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4017 driving multiple LEDs

Discussion in 'Electronic Basics' started by pstnly, Jan 12, 2006.

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  1. pstnly

    pstnly Guest

    I need a circuit which lights 4 LEDs upon 4 pulses on input. 4017 CMOS
    counter/decoder seems the obvious choice here, and there are tons of
    example circuits on the web. However, every circuit I've seen only
    drives one output at a time (a LED connected to each of the 4017
    outputs). What I want to do, though, is keep 'earlier' LEDs lit. So
    the following would be lit:

    count 0 - LED1
    count 1 - LED1 + LED2
    count 3 - LED1 + LED2 + LED3
    count 4 - LED1 + LED2 + LED3 + LED4

    I'm thinking I could use diode OR gates (none for LED1, 2 for LED2, 3
    for LED3, 4 for LED4 - 9 total), but not sure that this is the right
    answer. On count 4, 4 LEDS have to be lit - does the 4017 deliver
    enought current for this?

    Do I need to throw a transistor on each output to amplify the current?

    Is there a better way?
  2. Dan Hollands

    Dan Hollands Guest

    Another way is to use a shift register counter sometimes called a Johnson
    counter. The flip-flop outputs would give you exactly what you want

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  3. I would use transistors or MOSFETs. In general, logic IC's don't
    reliably comfortably source more than a couple milliamps of current,
    sometimes not even a milliamp (4000 series with 5 volt supply arguably
    could have their worst-case voltage drop of pullup output FETs higher than
    desirable with much more than half a milliamp.)

    I would rather use diodes than OR gates when things change infrequently
    enough to see things happen.
    With MOSFETs and with diodes rather than OR gates, add a pulldown
    resistor of 10K or 100K or so from gate to ground.

    Diodes used for such a purpose are called "steering diodes".

    Make sure the LEDs have dropping resistors. You may get away without,
    but I would not rely on this. 470 ohms should be sufficient with room
    for error with a 12V supply, and 150 ohms should do likewise with a 5
    volt supply.

    If driving bipolar transistors, have a resistor in series with the base
    to limit current to something that the IC can comfortably deliver -
    perhaps a 10K resistor. You can usually get away with 4.7K and possibly
    but nowhere near reliably with none.

    - Don Klipstein ()
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