Alarm Circuit for Water Level Display

Figure 1 

The circuit was intended for monitoring and measuring the fluid level or content in a storage tank or reservoir by providing visual display with an audible alarm that can be switched.

• Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it
• CMOS 4050B – non-inverting hex buffer where the inputs can withstand up to 15V even at lower voltage power supply while the outputs are capable of powering 74LS gate inputs directly with a 5V supply voltage
• Piezo Buzzer – a type of buzzer that can function as a speaker with piezoelectric disk attached where a mechanical oscillation of the piezoelectric diaphragm is damped properly to provide a wider frequency band of the sound pressure

From the diagram, the hex buffer CMOS 4050B is utilized for this circuit operated by a supply of 5 V. The buffer input and all the 10M ohm resistors connected to the ground are providing bias to all the gates while the topmost probe, or the common probe, above probe 1 is connected to the positive 5 V supply. The use of LEDs in the circuit is to indicate the level of the water by the activation of the gate. The top gate receives its activation when a tap water at 25º C is detected between the probes having a resistance of 20k ohms, thus lighting LED1. These probes are separated 1 cm apart as with probe 1 and common probe. Same thing goes with probe 2 as it is spaced 2 cm from the common probe. When water is detected at this level, LED2 will light and this goes with other LEDs. To choose which output level would trigger the Piezo buzzer or audible oscillator, switch SW1 is utilized which is made from CMOS 4011B IC.

During the placement of the probes, 7 or 8 wires may be used where the first 2 wires can be assigned as common probe by joining them together. The succeeding wires may be required to be cut on a specified length. The insulation may be stripped back for a couple of millimeters and acting as the probe would be the bare end of the wire. The distance and fluid between probe 6 and the common probe wire should be less than 900k ohms and any fluid under this resistance will trigger the circuit. This is due to CMOS IC giving the same value of logic 0 to any voltage below 0.5 V. to compute for the voltage using a resistance of 900k ohms while 10M ohm resistor is the divider at the input shows that: 5 X [0.9/(0.9+10)] = 0.41 Volt
The LED will light since the voltage is below 0.5 and is regarded as logic 0. The maximum distance between the top probe and the common probe should not exceed 45 cm when measuring tap water at 25º C. An ohmmeter could be used for measuring other types of fluids and temperatures.

The Piezo electric buzzers are used in switching sounds of home electric appliances like microwaves, washing machines, multiple sounds like in electronic games, alarm sounds of digital watches, calculators, card radios, speakers of crystal TV’s, key-in sounds of outdoor action equipment, pagers, smoke detectors, and telephone ringers, receivers and transmitters.

The water level alarm can also be termed as bath or sink water level alarm, water alert, overflow alarm, and leak detector, since they are mainly used in areas where detection of leaks and water spillage is necessary like kitchens, bathrooms, cellars or basements. It can also give benefits to nursing and care homes or care of the elderly at homes.

Source:www.zen22142.zen.co.uk/Circuits/Alarm/water_lev_alm.htm