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Circuits
The circuit uses a 555 timer wired as an
astable oscillator and powered by the emitter current of
the BC109C. Under dry conditions, the transistor will
have no bias current and be fully off. As the probes get
wet, a small current flows between base and emitter and
the transistor switches on. A larger current flows in
the collector circuit enabling the 555 oscillator to
sound.
An On/Off switch is provided and
remember to use a non-reactive metal for the probe
contacts. Gold or silver plated contacts from an old
relay may be used, however a cheap alternative is to
wire alternate copper strips from a piece of Vero board.
These will eventually oxidize over but as very little
current is flowing in the base circuit, the higher
impedance caused by oxidization is not important. No
base resistor is necessary as the transistor is in
emitter follower, current limit being the impedance at
the emitter (the oscillator circuit). |
