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7 Segment Display TOTAL confusion

Discussion in 'General Electronics Discussion' started by piczim, Dec 19, 2012.

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  1. piczim


    Dec 13, 2012
    I am trying to make a novilty number to be displayed on our post box. I have a dual seven segment display KingbrightDA56-11EWA. Common Anode right hand decimal.

    As I wanted to be able to display severalnumbers I am trying to drive the 7 segmentdisplay with a SN74LS47N Which is A BCD 7 Segment display driver Yje Datasheet shows a maximum 15 Volts. Is that the maximum supply voltage for the IC.

    I Have Pins 4 & 5 B1/RB0, RB1 ( both on data sheet have lines shownabove them and I have no idea what that means or how to show it here. ) connected to the positive.

    Pin 3 LT (Again with a line able it ) can be switched to goto the positive

    Pins 1,2 6,7 B,C,D,A Can be switched on to the positive or off .

    The pins to the 7 segment display are connected through a 150R resistor. The common on the Display is connected to the +ve.

    I have connected a 5 Volt supply andnothing happens.

    I have removed the IC and Taken a jumper from the Negative through each resistor and each segment lights up. But Icant get it to work from the IC and nervous to increase the voltage from 5 volts,I cant seemto findthe correct supply voltage.

    Assistance willbe appreciated.


  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The 74LS47N has open collector outputs for the segment outputs.

    These outputs connect to the cathodes of the display (via resistors) and can sink 40mA.

    The 74LS47 is powered from 5V, the common anode of the display may need a higher voltage (15V?)

    I can't find a good example on the net or in the datasheets. Maybe someone else can find one
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The line above the signal name means it is low active. You can oftne find the signal name preceded by an "N" since, as you have noticed, it is difficult to render a word with a line above. So instead of LT(+bar) you can sometimes find NLT. It's the same. I'll use that nomenclature here (I can't write words with bar either:) ).

    To see how the chip operates have a look at the table on page 3 of the datasheet.
    If you drive a single 7segment display, set
    - NLT = H
    - NRBI = H
    - NBI/NRBO = H
    You would need NRBI, NBI/NRBO if you want to cascade some 74ls47 for a multi-digit display with leading zero suppression. But let's get this one to work first.

    If you power up the circuit without any signal at the input, the 74LS74 will interpret an open input as H (this is a feature of TTL circuits, although I don't recommend to use it). An all-H input turns all outputs OFF, therefore you will not see anything on the 7segment display. Only if you apply a valif code to the inputs A...D (0...9) you will see a meaningful number displayed. Note that 10...14 will produce some rather meaningless result, see figure on page 3 of the datasheet. Only 15 doesn't produce any active output.
  4. Raven Luni

    Raven Luni

    Oct 15, 2011
    Possible hack suggestion: Butcher a cheap calculator. It's fairly likely your 7 segment displays wont be directly compatible but what you could do is flip the polarising plate of the calculator (that innocuous looking clear window is actually part of what makes the LCD work) so that the numbers are displayed grey on black, and add an LED backlight (several small LEDs around the perimeter) in any colour you like.

    Then all you need to do is put it in a custom case and hide the rest of the circuitry so that only you have access to the inputs - and just type in whatever number you want to display :)
  5. piczim


    Dec 13, 2012
    Many thanks for your assistance. Thus far, just so I am clear a H means I put 5 volts onto that pin.

    so in order to get a 3 this is what is needed

    A = 5 VOLTS
    B= 5 VOLTS
    C= 0 VOLTS
    D= 0 VOLTS

    I put the voltage on as shown above and then turn the supply onto the IC's and seven segment display and I should see a 3.

    Pins 4 & 5 are permenantly connected to 5 Volts.

    Thanks again for your help

  6. piczim


    Dec 13, 2012
    I have attached the drawing of the circuit.

    Attached Files:

  7. Relayer


    Dec 15, 2012
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Your circuit misses one important detail (4 to be precise): your switches can connect the inputs A..D to Vcc (5V). But what happens if a switch is open? As I told you, a 74LSxx will see an open input as H. Therefore it makes no difference to the decoder whether one of the switches is open or closed.
    You need to pull-down open ipnuts to GND. Use a 1kOhm resistor from each input A...D to GND.
  9. piczim


    Dec 13, 2012
    Please excuse my stupidity but how do I get the attachment and where do I find it
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