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555 timer

  1. Feb 13, 2008 #1
    ive basically put in one try a day at this, and interchange about 3 IC's possibly thinking thats the problem, but when push comes to shove, i cant make a basic 555 astable circuit powering a LED. Recently all ive been able to get out of it is a glorified circuit to just make the LED light up, no fancy blinking involved.

    Ive taken all the circuit diagrams off the net (top google results), and have even tried a few off youtube, but no success..... It may sound pathetic but some insight into what im doing wrong would be greatly appreciated. Heres a picture of my most recent disaster on a breadboard-


  2. jcsd
  3. Feb 13, 2008 #2
    Run a 1 K resistor between pin 7 and 8. There is nothing to charge the cap up the way you currently have it wired. Also, it wouldn't hurt to have a .1 uF cap between pin 1 and 8 for power supply decoupling. Pin 5 should probably have a .1 uF to ground.
    I just noticed a black wire that terminates somewhere under your 4.7k resistor. What is this doing here? Where does it go? I'm not sure it belongs there.
    Last edited: Feb 13, 2008
  4. Feb 13, 2008 #3
    thats the sending wire to the 10k pot between pin 7 and the positive rail.. i put a 1k in where you had said and a 1uf from 5 to the ground, but the same thing happened, just that steady defeating green glow of the LED.
  5. Feb 13, 2008 #4
    I'm not sure where you have the pot wired to. Is it to pin 6 or 7? It should be wired to pin Not sure what you have the 10k pot set at, but typically when a 555 is wired astable and you want a duty cycle close to 50%, a resistor is put between pin 7 and the positive supply rail. The size of this resistor should be quite alot smaller than the main resistor that goes between pin 7 and 6. (Like 10 times less) However, you need to be careful that you don't get the resistor so small that too much current flows through it into pin 7. Pin 7 is grounded via a transistor inside when the output of the 555 is low. So naturally, to small of a resistance will cause excessive current to flow into pin 7. I wired a 555 a couple of nights ago very similarly to the way you have it wired. I wanted about 300 hertz out of mine, but that is irrelevant. It should work the same. Can you verify what the voltages are doing on some of the pins? Do you have a digital voltmeter?
  6. Feb 13, 2008 #5
    yeah i do, ill work on it and post what im getting
  7. Feb 13, 2008 #6
    alright so ive got the same 4.7k between 6 and 7 and a 1k running from 7 to the ground rail. The voltage for each pin reads-


    Battery (power source)-6.61vdc

    now what would cause that? sure doesn't look normal to me...
  8. Feb 14, 2008 #7
    You are correct, it does not look right. I'd pull out the IC and measure again.
  9. Feb 14, 2008 #8

    Battery voltage- 7.27v
  10. Feb 14, 2008 #9
    You obviously have a problem with the breadboard, have something wired wrong or made a mistake in your measurements. Pin 2 and pin 6 should have the exact same voltage since they are tied together with a wire and they definately are different.
  11. Feb 14, 2008 #10
    yep, the breadboard is wacky. I triple-checked it and really cant understand how exactly that is happening.. might wanna go find a new one tomorrow..

    EDIT: with the quadruple-checking i discovered that the wire connecting them wasnt all the way in on the 7 side.
    Last edited: Feb 14, 2008
  12. Feb 14, 2008 #11
    Why can't you just slide the IC down to a new part of the board and rewire? At least temporarily. Give you some confidence wouldn't it? Seems like you're tired of having this thing kick your arse.
  13. Feb 14, 2008 #12
    well it wasnt the board in that case, the wire just wasn't all the way down.

    and yes, a middle-school bully cannot begin to compete with the devastating effect this is having on my self-esteem.
    Last edited: Feb 14, 2008
  14. Feb 14, 2008 #13
    Aha! there it is!!! the blinking light that has given me my self-respect back!!!

    i never thought id be that excited over a blinking light..... Thanks a ton man..
  15. Feb 15, 2008 #14
    Glad to have helped. I remember back on my first job doing electronic troubleshooting I made a comment to my trainer on the second day of work: "Things are sometimes not what they seem." He replied: "Things are NEVER as they seem." I will always remember that comment. In this case would you have expected to have a faulty wire/socket? You were most likely expecting that is was a mis-wire or faulty IC.
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