The figure shows a simple rain sensor design that incorporates pitch and volume controls in the alarm signal. Whenever the sensor is bridged by droplets of water, the Darlington transistor TR1/TR2 will conduct. This enables IC1, a 555 astable tone generator, to function, powering a small loud-speaker through a driver transistor (TR3). VR2 determines the pitch of the audio tone, which can be anything from about 25 Hz to 18 kHz, while VR3 adjusts the volume. The sensitivity of the circuit is set by VR1. The Darlington pair could be constructed with two separate ZTX300s or a single TlP122. For the sensor, use a small piece of stripboard, linking alternate strips into an interlocking design. The circuit will operate from a standard 9-V PP3-type battery.
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