24/06/2026
Water level indicator.
This is a transistor-switching circuit that provides a visual and audible warning of rising water levels in a tank. It uses the electrical conductivity of water to trigger NPN transistors, lighting up LEDs sequentially as the water climbs.
Power Supply Section
Β· 9V Battery & ON/OFF Switch: The battery provides the raw power. The ON/OFF switch is placed between the batteryβs positive terminal and the input of the regulator, allowing you to completely cut off power when the circuit is idle to save battery life.
Β· 5V Regulator (e.g., 7805): This IC steps down the unstable 9V from the battery to a steady, regulated 5V DC. This stable voltage is crucialβit protects the BC547 transistors from over-voltage, ensures consistent LED brightness, and provides a safe reference voltage for the sensing probes.
Sensing Probes & Mechanism
Four conductive probes (wires) are placed vertically inside the tank:
Β· Common Probe (VCC): Placed at the bottom, connected to the regulated +5V supply.
Β· Level 1 (Low), Level 2 (Medium), Level 3 (High/Full): Placed at increasing heights. Each is connected to the base of a separate BC547 transistor (via a small base resistor to limit current). When water covers a probe, it bridges the gap between the Common probe and that specific sensor. Because water contains dissolved ions, it completes the circuit, sending a small positive voltage (around 0.7V) to the transistorβs base.
Transistor Switching (BC547)
You use three BC547 NPN transistors, one for each LED. Their emitters are all connected to the ground (GND). When the base receives voltage from the water, the transistor turns "ON" and acts as a closed switch, allowing current to flow from its collector to the emitter.
LED Indicators (3 LEDs)
Each LED is connected to the collector of its respective transistor.
Β· The first LED (e.g., Green) lights up when water touches Probe 1 (Low level).
Β· The second LED (e.g., Yellow) lights up when water touches Probe 2 (Medium level).
Β· The third LED (e.g., Red) lights up when water touches Probe 3 (High level).
Role of the 330-ohm Resistors
You will need three 330-ohm resistors. One is placed in series with each LED between the +5V supply and the transistor's collector. These resistors limit the current flowing through the LEDs to a safe value (roughly (5V - 2V) / 330Ξ© β 9 mA), preventing the LEDs from burning out due to excessive current.
Buzzer Activation
The buzzer is wired in parallel with the third LED (High level) at the collector of the third transistor. When the water reaches the topmost probe, Q3 turns on. This not only lights the Red LED but also completes the ground path for the buzzer, triggering a loud audible alarm to alert you that the tank is full and it is time to turn off the pump.
How It Works (Step-by-Step)
1. Turn the ON/OFF switch ON. The regulator outputs 5V.
2. With no water touching the probes, all transistors are OFF; all LEDs and the buzzer remain silent.
3. As water rises and touches Probe 1, Q1 turns ON β Green LED glows.
4. Water rises to Probe 2, Q2 turns ON β Yellow LED glows (Green stays on).
5. Water reaches Probe 3, Q3 turns ON β Red LED glows and the Buzzer sounds, giving a clear "tank full" alert.
