Monday, September 12, 2011

Using Micro Switch for Mechanically Actuated Water Level Detection Float Switch

Another type of float switch for liquid level control and detection incorporates the use of the micro switch for its main electrical component. Unlike the mercury switch which is dependent on turning the position of the glass vial to cause the liquid mercury to fall by gravity to immerse its electrical contacts to provide a switch output, the micro switch is an electrical switching device fitted with both open and close contacts which is mechanically actuated to provide both open and close electrical switching output states via a pin button accessible on the upright side of its outer casing.

The drawing below illustrates how this typical float switch works. Similar to the principle of operation of the mercury switch liquid level float device, the micro switch liquid level float device also works in conjunction with the hanging and lifting of a counter weight acting on the movement of the lever mechanism to manipulate the switching of an electrical switch contact which is directly dependent on the condition of the water level acting on the buoyancy of a floating object.

The lever mechanism and the micro switch is housed inside a protective weatherproof plastic enclosure to provide protection from water splash for outdoor use. The lever is fixed on a center axis with a fulcrum to support its pivoting action. The plastic cylindrical tube floats due to the force exerted against its associated weight by means of the upward push of the rising water acting on the buoyancy of the plastic tube. The red pin button protrudes out of the micro switch casing due to the spring action provided by its internal switching mechanism.

mechanically actuated water level detection float switch with micro switch
Water Level Detection Float Switch fitted with Micro Switch

When the water level is below the plastic tube, the weight of the sand contained inside the plastic tube will pull down the lever which will eventually push the red pin button of the micro switch. In this condition, the internal switching mechanism of the micro switch is actuated resulting in changing the condition of the electrical contact points that provides a switching output, that is, the normally open contact will be placed in a closed state while the normally close contact will change to an open state.

When the rising water level reaches the plastic tube, it causes the tube to float, the tube slowly rises as the force of the rising water level carries the plastic tube upward, this relieves the weight off from the lever as the plastic tube is no longer hanging from the lever while it becomes lighter as it is being lifted upward by the rising water. This releases the lever form any weight burden leaving the red pin button untouched. In this condition, the internal switching mechanism of the micro switch is left at its normal state, retaining its internal electrical contacts in their normally open and normally close states.

To signal an action to the control circuit of a water pump, the contact points from the micro switch is directly connected to the start switch of the control circuit to provide appropriate action depending on the pump behavior required by the user. For automatic operation, to start the water pump when the water level is low, the normally open contact of the micro switch should be connected in parallel across the start push button switch of the water pump's control circuit. When the water level is low, the weight of the plastic tube will pull down the lever to push the red pin button of the micro switch which will change its open contact to a close contact to provide a complete circuit to the pump's control circuit, which runs the water pump. When the water level rises due to continuous pumping of water supplied to the water reservoir, the plastic tube floats as it is lifted by the rising water level, thereby releasing the lever from pushing the red pin button of the micro switch, hence returning the mechanically actuated close state contact back to its original state of being a normally open contact, placing the pump's control circuit in open state to stop the water pump.

Conversely, to stop the water pump during low water level in automatic mode of operation, for instances such as pumping water from a reservoir to fill a water tank, the normally close contact of the micro switch should be connected in parallel across the start push button switch of the pump's control circuit. This way, when the water level in the reservoir is high, the plastic tube floats which leaves the red pin button untouched by the lever, thereby causing the close contact of the micro switch maintained closed across the start button of the control circuit to continuously run the water pump to supply water from the reservoir to the tank. But when the water level in the reservoir falls down below a certain threshold, the weight of the plastic tube will eventually pull down the lever to push the red pin button of the micro switch which will open its close contact to render an incomplete circuit to the pump's control circuit, placing the pump in stop mode to discontinue the pumping of water from the reservoir to the tank.



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