A Cmos Based Motorcycle Alarm
This circuit features an intermittent siren output and automatic
reset. It can be operated manually using a key-switch or a hidden
switch; but it can also be wired to set itself automatically when you
turn-off the ignition. By adding external relays you can immobilize the
bike – flash the lights etc.
Any number of normally-open switches may be used. Fit “tilt” switches
that 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.
Once activated – the rate at which the siren switches on and off is
controlled by R9 & C5. For example – increasing the value of C5 will
slow it down – while reducing the value of R9 will make it faster.
While at least one switch remains closed the siren will sound. About
thirty seconds after all of the switches have been opened, the alarm
will reset. How long it takes to switch off depends on the
characteristics of the actual components used. You can adjust the time
to suit your requirements by changing the value of R6 and/or C4.
The circuit is designed to use an electronic Siren drawing 300 to
400mA. It’s not usually a good idea to use the bike’s own Horn because
it can be easily located and disconnected. However, if you choose to use
the Horn, remember that the alarm relay is too small to carry the
necessary current. Connect the coil of a suitably rated relay to the
“Siren” output. This can then be used to sound the Horn – flash the
The circuit board and switches must be protected from the elements.
Dampness or condensation will cause malfunction. Connect the 1-amp
in-line fuse AS CLOSE AS POSSIBLE to your power source. This is VERY
IMPORTANT. The fuse is there to protect the wiring – not the alarm.
Exactly how the system is fitted will depend on the make of your
particular machine – so I’m unable to provide any further help or advice
in this regard.
The quiescent (standby) current of the circuit is virtually zero – so
there is no drain on the battery. If you want to operate the alarm
manually use a key-switch or a hidden switch connected to the “off/set”
terminals. For automatic operation connect a wire from the ignition
circuit to the “ignit” terminal. Then every time you turn-off the
ignition – the alarm will set itself. Remember that this wire from the
ignition switch is not protected by your 1-amp inline fuse. So unless
its run is very short – fit the wire with its own 1-amp fuse as close as
possible to its source.
When you set the alarm – if one of the switches is closed – the siren
will sound. This could cause annoyance late at night. A small
modification will allow you to Monitor The State Of The Switches using
LEDs. When the LEDs are all off – the switches are all open – and it’s
safe to turn the alarm on.