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555 timer power prob

Discussion in 'General Electronics Chat' started by Spellman, Dec 18, 2003.

  1. Spellman

    Spellman New Member

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    i've made a timer cct with a 555 timer and its works ok but the problem is when i turn on the power. it activates the relay even though the switch from pin two hasn't made contact

    could anybody help me with this prob it would be great
     
  2. kinjalgp

    kinjalgp Active Member

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    Can you show us your circuit diagram?
     
  3. laroche73

    laroche73 New Member

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    555 output level

    Is your load referenced to ground? The 555 output (pin 3) is driven high until the threshold comparator is triggered. You can either use a load connected to VCC or drive a NPN transistor from pin 3 to invert the output (use an external transistor for load currents over 200mA).

    The following link has an animated display of the timers' operation in astable mode, and shows both VCC and ground referenced LEDs:
    http://www.williamson-labs.com/555-tutorial.htm
     
  4. dave

    Dave New Member

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  5. Spellman

    Spellman New Member

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    timer cct

    thanks for the replys i'll check them out when i get home heres an address for the circuit diagram

    http://www.aaroncake.net/circuits/relaytim.htm
     
  6. k7elp60

    k7elp60 Active Member

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    Timer circuit

    I think the problem is some false triggering of pin 2. Try a 0.1uF capacitor in parallel with R2, it will keep pin to at Vcc on power up.
    I think in the monostable mode and the circuit shown, pin three is low until pin 2 is grounded, then pin three stays high untill the capacitor on pin 6 and 7 charges to 2/3 Vcc, and then pin three goes low again
    :D
     
  7. Spellman

    Spellman New Member

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    thanks k7elp60

    i tried that and it seems to work ok but its hard to know cause it only happens the odd time
    i'm all out of 1uF caps i just had the one
    would it matter if i used a 10uF cap or if i used a 10nF cap is there any risk of damaging the timer by using these or which would be best
     
  8. laroche73

    laroche73 New Member

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