The water level indicator employs a simple mechanism to detect and indicate the water level in a tank or any other container. The level sensing is done by a pair of probes, the resistance between which depends upon the water level in the tank. These probes can be placed in two ways on the walls of the tank or container
1. Both the probes can be placed at different points on the wall lying at the same level/height from the bottom. This height determines the minimum detectable water level, or
2. The probes can be placed at different points lying at two different heights from the tank bottom. In this case, the probe which is placed higher than the other determines the minimum detectable water level.
When the water-level is below the minimum detectable level (MDL), there is infinite impedance between the two probes. Hence, transistor T2 remains in the cut-off mode and does not conduct. However when water-level reaches MDL or is above it, the connection between the probes gets completed (through the conducting medium o water) and the base voltage of T2 increases. This causes the base-emitter junction of T2 to get forward biased and the piezoelectric buzzer is turned on to generate an alarm sound. The path from the emitter of T2 through resistance R3 is responsible for forward biasing the base of transistor T1. This switches T1 from cut-off to conduction mode, thereby driving the red LED to glow. Hence at each instance when water reaches the MDL, both the buzzer and LED indicate this simultaneously.
The purpose of the water level indicator is to caution a person that a water tank has been filled up to a particular level. This circuit exploits the simple fact that water is a good conductor of electricity. The system is capable of indicating the water level in two ways- visual (by the means of an LED) and audible (by the means of a sound generated by a piezoelectric buzzer). The placement of two probes (with connections from the circuit) inside the water tank determines the minimum detectable level (MDL). When the water reaches this height in the tank (or any other fluid container), the buzzer is activated and the LED is turned on. Number of ICs used: None.