Specifications

CIRCUIT IDEAS
hence the melody generator UM66 (IC1)
does not get any power supply and is
thus off.
When bell switch S1 is pushed
(closed), the audio output from
module 1 is shorted to ground and at
the same time transistor T1 base is
pulled to ground via resistor R3. As a
result, transistor T1 is cut off, to pull
its collector high. The voltage at collec-
tor of transistor T1, after limiting by
zener ZD1 to 3V, serves as power sup-
ply for melody generator UM66. The
output of melody generator is directly
coupled to the input of audio amplifier
and hence only melody is reproduced in
the speaker, when switch S1 is pushed.
The main advantage of this bell is
S
everal circuits have been pub-
lished in earlier issues of EFY
for producing eye-catching light-
ing effects, such as lighting up of char-
acters one-by-one and their going off
one-by-one in same direction, i.e. first
character goes off first and so on (first-
in first-out, or FIFO). In the present cir-
cuit, an attempt is made for changing
this sequence, i.e. the first character to
go off last (last-in first-out, or LIFO).
Easily available ICs are used and
the number of characters is limited to
five. Effects like ‘curtain opening and
closing’ and ‘peacock feathers spreading
and closing’ can be produced. Wiring of
the same is shown in separate figures.
Each line is lit up by 6-volt or 12-volt
bulbs connected in series for 230V AC
operation and controlled via triacs. LEDs
can also be used with proper driving
circuits. (Please ensure that sum of volt-
age drop across the series-connected
bulbs is equal to around 230V.)
Flip-flops formed by NOR gates are
used for controlling the sequence. NOR
gates N1 and N2 form an oscillator
whose period can be controlled using
preset VR1. Oscillator’s output is fed to
clock input pin 14 of IC1 (CD4017),
which is a decade counter.
Initially when output Q0 of IC1 is
high, the output of flip-flop formed by
NOR gates N3 and N4 goes high, thus
triggering triac-1 through driving tran-
sistor T1. In the same way, when Q1 to
Q4 outputs go high sequentially, corre-
sponding triacs get triggered and all the
five groups of bulbs light up. The first
part of the sequence is over.
When output Q5 of IC1 become high,
the flip-flop formed by NOR gates N11
and N12 is toggled and the output
is pulled to logic 0, removing the neces-
that if there are strangers outside the
door, conversing before gaining entry
by pressing the bell switch, you can
eavesdrop and hear their conversation
to guess their intentions.
Lab Note. This circuit can be easily
modified to act as door-phone cum door-
bell. So try it out!
S.C. DWIVEDI
K.P. VISWANATHAN
MOVING CURTAIN DISPLAY
188