1. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Hi
    Have a Q.,
    if i set this up, will the LED's be turned on one at a time, each second ?
    If you can understand my schematics.
    I know there is no reset, but in princip, will it Work ?
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The output will be like this:
    Code (Text):
    1.  
    2. D5 D4 D3 D2 D1 D0
    3. =================
    4. 0  0  0  0  0  1
    5. 0  0  0  0  1  1
    6. 0  0  0  1  1  1
    7. 0  0  1  1  1  1
    8. 0  1  1  1  1  1
    9. 1  1  1  1  1  1
    10.  
     
    Last edited: Nov 23, 2014
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Yor LEDs are connected upside down. So LED will never light.
     
    absf likes this.
  4. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Actually these 74xx chips dont need resistors. However, the voltage level will be off, so, using high brightness LEDs + 2.2 K resistors also isnt wrong for some purposes.
     
  5. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    If i reverse the LED's , and connect 10 in series, then i will light up all 60 LED's one at a time, and then commom reset and all is off.

    Wich resistors do i not need ?
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    You can connect LEDs directly, they wont burn out, neither the 74XX IC.

    Mostly 74HC is used nowadays, and these have about 150 Ohms internal resistance. There's also LCX which is designed for 3.3 volts, however, it also doesnt burn up at 5 volts. Many HC ICs do work with less than 5 volts too.

    If you are scared, try just one LED, measure the current, and reduce the resistor gradually on a breadboard, and you'll see its not needed, except you need the output for other ICs (then you need the correct voltage, which breaks down to the LED forward voltage if no resistor is used).
     
  7. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    So you are saying that this will Work ?

     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    You need to in series a resistor for each output, the resistor can be use 330Ω for 2V/20mA/5mm LED, 220Ω for 3V/20mA/5mm LED.
     
  9. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Ok.so the transistor is not needed ?
    Just a 220-330 ohm resistor in series with the led.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Unless the brightness is not enough for you, or the output all output the high level all the time, otherwise it's ok for those values, and no need to use the bjts, so you can try that there is no bjt first.
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    When you need to connect output to another TTL input, its not OK to leave out resistor. As voltage will break down to LED forward voltage. You can at 3 volts, with blue LEDs, its still 0.5v loss, could be a problem also, but usually works.

    Same with PIC microcontrollers. ATMEL (used for Arduino) however, I measured, produce 60 to 70mA, which is too much for 20mA LEDs, unless you do a 6-phase or 8-phase multiplex.

    TTL is really no problem- you can drive a 4-digit segment with 2x HC595, and 1x LCX244, 2 bits in parallel for each cathode sink. With HC, you can lower the voltage, downto the brightness which is appropiate for you, at low voltage, its really just a dim glow, the current is low too, no damage possible.

    I have researched it in detail. The first 2 pictures show 74HC595 driving LEDs with no resistors, the other 2, with PICs directly/
     
  12. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    I play it safe and use resistor (10 k ? ) at base of transistor and resistor (330) in series with LED.
     
  13. absf

    Senior Member

    Dec 29, 2010
    1,492
    372
    Well, if you're new and dont have a lot of parts to experiment, it's better to follow scott's advice. But if you are a seasoned & experienced with electronics like takao then you can take his advice and make your circuit board as small as you want. As for me, I have done both. Cause I have a whole box of TTL chips and thousands of LED to spare.

    "May the force be with you"......

    Luke Skywalker.