I constructed a simple but effective alarm-system from a (extremely loud) siren, a 12 volt relays and the mercury-switch from a central-heating system. The moment the bike is 'straightened up' (taken from the side-stand) all hell brakes loose.  It has two downsides:  First of all it does not 'reset' after it is triggered. It will continue to 'howl' until the battery is empty.  Secondly (and more important) I discovered that hotel-clerks and security-guards are curious people and would like to know how it feels to sit on a bike like Pam. As a result the entire hotel wakes up and I have to get out of bed to 'reset' the alarm. Naturally everybody swears it just started by itself.