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posted Feb 7, 2013, 9:10 PM by Bill Bai
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I'd drop the current on the colons. You've got at least 80mA going through those LEDs. They would be really bright with modern high-efficiency LEDs.  I usually use 1K resistors for LEDs off 5V. Perceived brightness also varies by colour of LEDs, and angle. But I find a couple mA is fine for most designs. At 20mA/LED, the colons will probably outshine the 7-segs (which tend to be less bright). You'll have to "tune" this to get even brightness across the whole display.

Where are the current limiting resistors between the 4511 chips and the displays? Look at the datasheets for the chip, in the app-notes section it shows resistors for driving LEDs. Most people/designs mux the displays so you only need to provide one set of resistors for a whole row of displays. Modern LED driver chips are constant current source/sinks, so the resistors are omitted in these cases.

You're going to have change the layout to fit the resistors in (you'll need 7 resistors per display, so 42 in total). Normally I'd suggest SMD resistors because that would be trivial to fit on to such a large board but it might not fit in with the 70s retro look of the thru-hole parts. You could put then on the back? That wouldn't be a bad cheat.

You can get resistor arrays in single packages, that might work.

Now for the regulator ... 12V in , 5V out. Why 5V? The beauty of the CMOS chips is that they can run at up 18V!

In fact, you could power this design with almost anything, 15V to ~3V directly in. Check the specs on the chips, and simply write the maximum allowed voltage on the input. Ditch the regulator, maybe just add polarity protection.

But if you want to keep the regulator ... 

So far your current consumption is:

4 x 20mA on the LEDs = 80mA

Then you've got 6x 7-seg displays each drawing the maximum current per pin on the 4511. So worst case is: 42 x 25mA = 1.05A

Ignore current consumption of chips. 

Total current (worst case) is 1.13A, you've got a 7V drop across the regulator. So the regulator will need to dissipate roughly 8W. 

The LM7805 has a thermal resistance junction-to-air of 65 °C/W.  So that's 8 x 65 °C/W = 520°C above ambient.  The regulator will burn up before it reaches this temperature of course.

I see that the regulator is bonded to the PCB, with a large ground plane, but this ~~won't~~ might not be be enough. Also, you need to provide a solder mask aperture so the regulator can touch the copper for better thermal relief. Use thermal paste! Also, check the electrical connection for the back of the regulator. Is it ground? is it input/output? Don't want to short circuit it.

Even if half the LEDs are illuminated, the current consumption will be 0.5A, and the regulator will be at a temperature of 260°C above ambient. Still not good.

Muxing the LEDs reduces the current consumption of the design, as does PWM cycling the LEDs. This is usually done on the cathode of the 7-seg displays. That's why 7-SEG displays "flicker" when you saccade your eyes past them.

Just noticed, there are no decoupling capacitors for any of the chips! Those chips are digital, with fast edges and some draw large amounts of current. You'll need those caps.

**EDIT:** sorry to piss on your party

Technically the circuit can function, but it wouldn't be reliable or be consistent between builds.

**EDIT:** I looked through the circuit's author's history and saw that the circuit was put up for review, there were also comments like this:

> [–]tty2 2 points 7 months ago

> To rain on some parades, this is the sort of thing I would use as a lot of examples what not to do in electronics design...

and this:

But he didn't make any of the (necessary) changes to the circuit.

Personally, I wouldn't pay to get this PCB made without fixing the problems. But if he author wants to pay to spin this board, it's up to them.

**EDIT:** I like the retro design and general idea of this, I'd like to see it working. 

Nice layout, and I have some constructive feedback for you based on the gerber files.

There's 4 resistors on the bottom. Their leads come up underneath the LED displays, which makes them extremely difficult to solder. The 4 LEDs on the top side will also be hard to solder since these same resistors lay over the LED leads. The best solution is to put these resistors on the top. There's room to place them vertically, and then all the parts will be on the top too, another plus.

As a general note, all silkscreen markings for the orientation of parts like polarized caps, diodes, and ICs should be visible after the part has been assembled. This is a major help for troubleshooting.

Also, there's a lot of silkscreen on top of exposed pads. This isn't a problem for most board houses, as they routinely remove the overlapping silkscreen by subtracting the soldermask layer. But the quick turn and extreme low cost houses don't always make this check, and this can cause soldering problems. And it gives them an excuse to charge you more, or delay delivery if they choose to complain about it.