Friday, October 30, 2015

Audio Multiplexer with Integrated Amplifier

This post is a summary of the project portion of my Capstone for my masters in EE.

The audio multiplexer (mux) with integrated amplifier is a device that amplifies the audio signal with a volume control knob and has four selectable audio inputs and outputs.  The inputs and outputs are selected by button presses.  A microcontroller is programmed to take the button presses and power a corresponding set of relays to activate an audio jack.  A power supply needs to be attached to the micro USB port on the microcontroller to power the relays and supply power to the amplifier circuit.  A custom printed circuit board (PCB) has been created for this project to accommodate for functional use and be compact.  The final project is a working device that satisfies all of the requirements.

This post made it on Hackaday!

This project is to act as a central hub for switching between audio sources and listening devices. Multiple audio sources can include a phone, computer, TV, etc. The listening devices can include different sets of speakers, headphones, etc. This project aims at reducing the hassle of unplugging and reconfiguring the desired audio source and listening device to just a button press. There is also an integrated volume control knob on the audio mux to conveniently change the gain in the same location as the buttons.

Here is the link to the project files on github.

Project Files

Parts List
The entire project cost just under $130.

Part Quantity
Arduino Pro Micro 5V 1
Relay TLP172AF-ND 16
Headphone Jack CP1-3513N-ND 8
Capacitor 1uf 4
Resistor 20k Ohm 6
Resistor 270 Ohm 8
Resistor 100 Ohm 2
Resistor 90 Ohm 8
Capacitor 330uf 2
op amp TPA152D 1
Dual Gang Potentiometer RK09L1220A1B 1
Knob 1
Buttons COM-00097 8
Headers 1
1206 Blue LEDs 8
Custom PCB 1


Eagle was used for the PCB design.  The schematic is relatively simple.  The amplifier circuit is the reference schematic in the datasheet for the op amp, TPA152.

Here is the Eagle schematic for the project.

The microcontroller is the central hub for the whole project. It takes input from the buttons and provides power to the relays and amplifier circuit.
Here is the Eagle board layout with the ground planes removed for clarity.

OSHPARK was used to manufacture the board. You have to buy them in sets of 3. The overall quality is pretty nice and the interface for ordering the PCB is really simple.

Final board after soldering on the components. Most of the components are smd with an 0805 package size.

I did not check to see what thee smallest package size for a 330uF cap was when the board was laid out. I was able to bend the leads down and connect to the solder pads. The leads were wrapped with electrical tape.

Again, the package size was not checked with the actual size of the component for the relays. A similar part from the same manufacturer was used in the layout and the package size was assumed to be the same. The relays were slightly off so I just bridged the gaps with solder.

The lesson learned here is to always check the actual package sizes of ALL the components. If there is a next revision, these changes will be implemented.

Below is a video of the final project. It is the video I submitted to my college to complete my capstone project. It demonstrates having four different audio sources and four different listening devices hooked up to the audio mux and changing the active inputs and outputs while pressing buttons. It also shows how the knob controls the overall gain of the system.

The next version of this will have a better amplifier circuit in it. No real testing was done on the amp circuit since it was just a reference design. I will also make it easier to mount into an enclosure. The number of inputs and outputs can be increased/decreased depending on the requirements. This was just a proof of concept and all of the features worked and satisfied the current requirements.

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