This circuit is for connecting the stereo outputs of four different channels as inputs and only one is selected for output at any time.
When the circuit breaker on, the channel A (AR and AL) is selected. If there is no audio on the channel A, the circuit waits for some time and then select the next channel (channel B). The search operation continues until it detects an audio signal in one channel. The time delay between channels or wait can be adjusted with the help of preset VR1. If time is no longer necessary, you can replace a capacitor C1 with a higher value.
Manually jump from one channel to another channel active active, just press the jump switch (S1) to the desired channel input is selected. The selected channel (A, B, C or D) is indicated by the corresponding bright LED (LED 11, 12, 13 or 14).
IC contains 4 analog switches CD4066, these switches are connected to four channels. These analog switches controlled by IC CD4017 outputs. CD4017 is a 10-bit ring counter IC. Since only one of its products is high at any instant, only one switch is closed at a time. CD4017 IC is configured as a ring counter 4-bit by connecting the fifth exit Q4 (pin 10) on the reset pin. Capacitor C5, in connection with the resistor R6 form a power circuit-on-reset IC2, so that the initial change of the power supply, output Q0 (pin 3) is always high. The clock signal is provided by IC1 CD4017 NE555 which acts as an astable multivibrator when transistor T1 is in state court.
KA2281 IC5 is used here not only to indicate the levels of stereo audio channels selected, but also to bias transistor T1. As soon as a specific audio threshold is detected in a selected channel, the pin 7 and / or pin 10 of IC5 goes low. This low level is coupled to the base of transistor T1 through a combination of diodes D2-R1/D3-R22 resistance. As a result, transistor T1 conducts and causes the output of IC1 to remain low as long as the output of the selected channel audio level exceeds the threshold.
VR3 VR2 and Presets are included for adjusting each of the threshold levels for audio left and right stereo channels as desired. Once the action multivibrator IC1 is disabled, the output of IC2 is not changed more. Therefore, searching through the channels continues until it receives an audio signal above the threshold. Jumping the switch S1 is used to jump from one audio channel, even if present in the selected channel. The number of channels can easily be extended to ten by 4066 the use of additional integrated circuits.
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