Specifications

CIRCUIT IDEAS
ADD-ON STEREO CHANNEL
SELECTOR
CIRCUIT IDEAS
PRABHASH K.P.
RUPANJANA
T
he add-on circuit presented here
is useful for stereo systems. This
circuit has provision for connect-
ing stereo outputs from four different
sources/channels as inputs and only one
of them is selected/ connected to the
output at any one time.
When power supply is turned ‘on’,
channel A (A2 and A1) is selected. If no
audio is present in channel A, the cir-
cuit waits for some time and then se-
lects the next channel (channel B), This
search operation continues until it de-
tects audio signal in one of the chan-
nels. The inter-channel wait or delay
time can be adjusted with the help of
preset VR1. If still longer time is
needed, one may replace capacitor C1
with a capacitor of higher value.
Suppose channel A is connected to
a tape recorder and channel B is con-
nected to a radio receiver. If initially
channel A is selected, the audio from
the tape recorder will be present at the
output. After the tape is played com-
pletely, or if there is sufficient pause
between consecutive recordings, the cir-
cuit automatically switches over to the
output from the radio receiver. To
manually skip over from one (selected)
active channel, simply push the skip
switch (S1) momentarily once or more,
until the desired channel inputs gets
selected. The selected channel (A, B, C,
or D) is indicated by the glowing of cor-
responding LED (LED11, LED12,
LED13, or LED14 respectively).
IC CD4066 contains four analogue
switches. These switches are connected
to four separate channels. For stereo
operation, two similar CD4066 ICs are
used as shown in the circuit. These ana-
logue switches are controlled by IC
CD4017 outputs. CD4017 is a 10-bit ring
counter IC. Since only one of its out-
puts is high at any instant, only one
switch will be closed at a time. IC
CD4017 is configured as a 4-bit ring
counter by connecting the fifth output
Q4 (pin 10) to the reset pin. Capacitor
C5 in conjunction with resistor R6 forms
a power-on-reset circuit for IC2, so that
on initial switching ‘on’ of the power
supply, output Q0 (pin 3) is always
‘high’. The clock signal to CD4017 is pro-
vided by IC1 (NE555) which acts as an
astable multivibrator when transistor
T1 is in cut-off state.
IC5 (KA2281) is used here for not
only indicating the audio levels of the
selected stereo channel, but also for for-
ward biasing transistor T1. As soon as
a specific threshold audio level is de-
tected in a selected channel, pin 7 and/
or pin 10 of IC5 goes ‘low’. This low
level is coupled to the base of transistor
T1, through diode-resistor combination
of D2-R1/D3-R22. As a result, transis-
tor T1 conducts and causes output of
IC1 to remain ‘low’ (disabled) as long as
the selected channel output exceeds the
preset audio threshold level.
Presets VR2 and VR3 have been in-
cluded for adjustment of individual au-
dio threshold levels of left stereo chan-
nels, as desired. Once the multivibrator
action of IC1 is disabled, output of IC2
does not change further. Hence, search-
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