Triggering a 555

[98badcocks]

Hi. Ive designed a motorbike alarm circuit but have come across a problem. I ma using a mercury tilt switch to trigger the 555 timer but have encountered the problem that if the imput is held on, i dont get the pulse i need to start the timing.

what i really want to happen is that when then switch it closed, the alarm will go off and when the switch is openened again, it will start timing.

Any help would be greatlt appreciated. This is for my as level technology project and the deadline is looming.

Cheers,

SI

[electrodoc]

Hi Si..  you need to have the tilt switch normally open when the bike is on one side,  then have the tilt switch put a + supply onto the trigger (pin 2) of the 555. the output will then go high  for the duration of your CR network after the bike is left alone.  I'd also put a high value 4k7 resistor from pin 2 to ground to prevent mistriggering.  to prevent any damage due to voltage spikes I'd use a diode or a bridge rectifier and a smoothing capacitor before the supply gets to the circuit. A good book on this subject is Bernard Bapanni 555 timer projects