This is a photograph of the transistor based Motorcycle Alarm No.1
. The circuit features a timed output and automatic reset. It can be operated manually using a key-switch or a hidden switch. But add a small external relay and the alarm will set itself automatically - every time you turn-off the ignition. It's easily adapted to a 6-volt system - so it will protect your "Classic Bike". And the standby current is zero - so it won't drain your battery.