Specifications
CIRCUIT IDEAS
fan run at the three speeds for an
equal amount of time by connecting
three decimal decoded outputs of IC3
to each of the transistors T1 to T3.
One can also get more than three
speeds by using an additional relay,
transistor, and associated compo-
nents, and connecting one or more
outputs of IC3 to it.
In the motors used in certain coolers
there are separate windings for separate
speeds. Such coolers do not use a rheo-
stat type speed regulator. The method of
connection of this device to such coolers
is given in Fig. 4.
The resistors in Figs 2 and 3 are the
tapped resistors, similar to those used in
manually controlled fan-speed regulators.
Alternatively, wire-wound resistors of suit-
able wattage and resistance can be used.
G
enerally, when an equipment in-
dicates no power, the cause may
be just a blown fuse. Here is a
circuit that shows the condition of fuse
through LEDs. This compact circuit is
very useful and reliable. It uses very few
components, which makes it inexpensive
too.
Under normal conditions (when fuse
is alright), voltage drop in first arm is 2V
+ (2 x 0.7V) = 3.4V, whereas in second
S.C. DWIVEDI
used to trigger the siren. When the fuse
blows, red LED glows. Simultaneously it
switches ‘on’ the siren.
In place of a bicolour LED, two LEDs
of red and green colour can be used. Simi-
larly, only one diode in place of D1 and
D2 may be used. Two diodes are used to
increase the voltage drop, since the two
LEDs may produce different voltage
drops.
arm it is only 2V. So
current flows through
the second arm, i.e.
through the green
LED, causing it to
glow; whereas the red
LED remains off.
When the fuse
blows off, the supply
to green LED gets
blocked, and because
only one
LED is
in the
circuit,
the red LED glows. In case
of power failure, both LEDs
remain ‘off’.
This circuit can be easily
modified to produce a siren
in fuse-blown condition (see
Fig. 2). An optocoupler is
H
ere is an inexpensive auto cut-
off circuit, which is fabricated
using transistors and other dis-
crete components. It can be used to pro-
tect loads such as refrigerator, TV, and
VCR from undesirable over and under line
voltages, as well as surges caused due to
sudden failure/resumption of mains power
supply. This circuit can be used directly
S.C. DWIVEDI
as a standalone circuit between
the mains supply and the load, or
it may be inserted between an ex-
isting automatic/manual stabiliser
and the load.
The on-time delay circuit not
only protects the load from switch-
ing surges but also from quick
changeover (off and on) effect of
over-/under-voltage relay, in case the
mains voltage starts fluctuating in the vi-
cinity of under- or over-voltage preset
points. When the mains supply goes out of
preset (over- or under-voltage) limits, the
relay/load is turned ‘off’ immediately, and
it is turned ‘on’ only when AC mains volt-
age settles
within the pre-
set limits for a
period equal to
the ‘on’ time
delay period.
The on-time
delay period is
presetable for 5
seconds to 2
minutes dura-
Fig. 1: Power
supply
ASHUTOSH KUMAR SINHA
BLOWN FUSE INDICATOR
K. UDHAYA KUMARAN, VU3GTH
OVER-/UNDER-VOLTAGE CUT-OFF
WITH ON-TIME DELAY
153