7 Segment LED Help

Discussion in 'The Projects Forum' started by DMC-12, Oct 2, 2012.

  1. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Hello everyone,
    I am new here and come looking desperatley for help. I've looked and asked around many places but did not get the help I needed. I plan on working on small film props, etc. that require electronics. Unfortunatley, I have very limited knowledge of electronics.
    Basically, what I want to learn how to do is: How to set up a single 7 Segment LED to display a number. Let's use 3, for example. I want to know how to simply turn on the 7 Seg to only display the number 3, and do nothing else. Just turn on, display number, turn off.
    Schematics/a 'how to' would be great! Also, a parts list would be splendid!

    Thanks guys a lot guys! Your time and effort is greatly appreciated! :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    Not a problem.
    Choose the LED. Provide part number.
    Choose supply voltage.
    Add 5 resistors and presto! You're done.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
  4. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Thanks for the responses. They do help a bit, but not completely. :p

    Would I use a 7 Segment that is Common Anode or Common Cathode? (Sorry if I'm asking a lot here. It's something that I've been wanting to do for a long while now, but never seem to be able to find the right information.)
    Maybe this, pathetic diagram will show you what I need help in doing, and will show you just how new I am to this:
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Each of the seven segments is a separate LED, so you need power going to every segment you want to light, and every one of those power leads must have a current limiting resistor. Of those seven segments, one side of all seven is common; that common side may be the anode or the cathode; the only difference that makes is the direction you connect the DC power. You can use either type.

    If the display is common anode, you connect the positive side of your power to that common lead, and the negative side of your power through a current limiting resistor to each of the segments you want to light. If it's common cathode, you connect the negative side of your power to that common lead, and the positive side of your power through a current limiting resistor to each of the segments you want to light. The number "3" requires five segments to be lit, so you need five resistors, plus a common lead, i.e., six connections to the display.

    Each segment has a letter to designate it; here's a diagram that shows the letter designation for each segment.

    [​IMG]

    So, for the numeral "3" you would need to light segments A, B, G, C, and D. Notice that this display also has a decimal point, which is a separate LED and will have its own lead and will require its own current limiting resistor.

    The datasheet for the display you are using will identify which pins are for each segment, the decimal point, and the common lead.
     
    Last edited: Oct 2, 2012
  6. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Tracecom, I feel that helped a fair bit! Now I'm just wondering, what is a Common Lead? What resistors will I need to connect a 9V battery with a .56" CC 7 Segment? And would it be hard to add a simple switch, like a DIP switch? What should I search on eBay to find the right wires that are required, or would I just use solder?
    Thanks a lot guys!
    This poor quick sketch if for a Common Canode, minus the Common Lead :p :
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    What is the part number of the display you are going to use and what color are the LED segments?
     
  8. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I didn't find reliable specifications for the parts on the ebay ad (which is not unusual for imported parts.) When you receive the parts, they might have a datasheet with them, but if not, you will have to experiment to determine which pins are which, and what resistors you will need to use.

    Thus, the following information is just an educated guess.

    To light the LED's to optimum brightness (not necessarily maximum), I would suggest a maximum current of 20mA. Using 2.1V as the drop across each LED, and a supply voltage of 9V, that would call for a 360Ω (1/4 watt) resistor for each segment. When you get the parts, you might want to try a 1000Ω resistor first, and if the display is bright enough, use 1000Ω resistors. I would be very careful about a resistor less than 360Ω.

    An 8 position DIP switch would work, or just eight eight individual switches (SPST). Wikipedia has a pretty good article on 7 segment displays; I suggest you study it.
     
    Last edited: Oct 2, 2012
  10. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Closest datasheet I could find:

    http://www.taydaelectronics.com/datasheets/A-1712.pdf


    There are a number of ways to do what you ask, but I've kept it simple with the attachment. You can use an 8-position DIP switch to turn on and off the individual segments and a master switch to turn the display on and off. As you get more into electronics, we can show you how to:
    • Change the number using a single IC and BCD (rotary) switch
    • Count up and down with a single button press
    • Add a clock signal to automatically count up or down like a timer or clock
    • Add multiple displays together to form a larger number
    • And so on
    Hope this helps.
     
  11. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Thanks a bunch!
    I think I MAY be able to do this now.
    ..Hopefully..
    :)
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    For your application it doesn't matter if you get COMMON ANODE or COMMON CATHODE.
    Either will work but the connections will be different.
    Get a whole bunch of 1kΩ resistors (10 at least). This will work over a wide voltage supply range (5-12V).
     
  13. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Sorry for the double post, but this master switch to turn the the display on/off sounds even better! What exactly would I be looking for and how hard would it be to add to the circuit?
     
  14. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    It is just a toggle switch (or any switch that latches) wired in series with the power supply, a 9V battery in this case.
     
  15. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
  16. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Yes, that battery holder with switch would work.

    I have added a master switch to the schematic in my previous post. My schematic agrees with the datasheet that elec_mech posted. For wire, I suggest 26 awg stranded, but 24, 28, or 30 would also work.
     
  17. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Thanks a billion everybody!! :D
     
  18. DMC-12

    Thread Starter New Member

    Oct 2, 2012
    19
    0
    Could the same process be used on a 2 digit 7 Segment?
     
  19. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Yes, just make sure the two digits have 18 pins or so. Some two digit displays will have ~10 pins to use with an IC to multiplex - these will not work with switches as we've shown you so avoid them.
     
  20. Rbeckett

    Member

    Sep 3, 2010
    205
    32
    DMC,
    I asssemble a paif of LEDs to simulate the red eyes of a predator and place it in or near the chicken coops. Works better than you would ever believe. If you are just lighting the number three, apply Pos volts that have passed through a resistor to the appropriate segments and connect Neg volts to the common leg of the display. It will glow the number you choose for about 6 months without disconnecting. Thats the Ghetto butched together way, there are many other options that would be technically more correct. Hope this helps.
    Wheelchair Bob
     
Loading...