BCD for LEDs

Discussion in 'The Projects Forum' started by dyeraaron, Oct 28, 2008.

  1. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
    I have 8 LEDs coming off of a 7490 Decade counter......now one decade counter only has 4 outputs for ABCD......Does a second 7490 pick up the EFGH or is that just another ABCD, Obviously, you can't jumper each one because they have specific pins such as the 4 inputs and 4 outs, the Vcc and GND, along with the 2 clocks....so my question is this....I have a Frequency input going into the decade counters....do I jumper each pin with the input so I can get my BCD @ my LEDs?
  2. SgtWookie


    Jul 17, 2007
    A single 7490 has ABCD outputs that represent a decimal digit from 0 to 9. If you have two of them that you want to cascade, you'll need more gates. It would be easier to use something like a couple of 74160's. The 2nd counter would have bits ABCD of the 2nd decimal digit (0 to 9).

    As far as the LEDs - TTL is very poor at sourcing current; less than a milliamp. The outputs might sink up to 16mA, but that would mean that your LEDs would be ON when the output was low. You'd really need to use NPN driver transistors to invert the logic signal.
  3. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
    So I use (1) 7490 and my ABCD output goes to a 7447 but before those inputs, I paralleled the 4 into a resistor pack and into LEDs grounded by an NPN with a VCC into the base along with a resistor at the base. In the beginning though, I was thinking I'd need 8 inputs to the resistor pack to my 8 LEDs.....not so clear about that yet.....but back to the 7447s....looking at those datasheets and some text in my books....theres pins labeled RBI/RO or something like that...I know it stands for Ripple banking or something along that nature.....is this the output from one to the other 7447 so these 2 decoders can drive 2 seperate 7 segment displays?
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Ripple blanking is for leading zero suppression, so the display doesn't read 0001, but only 1.