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Alarm circuit triggered by fluorescent tubes

Discussion in 'Electronic Basics' started by Mike G, Aug 25, 2003.

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  1. Mike G

    Mike G Guest

    Hello all. I hope you can help me solve this annoying problem.

    I have built a simple burglar alarm for my caravan. It works off the 12V
    caravan's system which is supplied by its own leasure battery. The circuit
    uses a 555 timer. The normally closed sensors are used to by pass (or
    short??) a resistor. Therefore if a sensor contacts open, the resistor
    creates a voltage dip on the trigger pin of the 555 timer, thereby
    activating the alarm in monostable mode. Hope all that is clear???

    Anyway, here is the problem. When at home, I plug the caravan into the mains
    from the garage. This charges the battery in the caravan, and also supplies
    a mains voltage socket, in the caravan. However, if I switch on the lights
    in the garage (fluorescent tubes), whilst the caravan alarm is set and its
    connected to the mains, it triggers the alarm. I am guessing, that the tubes
    are somehow sending a spike or something into the caravans electrics. I
    tried inserting a 22 microfarad capacitor across the supply of the alarms
    circuit, but it had no effect. I'm new to electronics so any suggestions how
    to prevent this would be gratefully received!!

    Many thanks in advance.
  2. Do you have a bypass capacitor on pin 5 of the 555 chip?


    Chip Shults
    My robotics, space and CGI web page -
  3. Mike G

    Mike G Guest

    No! Pin 5 is not connected to anything! What value capacitor should I use,
    does it matter if its electrolytic, or not??

    Many thanks for you reply.

  4. Mike G

    Mike G Guest

    Many thanks for the explanation Jon. Cheers.

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