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
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
important.
No base
resistor
is
necessary
as the
transistor
is in
emitter
follower,
current
limit
being
the
impedance
at the
emitter
(the
oscillator
circuit).
