the circuit uses a 555 timer wired as an astable oscillat powered by the emitter current of the bc109c. under dry conditions, the transist will have no bias current be fully off. however as the probes get wet the transist will conduct sounding the alarm.an on/off switch is provided remember to use a non-reactive metal f the probe contacts. gold silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of veroboard. these will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not imptant. no base resist is necessary as the transist is in emitter follower, current limit being the impedance at the emitter (the oscillat circuit).
