Home > Circuits > Bilge Pump Sensor
Modified:20:32, 22 October 2013
This year I bought a 20ft Bayliner which is great fun and moored up in a harbour, but water tends to build up in the engine bay when left in the water for weeks at a time.

It has a bilge pump but it is manual and the cost of automatic ones is around £100! So a simple timer activated by a float switch is a cheap and easy project - it even switches itself off afterwards, drawing no standby current at all.

The circuit is based around a 555 monostable which is triggered by a float switch. When the ignition is off, power is applied to the float switch and to Relay 1 ready for it to activate.

When the float switch is triggered, the driver activates Relay 1 which supplies power to the bilge pump and the monostable section of the circuit. The trigger is a power-on type which starts the timing immediately.

When the time period ends, Relay 1 switches off, the bilge pump stops and the circuit switches itself off.


I wanted to control the bilge pump manually when the engine is running so Relay 2 is activated by the ignition switch of the boat which removes power from the entire circuit. This means that the circuit is fully automated and once fitted, you will not even notice it's there - apart from having a dry engine bay.


The monstable is formed by IC1. R1 and C1 trigger the monostable when the power is connected. D4 helps to stabalise the IC supply from the motor when running. VR1 alters the time duration from:

T = 1.1 x C x R
T = 1.1 x 0.00047 x ( 47000 + 0 ) = 24s (with VR1 at minimum)


T = 1.1 x 0.00047 x ( 47000 + 470000 ) = 268s (with VR1 at maximum)

Q1 drives RL1 while D1 reduces EMF from the relay coil. As long as the ignition is off and RL2 is off. The NC contact of RL2 will supply the power for RL1 and the sensor only. These draw no current when off and so no current is drawn at all until the sensor is activated - saving a flat battery if left for many weeks.

When the sensor is activated, RL1 is turned on by Q1. The NO contacts supply the power to the rest of the circuit, which immediately triggers the monostable sending pin 3 high and keeping Q1 turned on - thus a latching effect. When the time duration is over, Q1 switches off, turning off RL1 and the rest of the circuit.

Supply to the mortor is through D3. This is needed so that is the pump is activated manually, the circuit doesn't activate also and start to provide a timed ouotput. If this happened, when you switch the pump off, it would still be running if you activated it for less time than the timer did.

It is rated at 3A which is fine for smaller bilge pumps like mine which only draws about 1.3A under load. If this is used for a bigger pump, then either add another diode or 2 in parallel with D3, or use a DPDT relay for RL1 where the second set of contacts activate the pump only and the first set activate the circuit as before.

The PCB measures 2.5" x 1.45" (64mm x 37mm)

The underside of the PCB has the connections written on and all connections are via the 6way terminal block.

The float switch was mounted on a piece of 3mm acrylic which was bent at 90 degrees using a strip heater. Then holes drilled carefully for the switch and 2 screw mounts.

This was fixed as low in the engine bay as possible. It had to be mounted upside down (wires coming out the bottom) since the switch is meant to work when a tank is empty whereas I want it to work when the bay fills. It uses a magnet to activate a sealed reed switch which is ideal as sea water can be very corrosive.

OK an admission that I didn't make the PCB above as I didn't have enough etching materials so I resorted to stripboard, 2 in fact. One for the circuit and another to hold the relays. It fitted quite neatly in this little white ABS box I had. I haven't tested the PCB shown above so you will need to go over it first to check it - download below - but it should be correct.


The power came from the supply to the Engine Trim hydraulic pump with an inline fuse of 3A. This is on all the time and perfect for the AutoBilge sensor. The ignition connection was attached at the key switch terminal. The wire for the motor was simply spliced into the existing bilge live side as the circuit and the pump share the same 0v connection only one wire is needed.

The circuit boards and components were sprayed liberally with laquer and the box sealed with silicon sealant and were the wires protruding from the case. This should protect the circuitry from the harsh sea air - time will tell....


Please note: Take care when modifying any boat wiring. There is the potential for high currents to be generated if connections are incorrect, which could cause fire and injury. I take no responsibility for modifications made and you should always seek a second opinion if you are unsure.

Designed and Written by Phil Townshend - August 2011
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