A $6.47 Gadget That Could Save Your Boat

From automatic bilge pump activation to dry on PROMISE is about 30 seconds. It's safe to assume that any longer activation is caused by a large amount of water taken aboard, a leak that exceeds the ability of the pump to keep up, or a failed pump. Any is cause for concern.

This is an addition to, not a replacement for, a high bilge water alarm. Examples of this are a Rule float switch connected to an alarm mounted much higher than the "working" bilge pump switch or a solid state equilvalent.

I designed a circuit that will sound a loud alarm if the pump is continuously activated for more than approx 60 seconds. The alarm resets each time the pump is off. There is only a simple 2-wire connection to the pump side of the automatic switch. There is no power consumption when the pump is not activated. The schematic diagram is shown below. The parts list with prices and Radio Shack part numbers is here:

Theory of Operation

When power is applied, the capacitor momentarily clamps TRIG to ground thereby setting the internal bistable in the 555 to OUT high. This keeps the alarm silent. At the same time, the capacitor starts charging through the resistor toward Vcc. When the voltage reaches 2/3 Vcc the bistable resets and OUT goes low turning ON the alarm. The time delay from initial power to alarm is 1.1 RC or a bit over 50 sec for the components shown. OUT stays low and the alarm stays on until power is removed. If power is removed before the bistable resets, there is no alarm.


The 555 is a 8 pin DIP. I used a 14 pin DIP socket from my junk box and put the 555 in the upper part and the resistor and the capacitor in the left over pins. I soldered the external and internal connections to the back. I intend to pot the whole mess except for the piezo and fuse.

A small PC board would be a good idea. Any volunteers to lay out and make a few for all of us? 

Here is a pix of the circuit:

Modify Timing

I set the 555 timing to be approx 2X the normal ON to EMPTY pump time. By choosing different R or C or both, you have a wide choice of times in the range of tens of secs to up to maybe 5 minutes. For various reasons, avoid C > 100mfd and R > 5 megohm. Here are a few choices:

Louder Alarm

The 273-0059 Radio Shack piezo alarm is loud and should work OK if it isn't installed in the bilge. A louder alarm would be better though. The RS web site has a good selection. The CMOS 555 can sink up to 100ma in the OUT low state so any alarm rated less than this should work. 273-075 is 20db louder(that's a lot) and costs a little more at $7.49. I haven't tested this alarm with my circuit but watch this space.
de W4AVO 3/16/2009