7555 timer circuit

Discussion in 'General Electronics Chat' started by Dritech, Oct 12, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The 7555 should work the same whether 3v or 18V).

    What voltage does your relay's coil need? What current?

    Post a picture of your circuit (on the breadboard) - clear as possible.

    Also, the 7555 can only handle 100 mA (in/out of pin 3) the 555 can work with 250 mA.
     
  3. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    41.2 megs to a 33 uf? You are asking that capacitor to be a very low leakage capacitor! (146 nanoamps is the current flowing through the resistors with a 9 volt supply.) This is why we use counter chips on a short interval 555 chip.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    I calculate 49.2 megs with 33μF to give a 27 minute time-constant or a timer period of over 29 minutes.

    You will likely need very low leakage film capacitors (which are physically large) to achieve that period.
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
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    Yeah, 5 x 8 megs plus 6 times 0.2 megs = me getting old.:p

    The fact that I can remember what I was thinking when I blew that calculation (about 100 minutes ago) is amazing in itself!
     
    Last edited: Oct 12, 2015
  6. Wendy

    Moderator

    Mar 24, 2008
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    The 7555 has incredibly weak drive at lower voltages, it is proportional to the supply voltage. At 3V it is about 2-20ma, while at 15V it is nearer to 100ma. You have to allow for this weak drive when using this circuit with a much higher gain transistor, among other things.

    A conventional 555 does not have this problem.
     
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