Schematic Reference

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 role.

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.


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.