This invention relates to a level control switch, and in particular to such a switch for use with a pump to control the level of bilge-water in a boat.

Present level control switches for bilge pumps suffer from damage caused by their harsh operating environment and also from false readings caused by the rolling motion of a boat under the influence of wave action. ^' Typically, such switches are of a mechanical or electrical nature. Environmental damage to a switch may, for example, take the form of fouling by bilge refuse such as oil or debris or of condensation obscuring optical sensors. For electrical switches, particularly those using single-pole switching, damage may also take the form of electrolysis, that is, the deposition of contaminants by electrolytic action, which may adversely affect the operation of the electrical circuits and devices contained within such switches. Furthermore, present switches have not been able to operate in depths of bilge-water greater than 10cm and frequently cannot continue to operate as the bilge-water level approaches the bilge pump inlet.

In addition, electrical switches often draw current even when simply monitoring the water level.

The present invention provides a switching circuit for connection to a first and a second liquid level sensor each having a first and second output state, characterised in that it comprises an electromagnetic switching device coupled to a semiconductor latch such that the switching device is caused by the latch to remain switched in a first state until each sensor is in a predetermined respective first output state and such that the switching device is not permitted by the latch to revert to the first state until each sensor is in the second output state.

Preferably, the electromagnetic switching device is a relay. The semiconductor latch may, for example, be a silicon controlled rectifier (SCR) or a triac.

Preferably, the gate of the SCR or triac is connected to an output of one of the sensors and the cathode or anode of the SCR or triac is connected to an output of the other sensor. Preferably, the switching device is arranged to be switched by current flowing through the anode and cathode and is arranged to interrupt a current flowing through the gate connection when switching from one state to another. In this way, the switching device may conveniently be used to generate a pulse for application to the gate terminal in order to trigger the SCR or triac into a conducting state.

Preferably, the first and second sensors are arranged in use such that they sense respective different levels of the same liquid and at least the first sensor comprises a coupling member which defines an air space having an opening disposable for submersion in a liquid and which is coupled to pressure-sensitive switching means comprising a first pressure-sensitive switch such that in use the pressure of air in the air space is caused to vary according to the depth of submersion of the opening, thereby causing the first pressure sensitive switch to operate.

This arrangement permits the environmentally sensitive components of the switch, for example the pressure-sensitive switching means, to be isolated from the bilge. The coupling member may be a length of tubing, preferably with an enlarged opening or a filter to prevent blockage by debris and also to provide an enlarged surface area for the surface of the liquid to act on the air in the air chamber.

In a preferred embodiment, the second sensor comprises a second pressure-sensitive switch arranged within the pressure-sensitive switching means and operable to switch at

a different pressure from the first pressure-sensitive switch.

Alternatively, the second sensor may comprise a second coupling member and second pressure-sensitive switching means. By providing two separate coupling members, the distance between the two respective openings may be relatively large, thereby allowing the switching operations of the two sensors to occur at relatively disparate liquid levels.

By arranging for the switching device to switch between the two states only when the outputs of both sensors indicate the same condition, namely, that the level is above both of them or below both of them, the sensitivity of the arrangement to wave movement is reduced. The circuit may be used to switch a bilge pump on and off by connecting its power supply through the switching device. Switching on will only occur when the bilge-water has reached the level of the higher sensor and switching off will only occur when the bilge-water has gone below the level of the lower sensor. This arrangement allows the bilge-water to be kept below a predetermined level. Alternatively, the switching circuit may be used to pump a liquid into a vessel in order to maintain a particular liquid level.

The switching circuit may also include a third sensor for detecting when a liquid level has reached an alarm level. When the alarm level has been reached, a warning signal may be generated to indicate a fault or, alternatively, the third sensor may be used to operate a second pump to aid the first pump if the bilge-water is rising too quickly or to replace a faulty first pump.

In a preferred embodiment, the SCR is triggered by a current flowing between two conductors positioned in the bilge of a

boat such that conduction occurs through bilge—water when the conductors are immersed.

This circuit may be used with any combination of float, pressure and/or conduction sensors.

The invention will now be described by way of example with reference to the drawings in which:

Figure 1 shows a pressure switch in accordance with the invention; Figure 2 is a schematic circuit diagram of a first embodiment of a switching circuit in accordance with the invention; Figure 3 is a schematic circuit diagram of a second embodiment of a switching circuit in accordance with the invention, and Figure 4 is a schematic diagram of a variant of the embodiment of Figure 2 or Figure 3.

