inverting delayed 555 output

Discussion in 'General Electronics Chat' started by DonLduk, Jul 15, 2017.

  1. DonLduk

    Thread Starter New Member

    Jun 12, 2017
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    Hello.
    I want to invert a delayed 555 output pulse.

    I'm triggering the 555 some 3 seconds after circuit power-on. Then I feed the signal to a NPN transistor (2N2222) to invert it. I need to have a ground pulse generated 3 seconds after circuit power-on.

    However, with the following circuit (image attached) I'm getting a ground pulse from the transistor on circuit power-on, and then again after the 555 triggers 3 seconds later. So my load is triggered not from the delayed 555 but from the power-on ground pulse.

    How do I prevent my transistor to send a ground pulse on power-on?

    Screen Shot 2017-07-15 at 4.33.56 PM.png
    thanks.
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    I'd try putting an RC on timer Reset to force the output low while power stabilizes.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Why is R2 such a low value?
     
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  4. DonLduk

    Thread Starter New Member

    Jun 12, 2017
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    what should it be?
    i only need a 300ms pulse
     
  5. dl324

    AAC Fanatic!

    Mar 30, 2015
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    R2 has nothing to do with pulse duration; it establishes base current, which we can't calculate because you didn't include supply voltage in your schematic. We also can't tell you what it should be because of the missing supply voltage.

    Pulse duration is controlled by the timer; which we also can't calculate because your schematic isn't complete.
     
  6. DonLduk

    Thread Starter New Member

    Jun 12, 2017
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    Hello, you are right, sorry for the incomplete information Find attached the complete diagram. Vcc is 5V.

    As recommended by dl324 , I added a 33k and 100uF to pin4 on the right 555 to stabilize power. These values chosen so it is larger than Ra2 and C2 (longer than the pulse).

    with this addition, sometimes trigger2 goes to ground after 3 seconds after power-on, but sometimes it triggers after immediate power on.

    thank you for your support.

    Screen Shot 2017-07-16 at 10.14.48 AM.png
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The usual rule is to make that base current about 1/10th of the collector current for good saturation of the transistor as a switch.
    You have that backwards with the base current about ten times the collector current.
    Thus Rb1 needs to be no smaller than 4V/(0.5mA*10) = 80kΩ.
    Try connecting pin 4 of the left 555 to pin 4 of the right 555 (with the R and C delay).
     
  8. dl324

    AAC Fanatic!

    Mar 30, 2015
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    The time constant for the reset is too long and it's connected wrong. Put the resistor between VCC and Reset and the cap between Reset and GND. Connect the negative terminal of the cap to GND. 100k and 0.1uF would give you a reset of several 10's of mS which should be sufficient.

    As shown, the reset capacitor polarity is reversed and polarized caps won't tolerate more than around half a volt of reverse bias.
     
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  9. crutschow

    Expert

    Mar 14, 2008
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    The 555 Reset pin has a low threshold voltage (basically one base-emitter drop above ground)
    upload_2017-7-16_10-14-11.png
    so you need a longer time-constant of a few hundred ms to get several tens of ms of Rset time.
     
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  10. EM Fields

    Active Member

    Jun 8, 2016
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    Try this:
    Negative pulse after power-up delay.png
     
    Last edited: Jul 16, 2017
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  11. DonLduk

    Thread Starter New Member

    Jun 12, 2017
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    wow thank you very much for your responses.

    I got it working with the transistors in saturation as recommended, and also the RC on Pin4 works flawless with 150k and 100uF .
     
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