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  • Author Author: phoenixcomm
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QUESTION: I think the sky fell on me. I need some help please. The tail of too, too many wires.

phoenixcomm

Ok I have been working with the NAV Computer Display Panel, for a while now an I run up to this wall. OK, this beast as there are 4 sixteen-segment alphanumeric displays and another 13 seven-segment displays, DISCLAIMER THEY ARE NOT LEDS THERE INCANDESCENT! They run at 4 vdc @ 15ma. per segment.

First, this is a whole lot of wires drivers, cussing, etc. Second, they run off the 5vdc supply. There are 4 sixteen alphanumeric displays (that is 64 wires), plus 13 more seven-segment displays that is another 91 wires and there are 4 decimal points (4 more wires, are driven via the GPIO on an Arduino.). When I first got them years ago I thought that I could run them with a MAX part but it only supports LEDs and it uses a technique called 'Charlieplexing'. 'To make this easier you can drive the 4 alphanumeric displays on one bus but then you need 2-bits to act as selectors these can be driven via an I2C 16-bit port IC. The 2-bits are driven via the GPIO on an Arduino. Now you have left, the 13 seven-segment displays.. 

Here is the caveat, the refresh rate of the Human Eye. 

Any Ideas please post a response. 

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  •    The PCA9634 is an I2C-bus-controlled 8-bit LED driver optimized for Red/Green/Blue/Amber (RGBA) color mixing applications. Good try but you must have missed the spec.."DISCLAIMER THEY ARE NOT LEDS THERE INCANDESCENT! They run at 4 vdc @ 15ma. per segment." This is a big swing and a strike (baseball)!!  try again. LOL the total displays = 17  16segment(4) 7segment(13) also please remember that this fits into a 3" deep box 4.2" high, and 4.75" wide.  so you need 5 bits for the 17 selects +  3 more bits for the decimal points(4), so all I need in the unit is the drivers (UJTs?)  seven seg 91 + 64 more for alphanumeric.  

    the controller or FRU/LRU is one of my plastic shoe boxes that can hold all of the logic the two CPUs and whatnot. The main Idea is to reduce the interconnect wires there are 81 7segment drives + 64 alpha drives(16x4) just to toss a wrench  13 select line + 4 for the decimal points so the load 91+64+4 = 159 drives, x .015ma segment = 2.385amps. Here on ebay I found 5A Input DC 5-30V Output DC 3V 3.3V 4V 5V 6V 9V 10V 12V Synchronous DC-DC Buck so I would most likely split the load between two of them just to be safe.  This will provide the 4volts that I need for the displays.  now about dimming the beast back in the FRU/LRU I could do something with PCM.. 

    plus 2  thumbwheel switches with BCD outputs image

    plus 2 rotary switches, one 6 positions and the other 7. I can use two  8:3 bcd priority encoders which will go to a mcp2017.

    plus a 14 keypad which I can use a 74C922 keyboard 

    thoughts to ponder. I was planning on using 2 processors maybe an Arduino for the lights and maybe the switches as well.

    Then I could use an esp32 (small)  to handle the canAerospace/over ethernet traffic. and use an every this runs on interrupts. 

  • in reply to phoenixcomm

    Hehe I didn't miss that requirement : ) I'm aware of what's written in the datasheet, but, despite being marketed as an LED driver, that chip actually has no constant-current drivers for LEDs. It's simply open-drain (or totem-pole; it is configurable), allowing the driving of all sorts of devices, including incandescent. 

    That's why the circuit snippet has resistors in series with each output, because using that chip is effectively the same as driving the connected indicators from a constant voltage source, and it requires the series resistors to limit current, which is 100% as compatible with incandescent filaments as with LEDs.

    It will easily fit the dimensions you mention, with a 5-board or 10-board sandwich, since only two wires (I2C) plus two or three more wires for the supply voltage(s), are required to be connected between the boards (can be done with jumper wires or connectors). Each board would only be 1 or 1.5 inch square.

    The wiring requirement is therefore very small.  What's more important is how you wire from the boards to the indicators. If you have a photo of the connections, it would be interesting to see if a PCB could be soldered directly to them, to cut down your wiring between the indicators and board to zero.

    Also: suggesting 20 of the chips might sound a lot, but that example board in the earlier reply has seven of those chips on it, easily controllable, so I can confirm it is very feasible to have three times that amount driven by a microcontroller, to control all your incandescent filaments with simple code.

  • in reply to shabaz

    There is also the PCA9685 which has 16 outputs which might be worth a look if you want to reduce the chip count. One per alphanumeric display.

    (There is the MCP23017 16-Bit I2C I/O Expander but are limited to 8 per I2C bus as standard whereas the PCA9685 allows 64 per bus.)

  • in reply to beacon_dave

    I've not looked in detail but at first glance they look very similar, and should be suitable for incandescents. Halves the chip count as you say! The MCP chip might be pushing it in terms of output current (it doesn't seem to be stated apart from the Absolute Max ratings but I may have missed it, am in a rush : (

    image

  • in reply to shabaz

    There is also the PCA9635 which looks like the 16 output version of the PCA9634

    image

    https://www.nxp.com/docs/en/data-sheet/PCA9635.pdf

  • in reply to beacon_dave

     ,    sad to say but this will not work as there are too many dives  13 7 seg, 4 16 segs, and 4 decimals and these I2C parts aren't cheap! 

    ---------------------------Inside the control unit ---------------------------

    just for the drives I am thinking of 4v DC-DC converters with a zener across the load set to 4.5 volts. I am looking into NPN Transistors wired to the ground and the segment between the Vs 4v and the collector. for the 7 segs I will most likely use CD4511 (13) addressing would be (2) SN74LS138  to handle the 16 bit are handled by (4) PCF8575 latches and the addressing is via (1) SN74LS138 total chip count = 20.

    now the PCF875 is I2C so that's 2 wires  (no decoding)

    decoding 7 seg is 4 wires for decoding 

    BCD for the 7 seq is 4 wires

     total wires that I have to worry about = 10

    ops I don't want to forget about power: (1) 5v & (3x) 4 volts and (1) ground  = 5 more  4v power is split into 3 (2 for the 7 segs & one for 16 segs)

    I forgot the(4)  decimals, Thinking hey I still fit onto a db25!!

    ---------------------------------- LRU ------------------ 

    Plus the switches bcd thumbwheel switches may 8 wires

    Plus 2 rotary switches 6 pole and 7 pole = 14

    plus debounce for all the switches 

    plus (2) SN74LS148 encoders which feed a PCF875

    This gives me a wire count for the LRU = 22 + 1 ground and 1 power = 24 hey I still fit onto a db25!!

    ------------------------------------------------

    BTW the only thing I am missing is the PWM drive Unamused

    I think this is better than 200+ wires 

    If I keep the LRU in the right side of the cockpit and use shield CAT6 cable I could go  1meter plus.. I have to test it and find out. 

  • in reply to phoenixcomm

    Hi Cris,

    That's a bit more circuitry regarding the LED driving, but if it's cheaper, then using CD4511 seems a very good idea. I didn't realize that part was still available, but I see it in stock at some distributors. It looks a great option.

Comment Children
  • in reply to shabaz

      yes, but the two PCxxxx parts do not have on board decoders! so that more work for the CPU.  I will prototype some of the 7 segs with alphanumeric displays like two of each before I comment I will most likely need to have the boards made (maybe 3) as I don't know what type of wire nightmare I'm in for!.  

    Doing it this way I don't have to refresh the displays!! driving the alpha parts will require software maybe just a look-up table.