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.