Specifications
CIRCUIT
IDEAS
S.C. DWIVEDI
C.K. SUNITH
T
he circuit described her is dif-
ferent form conventional door
bell circuits in the sense that it
can produce two different tones-one of
a lower frequency and the other of a
much higher frequency.
The circuit uses a timer IC 555,
which has been wired in free-running
mode. When switch S1 is depressed, the
circuit oscillates at around 1.5 kHz, re-
sulting in a higher frequency note. When
switch S2 is depressed, transistor T1 is
turned 'on' and thus it shunts resistor
R2 across capacitor C2. As a result, the
circuit now oscillates at approximately
150 Hz and a tone of much lower fre-
quency is generated.
The circuit can be conveniently em-
ployed as a doorbell for two separate
doors at the same floor level. Doorbell
switches S1 and S2 can be mounted
near the re-spective doors.
The circuit can be assembled on a
general-purpose PCB and housed inside
a small cabinet. Tone can be adjusted
with the help of preset VR1. Before
power is switched 'on' , it is advisable
to adjust VR1 to approximately 5 kilo-
ohm, that is, at the centre of the full
range of potentiometer VR1. A trimpot
can be used as VR1 for convenience of
assembly.
PRADEEP G.
A
major drawback of some pest
repellers is that their power out-
put is low and hence their effec-
tiveness suffers. The pest repeller cir-
cuit described here generates powerful
ultrasonic signals to repel pests. In ad-
dition to the ultrasonic frequency oscil-
lator, separate push-pull power ampli-
fier and transformer are used to boost
ultrasonic signals.
Ultrasonic frequency oscillator is
built around IC CD4047, which provides
complementary outputs. These comple-
mentary outputs are amplified by tran-
sistors T1 and T2 (BD139) to drive
transistorised push-pull power amplifier
stage comprising power transistors T3
and T4 (2N3055).
The output of the power amplifier is
coupled to a tweeter, through output
SWG while secondary winding comprises
40 turns of 24 SWG wire. Adjust potenti-
ometer VR1 for maximum effectiveness.
S.C. DWIVEDI
transformer X1. Transformer X1 is wound
over ferrite core (UU or CC core). Pri-
mary winding consists of 150 turns of 28
2
101