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555 timer circuit

Discussion in 'General Electronics Discussion' started by Heeran, Jan 14, 2015.

  1. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    Hello,

    I would like to know if you guys can assist me, I am building a night activated timer circuit, I have completed the circuit but I have a problem with the operation. I have used two 555 timer IC. One IC as the night activated circuit which the triggers the second IC to be my timer circuit. My problem is, that if a set the timer to only work for eg: 1 hour, the timer does not switch off due to the night activated circuit still providing a trigger signal after the hour. If I place light on the LDR after it triggers, the circuit switches off after the pre-set time. I need to know how to modify this circuit so it will still switch off after the set time although the night activated circuit is constantly providing a trigger signal at night. Also when the trigger signal is off during the day, it must reactivate the same process for the nights and days that follow.
    Kindly guide me to rectify my circuit. I would like if the correction can be made at the timer IC circuit so I can still design a more effective night activated circuit using transistors as the trigger circuit.

    Thank you.
     
  2. BobK

    BobK

    7,642
    1,662
    Jan 5, 2010
    How about a schematic?

    Bob
     
  3. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    dark-activated-relay-switch-using-NE555.gif timer circuit.png

    The circuit with the blue IC1 is the night activated circuit and the clear IC2 is your timer circuit. I have taken the output from IC1 pin 3 directly to IC2 pin 2.
     
  4. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    IC1 has no relay in the circuit, it is the night trigger circuit the trigger the pre-set timer that activates the relay on IC2 circuit.
     
  5. pebe

    pebe

    83
    11
    Sep 3, 2013
    Connect a 1nF capacitor between IC1 pin 3 and IC2 pin2. As the night sensor activates and takes pin 3 low, the cap will produce a negative pulse at pin 2 to trigger the timer. After the pulse has ended pin 2 will go high and the timer will work normally.
     
  6. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    I will have to get a 1nf, I tried a 103, 104 and 102k cap, the circuit does not work at all, there is not triggering.
     
  7. pebe

    pebe

    83
    11
    Sep 3, 2013
    I have usually found 1nF to be adequate if R2 is around 100k. What is the value of R2 in your circuit?
     
  8. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    1k
     
  9. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    10k
     
  10. pebe

    pebe

    83
    11
    Sep 3, 2013
    The time constant would be too low with a 1nF and 1k. There's no need for R2 to be so low. Try changing it to 47K and increase the cap to 10nF if necessary.

    Edit: I assume you have nothing else connected to pin 3 output that could slow down the fall time?
     
  11. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    So I should make R2 47k and use a 10nf which is a 103 cap.

    I do not have anything else on pin 3 of the night active circuit, that pin just triggers pin 2 on the timer circuit.
     
  12. pebe

    pebe

    83
    11
    Sep 3, 2013
    Yes, that should do the trick.
     
  13. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    I have tried what you have advised me to do, the circuit switches off after the timer but does not switch on again when night falls, it needs to be completely switched off and back on for it to function again.
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Upload a full and correct schematic of what you have that doesn't work. Otherwise we really don't know what you're doing.
     
  15. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    I have provided a drawing of what I have.
     

    Attached Files:

  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Scan0003.png
    • Each 555 needs a decoupling capacitor of around 0.1 μF ceramic, connected as closely and directly as possible between pin 8 and pin 1, for reliable operation. See the data sheet for more information.
    • The first 555 (not just the second 555) needs a decoupling capacitor of around 10 nF ceramic connected directly between pin 5 and pin 1, for reliable operation. See the data sheet for more information.
    • The capacitor between pins 1 and 2 of the second 555 must be removed. It will distort and reduce the trigger pulse from the first 555.
    • A diode, D2, is required across the relay coil to protect the second 555 against inductive kickback from the coil when the relay is turned OFF. Any diode from the 1N400x family is suitable.

    The 555 is not suitable for timing periods longer than a few minutes. See the data sheet for more information. A better way is to use a counter/divider IC such as a CD4040 or CD4020 clocked from an oscillator. See https://www.electronicspoint.com/resources/long-duration-timers-notes-for-beginners.4/
     
  17. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    Okay, I have corrected the circuit, it works much better know. I would like to know, is it not possible to make the 555IC time work between 1 hour to 12 hours?
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Not reliably, no.
     
  19. Heeran

    Heeran

    64
    0
    Nov 13, 2012
    So I will have to redesign a circuit using a CD4040/CD4060/CD4020?
    Do you perhaps have a any already designed basic circuits that I can replace the 555 circuit that I am currently using?
     
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    No, but I will draw something up if you tell me in detail what you want to use it for. This is so I can make judgement calls about how it should behave.
     
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