A Simple Transistor Based Motorcycle Alarm

A Simple Transistor Based Motorcycle Alarm

Circuit Description

Photograph Of The Prototype

This is a simple – easy to build – transistor based motorcycle alarm.
It’s designed to work at 12-volts. But – if you change the relay for
one with a 6-volt coil – it’ll protect your “Classic Bike”. The standby
current is virtually zero – so it won’t drain your battery.

Schematic Diagram



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 one minute after all of the switches have been opened again –
the alarm will reset. How long it takes to switch off depends on the
characteristics of the actual parts you’ve used. You can adjust the time
to suit your requirements by changing the value of C1 and/or R3.

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 – and use its contacts to sound the horn.

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
as close as possible to the power source. This is Very Important. The
fuse is there to protect the wiring – not the circuit board. 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’s 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 be necessary.

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.

Veroboard Layout

Veroboard Layout

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