Water Level Indicator with BC547 Transistors, LEDs and Full-Tank Buzzer
Overhead tanks overflow every day in homes where someone forgets to switch off the motor. A water level indicator shows how full the tank is and warns you when it reaches the top. This project uses four BC547 transistors, four LEDs and an active buzzer. Wires dipped into the tank at 25, 50, 75 and 100 percent height act as sensors, and the water itself completes each transistor's base circuit.
The circuit runs from a 9 V battery or a 9 to 12 V DC adapter and costs very little. More importantly, it teaches how a transistor acts as a current amplifier: a tiny base current through water controls a much larger collector current through an LED.
Components Required
| Part | Value / Type | Quantity |
|---|---|---|
| NPN transistor Q1 to Q4 | BC547 | 4 |
| Base resistors R1 to R4 | 1 kilo-ohm | 4 |
| Base pull-down resistors R5 to R8 | 470 kilo-ohm (optional, reduces false triggering) | 4 |
| LED current resistors R9 to R12 | 1 kilo-ohm | 4 |
| LEDs | Red, yellow, yellow, green 5 mm | 4 |
| Active buzzer | 9 to 12 V DC type with built-in oscillator | 1 |
| Probes | Stainless steel screws or rods | 5 |
| Probe wire | Multi-core cable, 5 or 6 cores | As needed |
| Power | 9 V battery with clip or 9 to 12 V DC adapter | 1 |
| Switch | SPST slide switch | 1 |
Circuit Explained Stage by Stage
Stage 1: Probes in the tank
Five probes go into the tank. The common probe C reaches the very bottom and is connected to +V. Probes A, B, C2 and D are placed at 25, 50, 75 and 100 percent height. When water touches a level probe, current flows from +V through the water to that probe. Tap water is a weak conductor, so this path may measure anywhere from a few kilo-ohm to over 100 kilo-ohm depending on water quality and probe spacing.
Stage 2: Transistor switches
Each level probe connects through a 1 kilo-ohm base resistor to the base of one BC547. The 1 kilo-ohm resistor only protects the base if the probe wires are accidentally shorted to +V; the water resistance itself limits normal base current. With 9 V and a 100 kilo-ohm water path the base current is roughly (9 - 0.7) / 100000, about 80 microamps. A BC547 with a current gain of 150 or more can then pass over 10 mA, enough for an LED.
The optional 470 kilo-ohm resistor from each base to ground keeps the transistor firmly off when the probe is dry but damp with condensation, which otherwise can give a faint glow.
Stage 3: Indicators and buzzer
Each collector drives an LED through a 1 kilo-ohm resistor from +V. With 9 V and a red LED dropping about 2 V, the current is about 7 mA, bright enough for an indicator. The transistor on the 100 percent probe drives its LED and the active buzzer in parallel, so the buzzer sounds only when the tank is full.
Connections
| Probe / point | Connects to | Indicator |
|---|---|---|
| Common probe (bottom) | +V | - |
| Probe A (25 percent) | R1 to Q1 base | Red LED on Q1 collector |
| Probe B (50 percent) | R2 to Q2 base | Yellow LED on Q2 collector |
| Probe C2 (75 percent) | R3 to Q3 base | Yellow LED on Q3 collector |
| Probe D (100 percent) | R4 to Q4 base | Green LED and buzzer on Q4 collector |
| All emitters | GND | - |
One channel (repeat four times):
+V --R9 1k--|>|-- LED --+
|
Q1 collector
Probe A --R1 1k--+-- Q1 base
|
R5 470k
|
GND -------------+------ Q1 emitter
Full channel: buzzer (+) to +V, buzzer (-) to Q4 collectorHow It Works
As the tank fills, water first reaches probe A. Current flows from the common probe through the water into Q1's base, Q1 turns on and the red LED lights. As the level rises, B, C2 and D are reached in turn and the yellow, yellow and green LEDs join in. At the 100 percent probe Q4 also powers the buzzer, telling you to switch off the motor. As water is used, the LEDs go out from the top down.
Assembly and Testing
- Confirm the BC547 pinout from the datasheet of your parts before soldering.
- Build one channel on a breadboard. Touch the probe wire and the +V wire together through your wet fingers or a glass of water. The LED should light.
- Build the remaining three channels and connect the buzzer to the 100 percent channel, observing its polarity.
- Test with a plastic bottle or bucket: fix the probes on a plastic strip at the correct heights and fill slowly. Each LED should turn on in order.
- Transfer to veroboard. Keep the LEDs and buzzer in a box near the motor switch and run multi-core cable to the tank.
- Mount the probes on a PVC pipe or plastic strip so they cannot touch each other, and seal all joints above the water line.
Troubleshooting
- LED does not light even when the probe is under water: the water path may be too resistive. Move probes closer to the common probe, clean the probe tips, or replace that BC547 with a Darlington such as BC517.
- LEDs glow faintly when dry: moisture or a dirty probe holder is leaking current. Fit the 470 kilo-ohm pull-down resistors and keep cable joints dry.
- Wrong LED order: check which cable core goes to which probe with a continuity tester.
- Buzzer silent but green LED on: the buzzer is reversed or is a passive type that needs an oscillating signal. Use an active buzzer.
- Readings fade after weeks: probes are coated by electrolysis. Clean them, use stainless steel, and consider AC probe drive.
Summary
Four identical transistor switches turn tank water into a visual level display. The water carries a small base current, each BC547 amplifies it to light an LED, and the top channel also sounds a buzzer. Once this works, you can add a relay to stop the pump automatically at full level, or move to AC probe excitation for long probe life.