This water activated alarm circuit uses a NE555 timer wired as an astable oscillator and powered by the emitter current of transistor BC109C. In dry conditions, the transistor having no bias current and be completely off. As the probe gets wet, a small current flowing between the base and the emitter and the transistor turns on. A higher current flow in the collector circuit allows the IC NE555 osillator sound.
Probe / contacts may use a nonreactive metal. Contacts gold plated or silver age relay can be used, however, a cheaper alternative is the thread of alternate strips of copper a piece of veroboard. These will eventually oxidize over but as very little current flowing in the base circuit, the higher the impedance caused by oxidation is not important. No base resistor is necessary as the transistor is in emitter follower, current limit is the impedance at the emitter (the oscillator circuit)
Probe / contacts may use a nonreactive metal. Contacts gold plated or silver age relay can be used, however, a cheaper alternative is the thread of alternate strips of copper a piece of veroboard. These will eventually oxidize over but as very little current flowing in the base circuit, the higher the impedance caused by oxidation is not important. No base resistor is necessary as the transistor is in emitter follower, current limit is the impedance at the emitter (the oscillator circuit)
