|
The add-on circuit
presented here is useful for stereo systems. This circuit has
provision for connecting 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 (AR and AL) is
selected. If no audio is present in channel A, the circuit waits
for some time and then selects the next channel (channel B). This
search operation continues until it detects audio signal in one of
the channels. 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
connected 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 completely, or if there is sufficient
pause between consecutive recordings, the circuit automatically
switches over to the output from the radio receiver. To manually
skip over from one (selected) active channel to another
(non-selected) active channel, simply push the skip switch (S1)
momentarily once or more, until the desired channel input gets
selected. The selected channel (A, B, C, or D) is indicated by the
glowing of corresponding 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
analogue switches are controlled by IC CD4017 outputs. CD4017 is a
10-bit ring counter IC. Since only one of its outputs 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 provided 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 forward biasing
transistor T1. As soon as a specific threshold audio level is
detected 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,
transistor 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 included for adjustment of
individual audio threshold levels of left and right stereo
channels, as desired. Once the multivibrator action of IC1 is
disabled, output of IC2 does not change further. Hence, searching
through the channels continues until it receives an audio signal
exceeding the preset threshold value. The skip switch S1 is used
to skip a channel even if audio is present in the selected
channel. The number of channels can be easily extended up to ten,
by using additional 4066 ICs.
|
