Low Cost Burglar Alarm For Boats

This low-cost burglar
alarm employs a 12V strobe light and a truck reversing horn as the
visible and audible alarm outputs while the alarm itself is a 12V horn
relay and some pressure mat switches. This simple approach has the
benefit that the alarm continues to operate even if the entry point is
immediately closed and it draws no current while in the armed condition.
To make it independent of the boat supply it runs from a single 12V or
two 6V alkaline lantern batteries connected in series. These should last
well in excess of two years. An advantage of a lantern battery is that
it will last less than an hour while powering the alarm in its active

This means the alarm system will not seriously contravene noise
pollution laws. If there are very strict noise regulations in your area,
you can fit an alarm timer, available from some electronic shops, in
the circuit between the battery positive and the key switch. The key
switch can be installed in the cockpit bulkhead and connects to two
normally open (NO) switches wired in parallel. One switch is a pressure
mat placed on the cockpit floor near the entry, underneath a suitable
piece of carpet or pliable cover. If a pressure mat is unsuitable, the
main entry can be protected by a mechanical switch such as an automobile
boot lid or door courtesy light switch.

Circuit diagram:

Low Cost Burglar Alarm Circuit

Low Cost Burglar Alarm Circuit Diagram

The second switch can be used to protect against entry through a
forward hatch or second wheelhouse door. Any number of normally open
(NO) switches can be installed in this system, all wired in parallel. If
the alarm is tripped, the relay closes and latches on due to the wiring
of its contacts and the horn and strobe light are powered. The
suggested truck reversing horn is the Hella 6023 intermittent reversing
buzzer which has an 85dB sound level and a current drain of 0.9A at 12V.
The suggested strobe is a Hella 1657 which has an amber lens in a
weatherproof housing. This strobe flashes about once per second and the
current drain at 12V is 0.25A. The strobe can be installed outside on
deck or in the main cabin where its flash will be seen through most
ports and windows. Wiring to both the warning horn and strobe light
should be concealed.

Author: Dave Jeanes – Copyright: Silicon Chip Electronics

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