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7555 flasher circuit

Discussion in 'General Electronics Discussion' started by Rob_K, Oct 12, 2013.

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  1. Rob_K


    Sep 20, 2013
    Hi guys,

    Struggling here, I have built this circuit with all the values as on diagram, but I am using a 7555 chip instead of a regular 555 as that is all I have right now connected to a 9 V battery, but it does not flash, led comes on and that's it, is this circuit correct?

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The circuit look correct, but:your duty cycle is heavily skewed. What this means is that the on-time of the output is muuuuuch longer than the off time. The LED may in fact blink but it will be off for such a short time, that you don't notice.
    Here is a calculator to check your values. For a quick test, swap the 22k and the 10k resistors and put the 1Meg poti into the lowest resistance position.
  3. Rob_K


    Sep 20, 2013
    Perfect, thank you Harald. I had three unknowns going on, first being that I am still a little new to all this, second unknown were whether resistor values were ok and thirdly, the 7555 chip is the one I used for my dissertation that suffered thermal runout and I didn't know if it was blown or not.

    So using this to test the 7555 was a bit, well, when i've got my head out of the clouds shall we say.
  4. Rob_K


    Sep 20, 2013
    Also, if you would permit me to take a little time to understand this, Pins 1 and 8 are self explanatory, power up the chip and pins 2 and 3 trigger and output fine. This leaves me with pins 4 and 5 being reset and control voltage respectively are at a guess providing constants of some kind maybe providing something to use as a comparator possibly????

    This leaves pins 6 and 7, threshold and discharge respectively. So it seems to me, that this chip is effectively producing some sort of wave, using the charging and discharging of the 1µF Capacitor pin 6 is providing voltage value at which point pin is to discharge.

    Is this something of how a 555 timer works, could someone clear some of this up for me a little better than my own analysis please.


  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Rob, Google for "How does a 555 work?".

    The 555 is such a well known and widely used chip (especially for beginners) that there are explanations all over the web about this. You can find a plethora of youtube videos as well if that's what you like.

    Here is just the first hit I got.
  6. Rob_K


    Sep 20, 2013
    Yes, you have a good point there, I immediately assumed it would be more complex than it is, but it seems a most logical chip. I am starting to see where the gap in my knowledge is - Maths.
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