This is a liquid level sensor circuit that sounds off an alarm when a liquid has reached a certain level. It can be easily constructed using a single IC L4620 and few external components and has an oscillating freq. of 1.6 kHz.
This signal is divided into 32 and the resulting 50 Hz clock signal is fed to the sensor through pin 2. The sensor is a simple moisture dependent resistance. It is usually made of 2 wires in the liquid which either shorts the pin 3 to the ground or not.
The sensor interface receives a 200 Hz signal and compares the voltage at pin 3 with a reference voltage which is dependent on pin 2. The reference voltage is within 0.2 and voltage of pin 2 when pin 2 is low. It is within 0.4 volts and voltage of pin 2 when pin 2 is high. Once these thresholds are exceeded, the sensor interface passes the information to the rest of the circuit.
The liquid level sensor circuit will sound an alarm when the threshold is exceeded, pin 8 must be high. On the other hand when pin 8 is low, the alarm output is active. The alarm will not trigger immediately but only when the alarm situation (liquid level is reached) remains constant at the sensors for 10 seconds (pin 7 is low) or for 20 seconds (pin 7 is high).
This liquid level alarm project can consume upt to 300 mA during an alarm but the current is dependent on the voltage supply. At 5 volts supply, the current in around 6 mA.