG	3~8μ"

The above mentioned are FYI, thank you.

Continue: Thinking About Panel Sizes for contral PCB Fabrication Costs

To make the best use of the available space on a panel (and thus lower your cost), carefully choose the size of your board. Ask you manufacturer for the details of the panel sizes they prefer, and if possible pick board dimensions that are an integer divisors of the length and/or width of the panel size. Don't forget to account for the margin around the edge of the panel and spacing between the boards.  Your manufacturer should be able to provide specific instructions for sizing your board for maximal efficiency -- if they can't (or won't), you may want to consider a more cooperative manufacturer.

The math behind finding the best board size isn't complex, but it's tedious.  So to save you a little time in a spreadsheet, we've added a little calculator at the bottom of this page. Before we get to that, however, I want to volunteer a couple examples of PCB panelization scenarios:

16 x 22

Layout of a 18x24 PCB fabrication panel containing 1 16x22 boards. The efficiency of this panel is roughly 81.5%.

5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1 5 x 3.1

Layout of a 18x24 PCB fabrication panel containing 18 5x3.1 boards. The efficiency of this panel is roughly 64.6%.

7.9 x 8.01 7.9 x 8.01

Layout of a 18x24 PCB fabrication panel containing 2 7.9x8.01 boards. The efficiency of this panel is roughly 29.3%.

The point of the above examples is that size really does matter when it comes to effectively using the space on panels. Assuming panels have fixed cost (and they should, when they are identically built), then size choices can also impact your price. The difference between getting 2 boards per panel and 4 boards per panel may be a tiny fraction of an inch. The designer of the board in the third example above could cut their per-board price by as much as 50% by shaving a tiny bit off each dimension. Here are another couple examples, this time with the exact same size board and panel, but with the board rotated 90 degrees:

5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45 5.25 x 3.45

Layout of a 18x24 PCB fabrication panel containing 10 5.25x3.45 boards. The efficiency of this panel is roughly 41.9%.

3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25 3.45 x 5.25

Layout of a 18x24 PCB fabrication panel containing 16 3.45x5.25 boards. The efficiency of this panel is roughly 67.1%.

What a difference!  Getting 60% more boards per panel seems like a win-win-win situation to me.  Is your fabricator quoting your order based on only one rotation?  You'll never know if you don't ask.

And, finally, keep the spacing between boards in mind.  You might be temped to think that smaller boards would always lead to better panelization, but that isn't the case.  As the board size gets closer to the inter-board spacing, efficiency drops like a rock.  Consider these three cases:

2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2 2 x 2

Layout of a 18x24 PCB fabrication panel containing 63 2x2 boards. The efficiency of this panel is roughly 58.3%. Alice's PCB wonderland  alicebrain1992@gmail.com

Thinking About Panel Sizes for contral PCB Fabrication Costs

Getting the most for your money in custom printed circuit board manufacturing requires a little up-front knowledge of how they are made.  Unfortunately, your manufacturing partner may not talk completely straight about their processes and pricing model (kudos to those that have live pricing on the websites). If you're new to the industry, or just looking for hints on how to lower your PCB costs, keep reading and perhaps you'll learn something new.  Nothing in this article is rocket-science, it's really a matter of some common-sense math.

While designed-in features are the predominant driver of PCB manufacturing costs, the more subtle factor of panelization efficiency can also have a dramatic impact. One of the key things to understand about your PCB order is that the manufacturer (some would say "fabricator") probably doesn't build individual boards. For the sake of automation and repeatability, their machinery and processes are setup to handle uniformly-sized "panels" of material. Unless your board is large, or requires special processing, it's likely it will flow through the manufacturing process on panels with other designs.

The second key thing to understand is that the cost of manufacturing panels is basically fixed for a given set of technology. This obviously doesn't include non-reoccurring charges (i.e. the "one time" setup required for a new design) but it is the case for the actual fabrication processes.  Other than the price of materials and labor, not much varies from panel to panel.

Working from a fixed panel cost, you can quickly see that more boards packed into a set of panels means more efficient (less costly) manufacturing. And, generally speaking, the more boards that fit on a panel the lower the per-board price.  This works out well for both customers and fabricators.  However, it's one of those things that seems to be missed during PCB layout.  Costs can skyrocket when your design differs from "what everyone else is doing" because your boards will need to be on panels all by themselves.  Boards that are done using "common" technology are easily aggregated; meaning the cost of manufacturing the panel can be spread among multiple customers.  This can be a huge cost saver.  But if you're boards are going to be on panels by themselves, you have to take a close look at panelization efficiency.

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