Alarm circuit triggered by fluorescent tubes

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.
Mike.

 

Do you have a bypass capacitor on pin 5 of the 555 chip?

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

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.

Mike.

 

Many thanks for the explanation Jon. Cheers.

Mike.