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Update - by al1

A project log for PICTIL

Remake of the TIL311 hex LED display with recent technology.

alexAlex 11/07/2015 at 15:0313 Comments

The comments on the last log were good. Thinks I changed:

So here are some pictures as an update:

Discussions

Andrew Starr wrote 11/07/2015 at 21:17 point

Nice work :) Considering a black solder mask for maximum contrast?

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Alex wrote 11/08/2015 at 11:31 point

Thank you! 

I will consider black solder mask later. In the rendering I used red to get them look more like the original TIL311. The first prototypes will be purple (OSH-Park).

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Eric Hertz wrote 11/07/2015 at 16:39 point

If you're clever with your software, you could probably get away with a single resistor for all the LEDs and save some tiny-soldering ;)

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Alex wrote 11/07/2015 at 17:14 point

mh.. the only way I could imagine to reduce the number of resistors is to use some multiplexing. 

What do you have in mind?

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Eric Hertz wrote 11/07/2015 at 17:54 point

just scan through 'em... only one LED is on at a time, but done so quickly there's no noticeable flicker. With the brightness of LEDs these days, and their often being rated for brief pulse-currents much higher than their constant-current rating... Definitely more work, software-wise, but once it's done it's done...

Erm, of course, the PIC will have to be able to source that much current on each of its pins... keep that in mind. Maybe group them in groups of four and use 3 resistors, etc.

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Yann Guidon / YGDES wrote 11/07/2015 at 23:46 point

PIC pins are rated at "25mA" so there should not be a problem on this side. If you run 1mA per LED, you'll have 20mA through the ground pin (that's fine) and more than enough light.

If you turn each LED one by one, use Charlieplexing instead of direct drive : this will save pins and a cheaper PIC with less pins will work.

Charlieplexing works great for 20 LED because 20=4×5=n²+n, so you need only 5 bidirectional pins. I've done this on a 8-pins PIC16F for another application, it's maybe a bit tricky in the beginning but that's not black magic.

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Yann Guidon / YGDES wrote 11/08/2015 at 01:00 point

Note : Charlieplexing also reduces the number of resistors. There would be one 100 ohms per pin, because to turn a LED on, the current passes through 2 pins (2 resistors) plus the (small) resistance of the IO pin.
This also removes the FET/diode/pull-up because "blank" would be a freeze of the CPU. And Vled would not be used...

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Yann Guidon / YGDES wrote 11/07/2015 at 16:30 point

Wow ! Slowly but surely, it's getting amazinger ! ;-)

The last PNG is black, is it an error ?

I still believe you could put two 0603 LEDs in parallel for the segments (even sharing the same resistor).

I'm not sure it qualifies for https://hackaday.io/project/7813-the-square-inch-project but if it does, you should enter your design :-)

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Yann Guidon / YGDES wrote 11/07/2015 at 16:32 point

Your design does ! The original TIL311 is 19.31×10.67mm, well under the 25.4mm limits.

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Alex wrote 11/07/2015 at 17:06 point

Yes the black was an error - I fixed that

I could only use two LEDs for some segments (all the "long" / non corner ones) and these than would need an different resistor value. This would be problematic with the resistor arrays.

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Yann Guidon / YGDES wrote 11/07/2015 at 17:10 point

My hypothesis is that it does not matter that much. The eye will only see a tiny change, which will be imperceptible with small resistor values. What value did you plan ? I'd shoot for 100 Ohms and favor low power :-)

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Alex wrote 11/07/2015 at 17:18 point

If powering it from 5V I would use >100 Ohms. Some LEDs are very bright. Sometimes I use 1K. So I would test 470, 680, 1K. Depending on color and supply voltage. 

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Yann Guidon / YGDES wrote 11/07/2015 at 18:18 point

At 5V the TIL311 burns a lot of power so it's wise to reduce the Vled. I drop to 3.5 and it still gets warm.
If you consider a 5V supply then the two red LED would be in series, like in the original.

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