LM555 timer circuit

Discussion in 'The Projects Forum' started by Kefka666, Jul 9, 2008.

  1. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
    38
    1
    I've been trying to get an astable oscillator timer circuit to work but have had no luck yet. I set up the following circuit on my breadboard using an NTE955M IC timer.

    Here is the circuit I'm attempting:
    http://home.cogeco.ca/~rpaisley4/LM555Astable.GIF

    With:
    R1 = 1000 ohms
    R2 = 5100 ohms
    C = 47μF electrolytic rated at 35V
    ...presumably resulting in approximately 2.7 cycles per second.
    Source = 3V. Also tried 9V.
    Output as LED (2.1Vf, 20mA) with resistor.
    The LED should blink 2.7 times per second when the circuit is active.

    I have not been successful. What am I doing wrong?
     
    Last edited: Jul 9, 2008
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You appear to have left the reset pin (pin 4) open. This pin needs to be tied to the positive power rail.

    hgmjr
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You forgot to say what happens:
    Is the LED always turned on?
    Is the LED always turned off?
    Is the LED too dim?

    You tried a supply of only 3V so you obviously didn't look at the datasheet (minimum supply of 4.5V).

    What is the value of the LED's current-limiting resistor?
    Does the LED work?
    Is the 9V battery dead?
    Correct polarity of the LED and battery?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    And pin 5 should go to ground through a .01 uF capacitor.
     
  5. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
    38
    1
    I just got the circuit to work, but the LED only blinked once and would not blink again until I removed the battery and re-applied it. The LED should continuously blink 2.7 times per second when the circuit is active. The LED and its resistors and 9V work fine. I tested them all separately so I know they are functional.

    hgmjr: Thanks, I added the pin 4.

    beenthere: Is the 0.1μF capacitor essential? I read somewhere that it's optional, but I'll try using one when I can pick one up.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It's only optional if you want the 555 to run in the astable mode. This is from National Semiconductor.
     
  7. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
    38
    1
    I made a circuit based on this diagram:

    [​IMG]
    (minus the 0.01µF cap)

    But the LED blinked once and would not blink again unless I removed the battery and then re-connected it. I've tested the circuit components individually and they all work. Any comments on what I could be doing wrong?

    Note: Removing pin 5 from ground allowed the LED to stay on continuously without oscillating, as if the IC wasn't there at all.
     
    Last edited: Jul 11, 2008
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    R1 is really too low; it should be 3k or larger.
    Instead, try:
    R1 = 10k ohms
    R2 = 47k ohms
    C = 47μF electrolytic
    You should get about 0.295Hz with that combination (a complete flash cycle every 3.4 seconds)

    If you change R1 and R2 to both be 5.1k with that 47uF cap, you'll get 2Hz.
     
    Last edited: Jul 11, 2008
  9. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
    38
    1
    Wow, it was the addition of the so-called "optional" 0.01µF cap to the 5-pin that made it work. Without it, the LED pulses once and then stops. I wonder why that happens?

    Surprisingly the 1K/5.1K/47µF combination and the 10K/51K/4.7µF both worked once the 0.01µF cap was in place. I didn't realize that the astable operation depended on that one element. I'll still stick with >1K for R1.

    Thanks for the help everyone.
     
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