Circuit : Ron J
A motorcycle alarm that uses a mercury tilt switch as the sensor. Anyone moving your motorcycle steering will activate this alarm. Works off a 6 Volt battery.
Any number of normally open switches may be used. Fit the mercury switches so that they close when the steering is moved or when
the bike is lifted off its side-stand or pushed forward off its centre-stand. Use micro-switches to protect removable panels and the lids
of panniers etc. While at least one switch remains closed, the siren will sound. About two minutes after the switches have been opened
again, the alarm will reset. How long it takes to switch off depends on the characteristics of the actual components used. But, up to a
point, you can adjust the time to suit your requirements by changing the value of C1.
The circuit board and switches must be protected from the elements. Dampness or condensation will cause malfunction. Without its terminal
blocks, the board is small. Ideally, you should try to find a siren with enough spare space inside to accommodate it. Fit a 1-amp in-line fuse
close to the power source. This protects the wiring. Instead of using a key-switch you can use a hidden switch; or you could use the normally
closed contacts of a small relay. Wire the relay coil so that it is energized while the ignition is on. Then every time you turn the ignition off,
the alarm will set itself.
When it's not sounding, the circuit uses virtually no current. This should make it useful in other circumstances. For example, powered by dry
batteries and with the relay and siren voltages to suit, it could be fitted inside a computer or anything else that's in danger of being picked
up and carried away. The low standby current and automatic reset means that for this sort of application an external on/off switch may not
The Support Material
for this alarm includes a detailed guide to the construction of the
circuit-board, a parts list, a complete circuit description and more.