large custom led 7 segment display and driver

Discussion in 'The Projects Forum' started by davidhoff, Aug 14, 2011.

  1. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    I'm trying to build a large display with 4 custom giant seven segment led numbers. I think I have a decent plan, but I'd like to get some input before I go and spend a bunch of money on something that might not work. I'm especially looking for input on the schematic in the file called "scoreboard wiring.png". I know it's going to be a ton of pieces and soldering, but I'm okay with that.

    requirements -
    4 digits
    able to control each segment of each digit separately
    pwm to adjust brightness of entire display

    my idea for how to do it -
    25 green leds per segment * 7 segments per digit = 175 leds per digit
    175 leds per digit * 4 digits = 700 leds
    5 constant current drivers driving 5 leds each per segment
    3.2V per led * 5 leds per driver = 16V
    700 leds / 5 leds per driver = 140 drivers
    140 drivers * 20mA per driver = 2.8 A
    18V 6A power supply
    shift registers turn on constant current drivers for each segment
    1 mosfet acts as low side switch for all constant current drivers to allow pwm dimming of entire display (without eating up a lot of computing time doing pwm over the shift registers)

    datasheets for components in schematic -
    NTD4858N Power MOSFET http://www.onsemi.com/pub_link/Collateral/NTD4858N.PDF
    2N3904 NPN General Purpose Amplifier http://www.fairchildsemi.com/ds/2N/2N3904.pdf
    TC74HC595AP 8-Bit Shift Register http://www.toshiba.com/taec/components2/Datasheet_Sync//151/397.pdf

    questions -
    Is this a retarded plan? (If so, can you suggest something better, or at least tell me why it's dumb?)
    Will the circuit do what it's supposed to?
    Will it let me control each segment individually?
    Will the leds get the proper voltage and about 20mA of current with this circuit?
    Will this circuit let me dim the display with pwm?
    Will this burn down my house if I build it?
    Do you have any suggestions on improving this project?
    Did I leave anything out?

    Any comments are appreciated. I want to do this right, so even if there is small stuff wrong, I'd like to know. Thanks for the help!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Regarding the scoreboard wiring; I think you're just a bit optimistic on the gain of the 2N3904 transistor; not a lot though. You'd be better off to reduce the 7.5k base resistor to 6.8k or even 6.2k; as otherwise you may wind up with some transistors on the low end of the gain curve, and those LEDs will be more dim. Decreasing the base resistance a bit will help to insure that you will get an even brightness. You really don't want to have to replace any transistors if you can avoid it. The increase in current requirement from the '595 shift register from 300uA to 500uA per base, from 1.5mA to 2.5mA per segment, won't present a problem for the '595 shift registers. You will be well under conservative limits.

    I hope that you are planning on making the circuit boards for the LEDs and constant current drivers as 25 LEDS and 5 drivers per board; that way if one segment has problems, you can quickly exchange just the one segment for another complete board for later repair. This will also decrease the complexity of the individual digits, and if you plan on having the PCBs made commercially, will decrease that expense considerably.

    How large do you plan on the digits being?
     
  3. MMcLaren

    Well-Known Member

    Feb 14, 2010
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    Couldn't you just use four (4) eight bit serial-to-parallel sinking drivers? The Micrel MIC5821 comes to mind. Each of the eight outputs can do 500-ma at up to 35 volts. Wouldn't each one of those outputs handle each 16 volt 100-ma segment directly?

    Driving the /OE (output enable) line on the driver ICs with a PWM signal works very well for PWM brightness control, though the PWM duty cycle is inversely proportional to brightness with an active high PWM signal driving the active low /OE pins.

    Good luck on your project...
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting point. There's also the STPIC6D595 that will sink up to 250mA per output; it is basically plug-compatible with the regular 74HC595, but open drain outputs. The constant current drivers could simply be re-wired so that the base resistor connected to the junction of the upper transistor's collector and the lower cathode of the string.

    That would also mean only two connections per segment; +18v power and the power shift register connection. Reducing connections by 1/3 is quite significant.
     
    Last edited: Aug 15, 2011
  5. MMcLaren

    Well-Known Member

    Feb 14, 2010
    759
    116
    Oh, there's a whole bunch of different 8 and 16 bit serial-to-parallel constant current sinking drivers too. The 16-bit 120-ma TI TLC5926 might be a good choice except the outputs are only 17 volts.

    Anyway, if you use a constant current driver IC, you won't need any those transistors.

    Cheerful regards, Mike
     
  6. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    Thanks for the input guys! I'll have to look into your suggestions. I probably won't have time today, but I'll be back when I've had a chance to examine these options.
     
  7. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    I do plan on having each segment with its leds and drivers on its own board, but I'm hoping to make the boards myself. One of the reasons I chose this project was to force myself to try making my own circuit boards. Since I need a bunch of them all the same, it should give me some practice. The numbers are going to be around 7-10 in tall. I'm going for a look like the led price signs at a gas station.
     
  8. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    I've been thinking about your suggestions, and I have a question. If I'm just regulating the current of each segment to 100mA, how do I make sure that each of the 5 strings of 5 leds will get the same current? I don't want some to be brighter than others. Or would I just need 5 times as many chips so I can regulate the current of each string? Or are these alternatives to the 595 just an alternative way to turn the segments on and off rather than regulate the current?
     
    Last edited: Aug 15, 2011
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Your schematic shows that each 5-LED string has its' own current regulation scheme. That will work fine. However, if your power supply is well-regulated, all you really need is a resistor. The Monte Carlo random distribution of forward voltages will be good enough with 5-led strings so that detecting any difference in brightness would be rather difficult.

    The 595 alternatives are to replace the single large MOSFET that you were planning on using for PWM. Rather than having the standard 595 switch the base of the upper transistor of the current regulator on and off, the base resistor is connected to the cathode of the lowest LED, and the power 595 output sinks the current from the segments. The power 595 enable/disable can be used for PWM functions instead. The power 595 can sink more than enough current to light your segments.
     
  10. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    Thanks for the clarification. I was misinterpreting the suggestions. Also, I hadn't considered the fact that you can enable and disable the output without shifting in new data. I think I'm ready to order parts now. I'll let you know how it goes when I'm done.
     
  11. davidhoff

    Thread Starter Active Member

    Sep 19, 2008
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    As much as I hate to admit it, it looks like just using resistors for current limiting might be the best solution here. Based on some tinkering with excel, it looks like about 85% of the leds would get between 19 and 21mA with just a 100 ohm resistor, and almost all of them would get between 18 and 22mA. After playing around with some leds, I can't tell the difference in brightness in those current ranges. Seeing as it would cost less than 1/4 as much, I think I'm going to go with that. Thanks to all the responders for your help, though. Going through this process has taught me quite a few things, which was the whole point of this project. :)
     
  12. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
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    your gaint 7-segments consume only 20mA per segment?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    No, 100mA per segment. There are 5 series strings drawing ~20mA nominal each per segment. So, from 200mA to 700mA per digit, 800mA to 2,800mA for the entire display - assuming all positions will always have a digit lit.
     
  14. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Thats Fair enough :)
     
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