Rain alarm: two bare wires, one transistor and a buzzer
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Know when the rain starts, before the laundry gets wet. The sensor is just two bare wires (or two interleaved copper tracks on a scrap of board) a few millimetres apart. Dry, nothing flows. A drop of rain bridges them with something like 20 kΩ, enough to push a fraction of a milliamp into the base of a BC547, which turns on and sounds the buzzer and the LED. R1 protects the base, and R2 keeps the transistor off when the probe is dry. On the bench the probe is shown wet (a 20 k resistor): click it and raise it to 10 M to see the dry state. A 9 V battery lasts months, because the circuit draws nothing until it rains. Tin the probe wires or use stainless screws, or they corrode in a few weeks.
What is on the bench
15 parts, 18 wires.
| Resistor | 4 |
|---|---|
| 9 V battery | 1 |
| NPN transistor | 1 |
| Active buzzer | 1 |
| LED | 1 |
Built for real
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Comments
⚠️ Mains wiring, big batteries and solar arrays can hurt you. Comments are other makers’ experience, not checked by the lab: check advice against the datasheets and your local electrical code, and report advice that looks dangerous. Be kind: community rules.
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Put it on your page
The project runs live inside the frame: readers can press buttons, turn knobs and read the meters.
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Link
https://zencircuitlab.com/p/zclrainalarm
Published by its author under the lab's community terms: it may be opened, copied and changed by anyone. Simulation results are for learning; check a real build against the parts' datasheets.