Water-level indicator
LEDs show tank level through probe wires. Water conducts a little. The common probe at the bottom of the tank is at +5 V; when the water reaches a probe, a tiny current flows through it into a transistor’s base, which switches on that level’s LED. Three probes, three transistors, three LEDs.
Build it in the labGuided build, test sheet and fault finding, free in your browser.
Parts list
- Bench power supply
- Water tank with probes
- 9 × Resistor
- 3 × NPN transistor
- 3 × LED
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Empty
- 60 %
- Full
How it works
What it does: LEDs show tank level through probe wires.
Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.
Background
On rooftop tanks and cisterns everywhere; add a relay and it becomes a pump controller.