Specifications

CIRCUIT IDEAS
transistor T2. The transistor T2 will be
enabled only when the charging capaci-
tor raises its base potential to overcome
the reverse bias voltage at its emitter.
Thus, capacitor C6 and resistor R6 deter-
mine the duration of the on-delay, which
is approximately three minutes for the
given values.
As soon as relay RL1 energises due to
the switching action of transistor T2, the
boosted voltage appears at the output. The
adjustment of trimpot VR2 controls the
bucking point. The output is isolated when
the input reaches prohibitive voltage (say
270V), over-voltage sensing being con-
trolled by trimpot VR3 to saturate tran-
sistor T4, which, in turn, cuts off relay
RL1 via transistors T5 and T6. As a con-
sequence, no output is available from the
auto-transformer.
The resistor R8 discharges the tim-
ing capacitor C6 when RL1 energises.
This is done to ensure that when capaci-
tor C6 is connected back to the base junc-
tion of transistor T2, on resumption af-
ter a power failure or an over-voltage
condition, repeatability of on-delay is
taken care of.
By selecting the current rating of re-
lay contacts (5A or 30A) and auto-trans-
former (500VA or 4000VA), the circuit can
be adapted suitably for a refrigerator or
air-conditioner to obtain a regulation of
200V to 240V for an input variation of
170V to 270V.
RUPANJANA
T
his circuit is an add-on unit for
radio receivers that lack band-po-
sition display. The circuit pre-
sented here can show up to nine bands.
It also incorporates a
novel feature to make
the display dance
(blink) with the audio
level from the receiver.
The power-supply for
the circuit can also be
derived from the radio-
set.
The conversion of
selected channel to
BCD format is
achieved using diodes
D1 through D15 in con-
junction with resistors
R4 to R7. The voltages
developed across these
resistors (R4 through
R7) serve as logic in-
puts to BCD inputs of
BCD to 7-segment de-
coder IC1 (CD4511).
When all switches are
in off state, the volt-
age across resistors R4
through R7 is logic
zero, but when any of the switches S1
through S9 is slided to on position, the
output across these resistors changes to
output proper BCD code to represent the
selected channel. This BCD code is con-
verted to 7-segment display by IC1. By
this arrangement of diodes, the need for
another decimal-to-BCD converter IC and
associated parts is obviated. Switches S1
through S9 are actually parts of existing
band-switch of the radio. Usually, one or
two changeover contacts would be found
extra in the modular pushbutton-type
band-switches of the radios.
IC1s display blanking pin 4 is con-
nected to a display-blinker-control circuit
wired around transistors T1 and T2. A
small part of the audio signal from the
speaker terminals is applied to rectifier
diode D16 and filter capacitor C1 to pro-
duce a pulsating DC across preset VR1.
The sliding contact of preset VR1 is con-
nected to the base of emitter-follower
stage comprising transistor T2. The out-
put of transistor T2, as amplified by tran-
sistor T1, is connected to pin 4 of IC1.
Thus turning on’/‘off of display is con-
trolled by the pulsating voltage developed
from audio output of radio.
The power-supply regulator stage is
needed only when radio power-supply is
greater than 6V DC.
RADIO-BAND-POSITION DISPLAY
M.K. CHANDRA MOULEESWARAN
134