Author :
Wichit
Sirichote
Introduction
This is
my long
history
the
device
that
controls
my
home's
night
light,
air-conditioner,
etc. The
device
is a
Miniature
Real-time
Controller.
The
circuit
uses
only
three
chips, a
89C2051,
DS275(or
MAX232),
and
74LS07
open
collector
driver.
The
scheduler
for time
on/off
of
6-channel
output
can be
made by
downloading
from PC
and
saved
into
onchip
RAM.
Each
output
provides
a 20mA
sinking
suitable
for
driving
a
homemade
opto-triac
or big
solid-state
relay
for
heavy
load.
Hardware
A
circuit
diagram
of the
Miniature
Real-time
Controller
is shown
in
Figure
1. A
89C2051
with a
low-cost
X-tal
3.579MHz
runs
timer6.hex.
The
6-channel
output
is P1.2
to P1.7
driving
with
sink
current.
A 74LS07
open
collector
provides
approx.
max 20mA
@12V
suitable
for
driving
a
homemade
opto-triac
shown in
Figure
2. Two
signal
diodes,
1N914,
provide
simple
backing
up
supply
for the
89C2051.
Since
RST uses
simple
RC
circuit,
there is
no
brownout
protection
and
reset
switch
thus
when the
controller
operated
with
battery
for long
time,
the chip
may
knock.
Simply
take the
battery
out and
put them
again.
The
RS232
level
converter,
instead
of
DS275,
any
converter
may use,
say
MAX232,
or
simple
circuit
using
two
transistors.
Figure
1:
Circuit
Diagram
of
Miniature
Real-time
Controller
(a nice
circuit
diagram
was
drawn by
my
student
Patra
Pienchob).
Homemade
Opto-triac
An
opto-triac
with
zero-crossing
can be
made
easily.
I have
used an
MOC3041
to drive
5A 240V
triac. A
night
light
used in
my home
mostly
be an
incandescent
lamp,
the
circuit
below
works
fine.
For such
heavy
load as
air-conditioner,
I used a
solid-state
relay,one
from
CRYDOM
D2410,
say.
Figure
2:
Example
of
Connecting
homemade
optotriac
and SSR
to the
RTC.
Click
here for
more
detail &
source
code
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