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NE555 Timer Trigger Problem

  1. Apr 30, 2006 #1

    My circuit is using NE555 timer to active a solenoid driver for some seconds. I have finally found out that any time we turn the power ON, the circuit is initially running. I have wondered that, when we turn the power ON, the power switch creates a signal as a voltage, so that can stimulate the NE555 trigger (pin #2) then run the solenoid driver.

    1. What kind of that signal?
    2. How to solve that problem?

    Thank you all.

    Mike Phan
  2. jcsd
  3. Apr 30, 2006 #2
    Post your complete circuit. What are you doing with pin 4?
  4. May 1, 2006 #3
    Pin 4 is connected to Vcc.
  5. May 1, 2006 #4
    It's been a while since I worked with a 555 so I'm just going by memory on the pinout. I believe pin 4 is a reset pin. Connect it to Vcc through a resistor and ground through a capacitor. This will cause the pin to be grounded briefly at power up which causes the 555 to go reset. Choose your resistor and capacitor values based on how long you want the 555 to be held in the reset state.
  6. May 1, 2006 #5
    You are right, pin 4 is a reset. My 555 is deisgned to hold for 2 seconds. I have tried to modify pin 4 as you just said. Let me double check: connect it to Vcc through a resistor 10M ohm, and gorund it (pin 4) through a 4.7uF capacitor. It is still problem! I am thinking if the values of resistor and capacitor are not correct. Do you think so?

    Mike Phan
  7. May 1, 2006 #6
    1 RC time constant at those values is 47 seconds. A bit long don't you think?
  8. May 1, 2006 #7
    Do you know how to attach a file? I want you to see my circuit.

    The purpose of this is to delay reset pin a while after the power switching ON which cause a "voltage spike" and stimulate pin2 (trigger), right?

    What is your suggestion? Thanks and I am waiting for your reply.
  9. May 1, 2006 #8


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    Staff: Mentor

    If the NE555 is a bipolar version of the 555 family, 10MegOhm is too big for an input resistor. Look up the input bias current on the inputs. If you use a CMOS 555 like the Philips ICM7555, the input bias current is in the 10pA range, and 10MegOhm would theoretically work (although it is poor design practice to use resistors that big -- quiz question: Why is it poor design practice?).

    Here's a good tutorial on 555 operation -- do you get waveforms similar to those shown here?

  10. May 5, 2006 #9
    Make sure to hold the trigger pin high with a 10k ohm resistor (depending on voltage). This way the trigger isn't as sensitive to stray pulses that maybe be caused from the circuit being energized.
  11. May 10, 2006 #10
    Thanks Triden and others. It has been done already.

    Mike Phan
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