It tells me how full the tank is
Three LEDs stacked like the tank they watch: each one lights when the water reaches its probe. Three transistors, no chip.
Free build sheet
The parts, every wire with its length and bend points, a bend guide, both sides of the board and checkboxes to track your progress. Print it or keep it open at the bench.
Open the build sheetWhat you need
14 parts on a perfboard 15 × 27 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 1 | Power Input (2-pin) J1 | |
| 1 | Screw Terminal 4P (5.08mm) J2 | |
| 3 | LEDred 5mm LED1, LED2, LED3 | Buy LED on Amazon |
| 3 | NPN Transistor (TO-92)2N3904 Q1, Q2, Q3 | Buy NPN Transistor (TO-92) on Amazon |
| 3 | Resistor (axial)4.7k R1, R3, R5 | Buy Resistor (axial) on Amazon |
| 3 | Resistor (axial)330 R2, R4, R6 | Buy Resistor (axial) on Amazon |
Check every pin against the datasheet of the exact part in your hands before wiring.
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Cut list
13 wires, 369 mm (14.5 in) in total, in the order the build sheet cuts them. Wire numbers match the build sheet. Each length includes 5 mm per end to go through the hole.
| Wire | Colour and net | Length | ||
|---|---|---|---|---|
| 12 | BlackGND I11 → I17 | I11 → I17 | 25mm | Straight |
| 6 | BlackPROBE_LOW N20 → I4 | N20 → I4 | 68mm | 8mm (90°), 48mm (90°) |
| 11 | BlueGND D5 → J10 | D5 → J10 | 38mm | 18mm (90°) |
| 5 | BluePROBE_MID N14 → K4 | N14 → K4 | 43mm | 30mm (90°) |
| 4 | GreenCOLL_MID E19 → I19 | E19 → I19 | 20mm | Straight |
| 10 | GreenVCC F14 → F20 | F14 → F20 | 25mm | Straight |
| 1 | RedPROBE_HI N8 → N5 | N8 → N5 | 18mm | Straight |
| 7 | RedVCC F8 → F5 | F8 → F5 | 18mm | Straight |
| 13 | RedGND K17 → K23 | K17 → K23 | 25mm | Straight |
| 3 | WhiteCOLL_LOW E25 → I25 | E25 → I25 | 20mm | Straight |
| 9 | WhiteVCC H8 → H14 | H8 → H14 | 25mm | Straight |
| 2 | YellowCOLL_HI E13 → I13 | E13 → I13 | 20mm | Straight |
| 8 | YellowVCC G4 → C3 | G4 → C3 | 23mm | 8mm (90°) |
About this build
A tank level indicator built from three identical transistor bands. Three bare wire probes hang at three heights in the tank, and a fourth probe at the bottom is the common, sitting at 5V.
Water conducts a little, so when it reaches a probe it pulls that band's 2N3904 base up through a 4.7k resistor, and the band's red LED lights through its 330 ohm resistor. The LEDs are stacked on the board low at the bottom and high at the top, so the board reads like the tank.
DC through bare probes corrodes them over time, so treat this as a demo, not a permanent install. The probes connect on a 4 pin screw terminal and 5V comes in on a 2 pin input.
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