Water Tank Full And Low Alarm Project | Using NPN Transistors

This mini electronics project is designed to educate students on creating a water tank level alarm system using commonly available NPN transistors and float switches. Through this hands-on project, readers will gain practical experience in assembling and understanding the circuitry that enables efficient monitoring of water tank levels, providing alerts when the tank is full or low. This project is ideal for fostering foundational skills in electronics and circuit design.

Circuit Diagram

Parts Required

Resistors:

• R1,R2 – 1K
• R6,R7 – 1K
• R3,R4 – 10K

Transistor:

Q1, Q2 – BC547

Buzzer:

12V continuous or intermittent buzzer

Float Switch:

• Normally Closed (NC)
• Normally Open (NO)

LED:

D1,D2 (Any Color)

Power Supply:

12V SMPS or 9v Stepdown Transformer

Parts for Power supply if Transformer is used:

• BR – W04
• R5 – 1K (1/4w)
• C1 – 470uF, 16V
• D1 – LED

Circuit Working

Transistors Q1 and Q2 are configured to function as an OR logic gate in this water tank level monitoring system. A normally closed (NC) float switch serves as the low tank indicator, while a normally open (NO) float switch acts as the full tank indicator. When the water level is above the low threshold, the NC float switch opens, preventing the base of transistor Q1 from receiving a positive voltage, thus keeping it in the OFF state. As the tank reaches full capacity, the NO float switch closes, delivering a positive voltage to the base of transistor Q2. This activates Q2, which in turn triggers the buzzer and illuminates LED D2.

Conversely, if the water level drops below the preset low threshold, the NC float switch, which was previously open, closes. This action supplies a positive voltage to the base of transistor Q1, turning it ON. Consequently, the buzzer sounds, and LED D1 lights up. This configuration ensures that an alarm is triggered whenever the water level is either full or low.

In the above circuit design, users are required to manually turn off the device whenever the alarm is triggered, and then turn it back on later to continue monitoring the tank level. This process is cumbersome and represents a significant drawback, making the circuit suitable only for project purposes and not for commercial use. However, by incorporating a momentary switch to silence the alarm without the need to power down the entire device, we can overcome this drawback. Here is a circuit to overcome this drawback and it can be used commercially as well.

Working Video