Connect a 5 V DC power supply to the breadboard.

• Positive (+) → positive rail
• Negative (–) → negative rail

Verify the voltage and polarity before continuing.

Place four 2N2222 NPN transistors on the breadboard.

For each transistor:

• Emitter → negative (–) rail
• Base → corresponding sensor probe
• Collector → LED/resistor circuit

Note: Check the manufacturer's datasheet for the correct transistor pinout.

For each stage:

1. Connect a 330 Ω resistor to the transistor collector.
2. Connect the resistor to the LED cathode (–).
3. Connect the LED anode (+) to the positive rail.

Connect a wire from the positive rail and place its stripped end near the bottom of the container.

This is the common positive probe used for liquid detection.

Connect one sensor wire to the Base of each transistor.

Place the probes at different heights:

• Stage 1 → Lowest
• Stage 2 → Higher
• Stage 3 → Higher
• Stage 4 → Near the top

Before powering the circuit, verify:

• Correct transistor pinout
• Correct LED polarity
• 330 Ω resistors are installed
• Sensor probes do not touch each other
• Power polarity is correct

Apply power and slowly add water to the container.

As the water rises, it connects the common positive probe to each level probe, activating the corresponding transistor and LED.

Red → Yellow → Blue → Green

Move the sensor probes up or down to change the detection thresholds for your container.

The lower the probe, the earlier its LED will activate.