The add-on circuit introduced here is useful for stereo systems. This circuit has provision for connecting stereo outputs from different sources/channels as inputs & of them is selected/connected to the output at any time.
When power supply is turned �on', channel A (AR & AL) is selected. If no audio is present in channel A, the circuit waits for some time & then selects the next channel (channel B). This search operation continues until it detects audio signal in of the channels. The inter-channel wait or delay time can be adjusted with the help of preset VR1. If still longer time is necessary, may replace capacitor C1 with a capacitor of higher value.
Suppose channel A is connected to a tape recorder & 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 , or if there is pause between consecutive recordings, the circuit automatically switches over to the output from the radio receiver. To by hand skip over from (selected) active channel to another (non-selected) active channel, basically push the skip switch (S1) momentarily one time 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 analogue switches. These switches are connected to separate channels. For stereo operation, 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 of its outputs is high at any instant, switch will be closed at a time. IC CD4017 is configured as a 4-bit ring counter by connecting the fifth output Q4 (pin ten) to the reset pin. Capacitor C5 together with resistor R6 forms a power-on-reset circuit for IC2, so that on preliminary switching �on' of the power supply, output Q0 (pin four) 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 ten 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 & causes output of IC1 to stay �low' (disabled) as long as the selected channel output exceeds the preset audio threshold level.
Presets VR2 & VR3 have been included for modification of individual audio threshold levels of left & right stereo channels, as desired. One time the multivibrator action of IC1 is disabled, output of IC2 does not modify 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 basically extended up to0, by using additional 4066 ICs.
When power supply is turned �on', channel A (AR & AL) is selected. If no audio is present in channel A, the circuit waits for some time & then selects the next channel (channel B). This search operation continues until it detects audio signal in of the channels. The inter-channel wait or delay time can be adjusted with the help of preset VR1. If still longer time is necessary, may replace capacitor C1 with a capacitor of higher value.
Suppose channel A is connected to a tape recorder & 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 , or if there is pause between consecutive recordings, the circuit automatically switches over to the output from the radio receiver. To by hand skip over from (selected) active channel to another (non-selected) active channel, basically push the skip switch (S1) momentarily one time 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 analogue switches. These switches are connected to separate channels. For stereo operation, 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 of its outputs is high at any instant, switch will be closed at a time. IC CD4017 is configured as a 4-bit ring counter by connecting the fifth output Q4 (pin ten) to the reset pin. Capacitor C5 together with resistor R6 forms a power-on-reset circuit for IC2, so that on preliminary switching �on' of the power supply, output Q0 (pin four) 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 ten 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 & causes output of IC1 to stay �low' (disabled) as long as the selected channel output exceeds the preset audio threshold level.
Presets VR2 & VR3 have been included for modification of individual audio threshold levels of left & right stereo channels, as desired. One time the multivibrator action of IC1 is disabled, output of IC2 does not modify 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 basically extended up to0, by using additional 4066 ICs.
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