Specifications
CIRCUIT IDEAS
switches to a high level when
the door is opened. (See wave-
form
B
in Fig. 2.) Thus, a low-
to-high going voltage transi-
tion is available at point
B
for
every opening of the door,
from the closed position. Ca-
pacitor
C
2 is connected at the
output to filter out unwanted
transitions in the output volt-
age generated at the time of
closing or opening of the door.
IC
7474, a rising-edge-sen-
sitive dual-
D
flip-flop, is used
in the circuit to memorise the
occupancy status of the bath-
room.
IC
1(a) memorises the
state of the door and acts as
an occupancy indicator while
IC
2(b) is used to control the re-
lay to turn on and turn off the bathroom
light.
Q
output pin 8 of
IC
1(b) is tied to
D
input pin 2 of
IC
1(a) whereas
Q
output pin
5 of
IC
1(a) is tied to
D
input pin 12 of
IC
1(b).
At the time of switching on power for
the first time, the resistor-capacitor com-
bination
R
3-
C
3 clears the two flip-flops. As
a result
Q
outputs of both
IC
1(a) and
IC
1(b)
are low, and the low level at the output
of
IC
1(b) activates a relay to turn on the
bathroom light. This operation is inde-
pendent of the door status (open/closed).
M
ost of the fluid level indicator
circuits use a bar graph or a
seven-segment display to indi-
cate the fluid level. Such a display using
LED
s or digits may not make much sense
to an ordinary person. The circuit pre-
sented here overcomes this flaw and dis-
plays the level using a seven-segment dis-
play—but with a difference. It shows each
level in meaningful English letters. It dis-
plays the letter
E
for empty,
L
for low,
H
for half,
A
for above average, and
F
for
full tank .
The circuit is built using
CMOS
IC
s.
CD
4001 is a quad.
NOR
gate and
CD
4055 is a
BCD
to seven-segment decoder and dis-
play driver
IC
. This decoder
IC
is capable
of producing some English alphabets be-
sides the usual digits 0 through 9. The
BCD
codes for various displays are given
in Table I. The
BCD
codes are generated
by
NOR
gates because of their intercon-
nections as the sensing probes get im-
mersed in water. Their operation being
self-explanatory is not included here.
Note that there is no display pattern
like
E
or
F
available from the
IC
. There-
fore to obtain the pattern for letters
E
and
F
, transistors
T
1 and
T
2 are used.
These transistors blank out the unneces-
sary segments from the seven-segment
display. It can be seen that letter
E
is
generated by blanking ‘b’ and ‘c’ segments
of the seven-segment display while it de-
codes digit 8. Letter
F
is obtained by
blanking segment ‘b’ while it decodes let-
ter
P
.
As
CMOS
IC
s are used, the current con-
The occupancy indicator red
LED
(
D
3) is off
at this point of time, indicating that the
room is vacant.
When a person enters the bathroom,
the door is opened and closed, which pro-
vides clock signals for
IC
1(b) (first) and
IC
1(a). The low level at point
C
(pin 5) is
clocked in by
IC
1(b), at the time of open-
ing the door, keeping the light status un-
changed.
The high level point
D
(pin 8) is
clocked in by
IC
1(a), turning on the occu-
pancy indicator
LED
(
D
3) on at the time of
closing of the door. (See waveform
C
in
Fig. 2.)
When the person exits the bathroom,
the door is opened again. The output of
IC
1(b) switches to high level, turning off
the bathroom light. (See waveform
D
in
Fig. 2.) The closing of the door by the
door-closer produces a low-to-high transi-
tion at the clock input (pin 3) of
IC
1(a).
This clocks in the low level at
Q
output
of
IC
1(b) point
D
to
Q
output of
IC
1(a)
point
C
, thereby turning off the occupancy
indicator.
TABLE I
D C B A DISPLAY
LLLL0
LL L H 1
—— — — 2
—— — — 3
—— — — 4
—— — — 5
—— — — 6
—— — — 7
HLLL8
HL L H 9
HL H L L
HLHHH
HHLLP
HH L H A
HH H L —
H H H H BLANK
THOMMACHAN THOMAS
SMART FLUID LEVEL INDICATOR
RUPANJANA
Fig. 3
20