8 Channel Audio Switcher Circuit

Description: This source is selected by pressing a momentary-contact pushbutton switch (SI). Switch SI is connected to the trigger of a 555 oscillator/timer (U1) configured as a monostable multivibrator, which generates one short output pulse for each press of SI. That pulse turns on LED1 to provide a visible indication that the 555 is functioning correctly. This pulse is also used to clock U2 (a 4017 CMOS divide-by-10 counter/divider). Both LED1 and its associated current-limiting resistor (R3) are optional and can be omitted from the final project without affecting circuit operation. The 4017 advances by one clock pulse each time SI is pressed, activating its corresponding output. Pin 9 (corresponding to output 8) of U2 is directly connected to its own reset terminal at pin 15. This configuration allows the counter to count from zero to seven and then reset to zero on the eighth count. Pin 13, the enable input of U2, is tied to ground to enable the counter's operation. Outputs zero through seven are connected to eight indicator LEDs and the control pins of two LM1037s (U3 and U4). When an output is selected, its LED lights up, and the corresponding control input on the LM1037 is brought high. The LM1037 features extremely high-impedance inputs and low-impedance outputs, ensuring compatibility between various types and brands of equipment. This, along with a wide frequency response and low distortion, makes it ideal for use with quality home entertainment systems. The prototype of the audio switcher exhibits a usable frequency response ranging from just a few hertz to over 100 kHz. Power for the switcher is supplied by a simple circuit. Since the switcher only draws between 20 and 30 mA, a straightforward circuit using the popular 7812 or 78L12 (a low-power version) voltage regulator is sufficient.

The circuit operates as a basic audio switcher, utilizing a momentary-contact pushbutton switch (SI) to initiate the process. Upon pressing the switch, the 555 timer (U1) generates a pulse, activating LED1 for visual feedback, indicating that the timer is operational. The pulse output from the 555 timer serves a dual purpose: it clocks the 4017 counter (U2), which counts the number of button presses. The 4017 is configured to count from zero to seven, resetting automatically after reaching eight, thanks to the connection of pin 9 directly to pin 15. This design ensures that the counter can cycle through its outputs effectively.

The outputs from the 4017 are linked to indicator LEDs and control inputs of two LM1037 audio switch ICs (U3 and U4). Each time the counter increments, the corresponding LED lights up, providing a clear indication of the active output. The high-impedance inputs of the LM1037 ensure that it can interface with various audio sources without loading them down, while its low-impedance outputs effectively drive the connected loads. This characteristic, combined with its wide frequency response and minimal distortion, makes the LM1037 an excellent choice for audio applications.

The power supply for this circuit is designed to be simple and effective. Given the low current requirements of the switcher (20 to 30 mA), a standard 7812 voltage regulator, or its lower-power variant (78L12), is employed to provide a stable 12V supply. This setup ensures reliable operation of the entire circuit while maintaining efficiency. The overall design is well-suited for integration into home entertainment systems, providing an effective and user-friendly audio switching solution. 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. 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.