Two stainless steel probes detect water contact, triggering a 555 astable oscillator that drives a piezo buzzer.
Use food-grade stainless screws as probes (galvanized rusts fast). Keep the timing resistor R1=10k, R2=10k, C=10nF for a ~7kHz buzz.
| Component | Qty | Spec | Cost | Source |
|---|---|---|---|---|
| NE555 timer IC | 1 | DIP-8 | $0.50 | — |
| Piezo buzzer | 1 | 5V passive | $1 | — |
| 10k resistor | 2 | 1/4W | $0.05 | — |
| 10nF ceramic cap | 1 | $0.05 | — | |
| Stainless steel probes | 2 | M3 screws | $0.50 | — |
| 9V battery + clip | 1 | $2 | — |
01
Wire 555 in astable mode
Pin 1 GND, pin 8 V+, pin 4 reset to V+, pin 7 to R1+R2, pin 6+2 to R2+C.
02
Wire probes to base of NPN
When water bridges the probes, the BC547 saturates and powers the 555.
03
Test in a glass of water
Probe contact -> buzzer screams. Mount on a 3D printed bracket near the rim.