There are a number of ways to provide bilge-water warning. One of the simplest is the float-actuated switch system shown in Fig. 10. Here, a sealed tube containing a reed switch is surrounded by a float with a built-in magnet. The float rides up and down the tube with increasing and decreasing water level, closing and opening the switch's contacts.
With the actuating switch assembly placed low in the bilge, the float lifts with rising water level. At some predetermined point, the contacts of the reed switch close and the alarm sounds on and an indicator light comes on. Alternatively, the system can be rigged to automatically turn on the bilge pump as well as sound an alert.
There is nothing electronic about the system shown in Fig. 10, but it is so simple that it is just about foolproof.
While you can fabricate your own float switch if you wish, it is hardly worth the effort be cause all-plastic units for boats are available from marine hardware stores at low cost.
A second bilge alarm is shown in Fig. 11. Here, a pair of electrodes is sealed in an insulating housing that is mounted low in the bilge. A small screen surrounds the probe-like elements to prevent bridging by debris.
In fabricating the probe shown in Fig. 11, two small brass bolts are mounted on a small disc of insulating board and are connected through a pair of resistors to a water-tight cable that goes to the instrument panel. The disc fits one end of a (19.1-mm) plastic plumbing fitting. Then the whole rear of the assembly is. filled with epoxy to seal in the probe ends, resistors, and cable connections. When potting is finished, there should be no place, except at the probe tips, where moisture can bridge the circuit.
When water bridges the probe tips, the SCR fires and actuates the alarm. The Sonalert alarm will continue to sound, even after the water level drops below the point where it bridges the probe tips, until the switch is opened. To rearm the alarm, simply close the switch.
A third type of bilge alarm is illustrated in Fig. 12. This system is designed for boats with multiple bilge spaces that are separated by watertight bulkheads. An audible, alarm and a visual indicator to tell you which bilge has water in it are required in this system.
The sensors in this circuit are LM1 830 fluid-detector ICs. When water bridges the probes, the output of the associated IC goes high and turns on the pair of transistors connected to it. Output connections to the transistor switches are arranged so that water entering any bilge space and bridging its probes will activate the Sonalert but will light only the LED labelled for that bilge. You can duplicate the circuit for each bilge to be protected. The only thing in common among the circuits is the Sonalert.
Shown in Fig. 12 is a method for marking the safety panel area where the LEDs are mounted. Using the layout shown, you know instantly which of the bilges is leaking (by its lighted LED) when the alarm sounds. The transistors can be replaced by a DIP transistor array, provided the outputs can sink enough current to drive the Sonalert. You can use a high-power alarm sounder by replacing the Sonalert with a relay whose contacts can handle the bigger alarm's current. If you use this arrangement, be sure to install a protective diode across the relay's coil.
Copyright by Bill Bytheway, K7TTY February 2012