Please Don't Bite - 555 Timer Question

Discussion in 'General Electronics Chat' started by daroc26, Jul 19, 2009.

  1. daroc26

    Thread Starter Active Member

    Apr 4, 2009
    Hi All,

    I'm stuck! I was previously able to breadboard a working 555 Astable Circuit, but now I can't figure out the correct component values.

    The constants: I'm using a 555 IC, 9v battery, red LED, C1 (pin 6 to earth) is .47uf, C2 (pin 5) is .01uf.

    Need values for these Variables: R1 (pin 8 to 7), R2 (pin 7 to 6), R3 (pin 3 to LED +).

    I know there are 1000 tutorials for the 555 timer, but any help would be appreciated.

    Thank you!
  2. AdrianN

    Active Member

    Apr 27, 2009
    You don't necessary need a tutorial for an astable circuit. Just look into the 555 datasheet.

    At page 8 they describe the astable waveforms and the time constants. Just figure out what is the frequency you need and the duty-cycle, and calculate the resistors based on the formulas:

    Output high, t1 = 0.693 * (Ra +Rb) * C
    Output low, t2 = 0.693 * Rb * C

    where Ra is the resistor between pin 8 to 7 and Rb is 7 to 6.

    Pin 3 is the 555 output. Calculate the resistor in series with the LED based on the recommended current for the LED. If this is a regular LED, I would use a 680 ohm to 1 kohm for a 9V battery. Still, look in the LED datasheet so that you do not go over the maximum current.
  3. mauro.laurenti

    Active Member

    May 8, 2009
    ...if you want to have some more fan, I would also suggest to read the Texas Instrument data sheet. They show more applications on the 555.

    ...then, on the web you find the rest...


  4. bertus


    Apr 5, 2008
  5. Wendy


    Mar 24, 2008
    I've done quite a bit of writing on the 555, and will do more before I'm done. You're schematic or attachment didn't make it through, you probably need to put the LaTeX codes around it, or the codes.

    You'll find we don't bite here, but welcome all questions. If you don't get an answer it is usually not distain, but we don't know the answer enough to speak with authority. If someone does bite, our mods will promptly muzzle them, their very good at their jobs.

    Here are my links.

    The 555 Projects

    Bill's Index

    Another trick to add pictures to posts (but please keep the width 800 pixels and under)...

    How to Display Attachments Full Size
  6. yasir_66

    Active Member

    Jun 25, 2009
    hi daroc26
    this might be helpful to u.
    it is 555 free running oscillator or astable mode.
  7. SgtWookie


    Jul 17, 2007
    Be careful that you do not make R1 too low of a value, otherwise when pin 7 goes low to discharge the timing capacitor, you may dissipate a lot of power across R1.

    For a general "rule of thumb" on that, use 5mA as the maximum desired current flow through R1 while discharging. Since R=E/I (Resistance = Voltage / Current), E=9v, I=5mA, then R1 should be 1.8k Ohms or more. Just using a single potentiometer for R1 is risky, as if you accidentally set it to a low value, you'll either burn things up or just run down your battery really quickly.
  8. daroc26

    Thread Starter Active Member

    Apr 4, 2009
    Hi All,

    Thanks for the replies. I realize that I was looking for shortcuts. Instead of learning the formulas and doing the math, I wanted the board to give me the values. Laziness...

    FWIW, here is the schematic I was following. I'll be sure to use the data sheets and review the blog posts as well.

    Thanks again!

  9. millwood


    that will work.

    c2 is just a few .uf to a few uf.
    R1/R2/C1 depends on how fast you want it to flash: you typically need to check with the datasheet of your chip to figure that out. But for the bjt types, r1+2*r2=10meg ohm + 0.1u c1 gives a 1hz frequency. or 1MOhm + 1uf as well.

    R3 depends on the supply voltage, and your LED. for 10v Vcc and 3.3v Vf for a 20ma diode (typical), R3=(10-3.3)/20ma=330ohm.
  10. someonesdad

    Senior Member

    Jul 7, 2009
    One thing that has to sometimes be learned over and over again is to keep a lab notebook when you're doing design work (be it mechanical, electrical, physics, etc.). It ultimately should be embarrassing to tell someone you tried to make up a circuit you made before, but couldn't remember the values.

    That said, you're free to flog me, as I don't always practice what I preach. I too have done some quick experimenting and not written things down. That you need the numbers/drawing/circuit later is mother nature's way of letting you know you've been bad. :) Of course, Murphy guarantees that if you diligently and meticulously write every last detail down, it will never be needed again.

    My wife has a very perceptive quote for this from when she was little. She said a cute little boy from next door came over to their house, crying because he had been spanked for something he didn't do. She quoted him, in a perfect little boy voice, "They beat you when you're bad and they beat you when you're good". This ranks right up there with something Einstein, Ghandhi, or Confucious would have said. This saying gets used around our house on nearly a daily basis.
  11. yasir_66

    Active Member

    Jun 25, 2009
    yes as millwood said that how fast you choose these components values(R1,R2 & C1) for your desired frequency by this relation,