Monostable 555 lockout?

Discussion in 'The Projects Forum' started by Midnightalaskan, Feb 13, 2011.

  1. Midnightalaskan

    Thread Starter New Member

    Feb 13, 2011
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    0
    I've tried to make a 555 timer in monostable mode that "locks out" a primary timer. This is what I'm trying to do:
    Have a timer that is triggered by a 5v positive signal from external sources, the time then runs for about .2-1 second and switch a 9v relay(supplying 12v external power)for said duration. That primary then activates a secondary timer that runs and "locks out" the first timer from re tripping from the external trigger for about 20 -360 seconds...

    I've tried rigging this time in tandem and just removing the light sensor and using a transistor to close the trigger.
    http://www.kpsec.freeuk.com/projects/lalarm.htm

    My problem is I'm getting a - feedback somewhere that is shorting my chips... I've burned through a few ic's

    I reduced the ohms on 6&7 to a 100k pot and added diodes to make sure the relays don't damage the transistors, I have four transistors total two that are called for (two seprate circuits) and the other two are for each relay (one provides the external power and the other is thrown by the secondary timer to cut the power to the transistor loop on the primary timer circuit.

    My questions are... Why am I frying my Ic's and is their a better way of doing this?
    Also I need to have two different triggered timers one that is tripped by the 5v positive external signal. And one that is triggered by the closing of a relay

    Any advice or help is appreciated!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    Try showing us the schematics and we might be able to help. Pin 7 MUST have resistance, zero ohms will always smoke a 555, since it is a simple transistor going to ground. You would get similar results shorting pin 3 (the output) to either rail of the power supply.
     
  3. Midnightalaskan

    Thread Starter New Member

    Feb 13, 2011
    2
    0
    I will try to get the schematic of the circuit I built soon
    Pin 7 has resistance of 47 k and if I put a relay on pin 3 with a simple diode would that still cause the feedback? (relay is backfeeding from ground)

    Thanks
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Again, without schematics we can only guess. Schematics are the language of electronics.
     
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