Parts List
R1 = 100K D1 = 1N4004, general purpose diode
R2 = 4K7 C1 = 47µF/16V
R3 = 10M IC1 = 555 Timer
P1 = 100K Q1 = 2N3904, 2N2222, or similar
Ry = Relay
Additional
Notes
Not much
to tell
here as
the
circuit
speaks
for
itself.
The 555
can be
almost
any
type,
they are
all
pin-compatible.
Although
some
CMOS
types
may not
have
enough
power to
drive
the
transistor,
in that
case use
an
ordinary
555.
C1's
working
voltage
should
be
increased
to 25V
if you
decide
to go
with a
12V
power
source.
Change
the
value of
C1 for
the
desired
output
pulse.
For the
timing
use this
equation:
T=1.1*(R1+P1)*C1
assuming
R1 + P1
= 150K,
then
select
C1 as
follows:
C1 = 6µF
for each
1-second
pulse
width.
For
example,
if you
want the
pulse
width to
be 5
seconds,
C1
should
be 30uf
or
nearest
value
like 22
or 33µF.
Additionally,
P1 can
adjust
the
rest.
Rule of
thumb:
the
working
voltage
of
capacitors
are at
least
double
the
supplied
voltage,
in other
words,
if the
power
source
is
9Volt,
your
capacitor(s)
is at
least
18V.
Transistor
T1 can
be any
approximate
substitute.
Use any
suitable
relay
for your
project
and if
you're
not
tight on
space,
use any
size.
I've
build
this
particular
circuit
to
prevent
students
from
fiddling
with the
security
cameras
in
computer
labs at
the
University
I am
employed.
I made
sure the
metal
casing
was not
grounded.
But as
the
schematic
shows
you can
basically
hook it
up to
any type
of metal
surface.
I used a
12-vdc
power
source.
Use any
suitable
relay to
handle
your
requirements.
A
'RESET'
switch (Normally
Closed)
can be
added
between
the
positive
and the
'arrow-with-the-+'.
The
trigger
(touch)
wire is
connected
to pin 2
of the
555 and
will
trigger
the
relay,
using
your
body
resistance,
when
touched.
It is
obvious
that the
'touching'
part has
to be
clean
and
makes
good
contact
with the
trigger
wire.
This
particular
circuit
may not
be
suitable
for all
applications.
Just in
case you
wonder
why pin
5 is not
listed
in the
schematic
diagram;
it is
not
really
needed.
In
certain
noisy
conditions
a small
0.01µF;
ceramic
capacitor
is
placed
between
pin 5
and
ground.
It does
no harm
to add
one or
leave it
out.
NOTE:
For
those of
you who
did not
notice,
there is
an
approximate
5-second
delay
build-in
before
activation
of the
relay to
avoid
false
triggering,
or a
'would-be'
thief,
etc.
AGAIN,
make
sure the
latch
(pin 2)
is not
touching
anything
'ground'
or the
circuit
just
keeps
resetting
itself
and so
will not
work. My
shed has
wooden
doors so
works
fine. If
you
can't
get
yours to
work,
check
the
trigger
input,
verify
there is
some
sort of
signal
coming
from
output
pin 3,
play
with the
value of
R3/C1,
etc. |
