A simple water-level indicator circuit walkthrough — components, circuit connections, working principle, wiring steps, and an interactive water level indicator you can explore in the browser.
R = 100Ω per stage · supply 5–9V · drag the level control to raise the water
When rising water bridges a probe to LB, current flows through that probe's resistor into its transistor's base. The transistor switches on and pulls the shared collector bus low, completing the path from the 5–9V battery through that stage's LED (or the buzzer) to ground. Drag the level control above to watch each stage light up live.