DIY Audio Input Switch Circuit with CD4028 & LM3915

If your amplifier only has one audio input, it can be inconvenient to have to unplug and switch wires every time you want to switch between CD, cassette, or Bluetooth sources. You can design this circuit to solve the problem. Instead of using Analog switch ICs to replace relays, relays can achieve lossless switching. Additionally, an LM3915 frequency indicator was added for visual effect.

The decimal decoder IC CD4028 controls the relays and can be expanded to up to 10 channels. Because it's an interlock circuit, only one relay can work at a time.

When the circuit is powered on, CD4028 input A, B, C, and D are all low voltage, and only the CD4028 0 output (CD4028 pin 3) is high voltage, at approximately the same voltage as VDD. Therefore, all four relays are inactive. At this time, the voltage from pin 3 is added to input A (pin 10) via D5, and pin 14 outputs high voltage to drive Q1, which causes relay K1 to close, selecting the CD source. At the same time, the high voltage from pin 14 enters pin 10 through R1, keeping both pins 10 and 14 at high voltage.

Pressing the "TAPE" button allows VDD to enter input B (pin 13) via the button, and pin 2 outputs high voltage to lock it in place, causing K2 to close while K1, K3, and K4 remain open. The other buttons operate in a similar manner. For specific details about CD4028, please search on Baidu.

The LM3915 is a 10-segment frequency indicator circuit, and you can follow the diagram to assemble it. Here, we will only explain pin 9, which connects to VDD for bar graph display or is left floating for dot display. Just have fun with it.

Before the finished product was completed, we did not consider the LM3915 generating excessive heat when using a high voltage for a long time, so we changed from a 12V power supply to a 6V power supply. This minor issue doesn't require the circuit board to be remade, just add an LM78M06. Of course, if you want, you can buy 5V relays, and the entire circuit can be powered with 5V, which works perfectly without affecting the sound quality, except that the RCA socket should be of good quality, and the power supply ripple should not be too high.