Specifications
CIRCUIT IDEAS
transistor
T
1. The reset keys
are wired to set pins 6 and
8 of each
IC
. (Power-on-reset
capacitor
C
1 has been added
at
EFY
during testing as the
state of
Q
output is indeter-
minate during switching on
operation.)
This circuit can be use-
fully employed in cars so
that the car can start only
when the correct code se-
quence is keyed in via the
key pad. The circuit can also
be used in various other ap-
plications.
T
his circuit is used to automate the
working of a bathroom light. It is
designed for a bathroom fitted
with an automatic door-closer, where the
manual verification of light status is dif-
ficult. The circuit also indicates whether
the bathroom is occupied or not. The cir-
cuit uses only two
IC
s and can be oper-
ated from a 5V supply. As it does not use
any mechanical contacts it gives a reli-
able performance.
One infrared
LED
(
D
1) and one infrared
detector diode
(
D
2) form the sensor part of
the circuit. Both the infrared
LED
and the
detector diode are fitted on the frame of
a reference potential set by preset
VR
1.
The preset is so adjusted as to provide
an optimum threshold voltage so that out-
put of
IC
2(a) is high when the door is
closed and low when the door is open.
Capacitor
C
1 is connected at the output
to filter out unwanted transitions in out-
put voltage generated at the time of open-
ing or closing of the door. Thus, at point
A
, a low-to-high going voltage transition
is available for every closing of the door
after opening it. (See waveform
A
in Fig.
2.)
The second comparator
IC
2(b) does the
reverse of
IC
2(a), as the input terminals
are reversed. At point
B
, a low level is
available when the door is closed and it
JAYAN A.R.
AUTOMATIC BATHROOM LIGHT
the door with a small sepa-
ration between them as
shown in Fig. 1. The radia-
tion from
IR
LED
is blocked
by a small opaque strip (fit-
ted on the door) when the
door is closed. Detector di-
ode
D
2 has a resistance in
the range of meg-ohms when
it is not activated by
IR
rays.
When the door is opened,
the strip moves along with
it. Radiation from the
IR
LED
turns on the
IR
detector di-
ode and the voltage across
it drops to a
low level.
Com-
parator
LM
358
IC
2(a)
compares
the voltage
across the
photodetec-
tor against
Fig. 1
Fig. 2
G.S. SAGOO
19