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4 inch seven segment display — Parallax Forums

4 inch seven segment display

BladerunnerBladerunner Posts: 25
edited 2004-11-30 19:19 in BASIC Stamp
I am new to this and have been trying to work with a large seven segment display that I purchased at Jameco (Part Number 105591 made by LIGITEK)
and am having no luck. I have been successful with following the projects in What’s a Microcontroller, Student Guide using a
small seven segment display from Parallax... I get power the segments light up etc.
But when I use the same Resistors – 1 k (brown-black-red) for the large seven segment display I get nothing...
Help...
Thank you

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2004-11-30 16:07
    You need to look at how much current the large seven-segment display needs. Typically, to make a large segment, you use multiple LED's in one segment. The result of this is what looks like a 'super' LED, but it takes lots of current (10 mA per LED). Your poor little BS2 can *only* source 20 mA per pin.

    To drive the large segments, you'll probably need darlington transistor package (whose part number escapes me right now) and perhaps a larger power supply.

    A 1K resistor is probably WAY too large. The result is the LED doesn't get enough current to light.
  • BladerunnerBladerunner Posts: 25
    edited 2004-11-30 16:27
    That makes sense thanks for the info but I am not sure how to set up the darlington transistor... I have used one before to control a stepper motor.
    Attached is the seven segment display info PDF

    Thanks!!!
    pdf
    pdf
    105591.pdf
    154K
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-11-30 16:42
    You could use a ULN2803 array -- but you'll have to use a common anode display so that the ULN2803 can switch the cathodes. The anode of the C/A display would be tied to a power supply that can source enough current to run the LED (at 20 mA per segement, 160 mA). Using 220 ohm resistors in the segment lines (between the ULN2803 and the display) will keep the current within specs.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • BladerunnerBladerunner Posts: 25
    edited 2004-11-30 17:25
    Thanks Jon
    So if it is a common cathode can I use it?

    freaked.gif
  • allanlane5allanlane5 Posts: 3,815
    edited 2004-11-30 17:38
    No, you can't use a common cathode with a ULN2803 array.

    Also, note there are 5 LED's *in series* for each segment. Each LED takes 1.4 volts. So you need at least 7 volts to drive this thing -- 9 volts controlled by a ULN2803 (with a current limiting resistor) would be better. Definitely do-able.
  • BladerunnerBladerunner Posts: 25
    edited 2004-11-30 19:12
    Hi
    can I use a ULN2003A with this? or only the ULN2803?
    Sorry I am new at this

    Thanks for your help!!!
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-11-30 19:19
    The ULN2x03 provides a high-current path to ground, so to use it you would need to use a common-anode display.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
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