8 Channel Audio Switcher Circuit

This source is selected by pressing momentary-contact pushbutton switch SI. Switch SI is connected to the trigger of a 555 oscillator/timer (Ul) configured as a monostable multivibrator, wliich generates one short output pulse for each press of SI. That pulse turns on LED1 to give a visible indication that the 555 is working correctly. That pulse is also used to clock U2 (a 4017 CMOS divide-by- 1 -counter/divider). Both LED1 and its associated current-limiting resistor R3 arc optional and can be left out of the finished project without any affect on circuit operation. The 4017 advances by one clock pulse each time SI is pressed, turning on its corresponding output. Pin 9 (corresponding to output 8) of U2 is directly connected to its own reset terminal at pin 15.
8 Channel Audio Switcher Circuit - schematic

This allows the counter to count from zero to seven, and then reset to zero on the eighth count. Pin 13, the enable input ol` U2, is tied to ground to allow the counter to operate. Outputs zero through seven are connected to eight indicator LEDs and the control pins of the two LM 1037s (U3 and U4). When an output is selected, its LED lights and the corresponding control input on the LM1037 is brought high. The LM1037 has extremely high-impedance inputs and low-impedance outputs, so interconnection between various types and brands of equipment should not be a problem. That, together with a wide-frequency response and low distortion, makes it ideal for use with good-quality, home-eriter-taiiunent systems. The prototype of the audio switcher has a usable frequency response of from just a few hertz to over 100 kHz. Power for the switcher is provided by a rather simple circuit. Because the switcher only draws between 20 and 30 rnA, a simple circuit using the popular 7812 or 78L12 (a low-power version) voltage regulator works quite well.

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