C I R C U I T I D E A S

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C I R C U I T I D E A S

PRABHASH K.P.

RUPANJANA

The add-on circuit presented hereis useful for stereo systems. Thiscircuit has provision for connect-

ing stereo outputs from four differentsources/channels as inputs and only oneof them is selected/ connected to theoutput at any one time.

When power supply is turned ‘on’,channel A (A2 and A1) is selected. If noaudio is present in channel A, the cir-cuit waits for some time and then se-lects the next channel (channel B), Thissearch operation continues until it de-tects audio signal in one of the chan-nels. The inter-channel wait or delaytime can be adjusted with the help ofpreset VR1. If still longer time isneeded, one may replace capacitor C1with a capacitor of higher value.

Suppose channel A is connected toa tape recorder and channel B is con-nected to a radio receiver. If initially

channel A is selected, the audio fromthe tape recorder will be present at theoutput. After the tape is played com-pletely, or if there is sufficient pausebetween consecutive recordings, the cir-cuit automatically switches over to theoutput from the radio receiver. Tomanually skip over from one (selected)active channel, simply push the skipswitch (S1) momentarily once or more,until the desired channel inputs getsselected. 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 analogueswitches. These switches are connectedto four separate channels. For stereooperation, two similar CD4066 ICs areused as shown in the circuit. These ana-logue switches are controlled by ICCD4017 outputs. CD4017 is a 10-bit ring

counter IC. Since only one of its out-puts is high at any instant, only oneswitch will be closed at a time. ICCD4017 is configured as a 4-bit ringcounter by connecting the fifth outputQ4 (pin 10) to the reset pin. CapacitorC5 in conjunction with resistor R6 formsa power-on-reset circuit for IC2, so thaton initial switching ‘on’ of the powersupply, output Q0 (pin 3) is always‘high’. The clock signal to CD4017 is pro-vided by IC1 (NE555) which acts as anastable multivibrator when transistorT1 is in cut-off state.

IC5 (KA2281) is used here for notonly indicating the audio levels of theselected stereo channel, but also for for-ward biasing transistor T1. As soon asa specific threshold audio level is de-tected in a selected channel, pin 7 and/or pin 10 of IC5 goes ‘low’. This lowlevel is coupled to the base of transistorT1, through diode-resistor combinationof D2-R1/D3-R22. As a result, transis-tor T1 conducts and causes output ofIC1 to remain ‘low’ (disabled) as long asthe selected channel output exceeds thepreset 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 multivibratoraction of IC1 is disabled, output of IC2does not change further. Hence, search-

C I R C U I T I D E A S

ing through the channels continues un-til it receives an audio signal exceedingthe preset threshold value. The skip

switch S1 is used to skip a channel evenif audio is present in the selected chan-nel. The number of channels can be eas-

ily extended up to ten, by using addi-tional 4066 ICs.