With reference to Figure 1 , a switching circuit comprises first and second pressure-sensitive switches 2, 4 each mounted in a respective gas-tight, switching chamber 6, 8. The switching chambers 6, 8 may be formed in a single housing which preferably also contains an electronic switching circuit coupled to the switches 2, 4. Each chamber 6, 8 is connected to a respective sensing tube 10, 12. At the other end of each tube 10, 12 is an air chamber 14, 16 open to the exterior.

The switching chambers 6, 8 may be mounted in a wheelhouse or cabin of a boat, for example, whilst the air chambers 14, 16 are mounted at different levels in the bilge of the boat. As the level of water in the bilge rises, each respective air chamber 14, 16 is successively submerged and air is trapped within. As the water level rises further, the air pressure within each air chamber rises and consequently the

pressure in each switching chamber also rises. Since the air chambers 14, 16 are mounted at different levels, the pressure in each switching chamber 6, 8 is different for a given level of bilge-water. This can be used to provided a hysteresis effect to prevent a pump from being repeatedly switched on and off according to small fluctuations in the bilge-water level.

In the preferred embodiment, and with reference to Figures 2 and 3, a single air chamber 114, switching chamber 116 and sensing tube 110 are used and the two switches 2, 4 are housed in the same switching chamber but are arranged to switch at different pressures. This has the same effect to that described above but is not suitable for use where a large difference in levels is anticipated; in such a case, the embodiment with two air chambers 6, 8 would be required.

The or each air chamber is preferably located as close as possible to the inlet of the pump (not shown) so that the sweeping action of the pump helps to keep the chamber mouth free from debris.

With particular reference to Figure 2, a switching circuit in accordance with the invention comprises a double-pole, double-throw relay 18 (shown within dotted lines) and an SCR 2 _. 0. Connections are provided for a pair of pressure switches 2, 4 of the type described above, for circuit power and for a load such as a pump motor.

As the level of bilge-water rises, the pressure in chamber 116 increases. The switch 2 is arranged to switch at a lower pressure than the switch 4 and, when the switch 2 switches, it completes a circuit supplying power to the relay coil 18A. However, since the SCR 20 is connected in series in this circuit and is not at this point in a conducting state, no current flows and the coil 18A is not energised.

As the water level rises further, the pressure further increases in the chamber 116. Eventually the switch 4 operates. When this happens, power is supplied via a resistance 22 to the gate of the SCR 20, triggering it into a conducting state. With the SCR 20 in a conducting state, the relay coil 18A is energised and the pump is switched on. At the same time, the relay 18 is arranged to interrupt the power supply to the SCR gate to ensure a stable conducting state. This last feature is particularly advantageous in that the trigger pulse required by the SCR 20 is generated mechanically, thereby obviating any requirement for further electronic components to produce a pulse.

Using a relay rather than the SCR 20 to switch power to the pump permits the full voltage available from the power supply terminals to be developed across the pump motor and hence ensures maximum performance of the motor.

The two-stage triggering and latching characteristic of the circuit minimises the effect of wave action on the switches 2, 4.

As the pump succeeds in reducing the water level, the pressure in the chamber 116 eventually decreases. to a level at which the switch 2 opens. When this happens, current no longer flows through the SCR 20 or the relay coil 18A. The relay then switches the pump off and reverts to the position shown in Figure 2. Since current is no longer, flowing through the SCR 20, this resets to a non-conducting state.

This cycle repeats each time the water level rises and falls.

The circuit and chamber 116 may be housed in a waterproof box, typically having dimensions of 100mm x 100mm x 50mm.

This is connected via a small plastic tube to the chamber

114, which is a plastic container and is located in the bilge.

With reference to Figure 3, in order to permit the switching of voltages other than the supply voltage of the circuit or to switch alternating current, a three-pole relay 118 may be used. The additional pole is used to generate the pulse for the gate of the SCR 20 whilst the other two poles are used for isolated, two-pole switching of a supply to the load.

In a preferred embodiment, the pressure switch 4 may be replaced by two conductors such as a pair of metal strips (40mm x 4mm) placed about 2mm apart. When these conductors are immersed, they conduct sufficiently to trigger the SCR 20. In this embodiment, the resistance 22 is reduced in value or omitted.

One or both of the switches may be replaced by float switches.

By reversing the switches 2, 4 and reversing their sense, that is, by replacing normally open with normally closed, the circuit may be used to detect a fall in liquid level and to operate a pump to fill a tank.

With reference to Figure 4, by providing a third pressure switch 5, set to switch at a higher pressure than the switches 2 and 4, an alarm signal may be generated when the bilge water has reached a predetermined level higher than the normal switch-on or switch-off levels. This third switch may be used to operate a visual and/or audible alarm 24 and/or to switch on a reserve pump (not shown).