Escape Cable Hell With An Audio I/O Multiplexer

If you ever find yourself swapping between a mix of audio inputs and outputs and get tired of plugging cables all the time, check out [winslomb]’s audio multiplexer with integrated amplifier. The device can take any one of four audio inputs, pass the signal through an amplifier, and send it to any one of four outputs.

The audio amplifier has a volume control, and the inputs and outputs can be selected via button presses. An Arduino Pro Mini takes care of switching the relays based on the button presses. On the input side, you can plug in devices like a phone, TV, digital audio player or a computer. The output can be fed to speakers, headsets or earphones.

At the center of the build lies a TI TPA152 75-mW stereo audio power amplifier. This audio op-amp is designed to drive 32 ohm loads, so performance might suffer when connecting it to lower impedance devices, but it seems to work fine for headphones and small computer speakers. The dual-gang potentiometer controls the volume, and the chip has a useful de-pop feature. The circuit is pretty much a copy of the reference shown in the data sheet. Switching between inputs or outputs is handled by a bank of TLP172A solid state relays with MOSFET outputs, and it’s all tied together with a micro-controller, allowing for WiFi or BLE functionality to be added on later.

[winslomb] laid out the design using Eagle and he made a couple of footprint mistakes for the large capacitors and the opto-relays. (As he says, always double-check part footprints!) In the end, he solder-bridged them on to the board, but they should probably be fixed for the next revision.

[winslomb] built the switch as his capstone project while on his way to getting a Masters in EE, and although the device did function as required, there is still room for improvement. The GitHub repository contains all the hardware and software sources. Check out the video below where he walks through a demo of the device in action. If you are looking for something simpler, here is a two input – one output audio switcher with USB control and on the other end of the spectrum, here’s an audio switch that connects to the Internet.

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A High-End Studio Multiplexer Surrenders To An Arduino

The equipment used in professional radio and TV studios is both extremely high quality and very expensive indeed, and thus out of the reach of an experimenter. Happily as studios are refurbished there’s a steady supply of second-hand equipment which can be surprisingly cheap, but as [Nathan] found out with a Quartz audio router, comes with no control software. What’s to be done with what’s essentially a piece of junk? Remove its brain and replace it with one that can be controlled, of course!

On the PCB alongside a bank of switch matrices is an FPGA which does the heavy lifting. That’s “heavy” in a limited sense, because all it does is handle the chip select lines for the matrices and write data to their registers. This is a task that can be handled by a microcontroller, so in goes an Arduino Nano, which along with a few other board modifications delivers a serial-controlled studio router.

The interesting part for us in this project comes from a look at the date codes on the board, they’re from the early 2000s. This is (roughly) contemporary with the ATmega chip on the Arduino, so we’re curious as to why the designers saw fit to use an FPGA when the microcontrollers of the day were clearly up to the task for much less outlay. We suspect a touch of millennium-era price inflation, but we can’t be sure.

Meanwhile, old broadcast kit has featured here before.

Arduino Analog I/O Multiplexer

[SeanHodgins] has a project in mind where he needs to sample over 500 analog sensors. To get ready, he made a breakout board for 32-channel analog multiplexer device he wants to use. He put the project out on and also has a video tutorial you can see below.

There are five input pins to the chip which lets you connect one analog pin to any one of 32 analog pins. Of course, in addition to the five control lines, you need some handshaking lines, too so you could use as many as eight digital pins to control the device.

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Tiny Audio Switcher Eliminates Repetitive Plug Swapping


[Phil] uses both his computer’s speakers and a set of headphones while working at his desk, but he was growing tired of constantly having to remove the headset from his sound card in order to insert the speaker plug. He’s been meaning to rig something up to make it easier to switch outputs, but never seemed to get around to it until he recently saw this LAN-enabled audio switcher we featured.

His USB-controlled switch features a single audio input and two audio outputs, which he mounted on a nicely done homemade double-sided PCB. The switch can be toggled using any terminal program, sending commands to the on-board ATtiny13A via an FT232R USB to serial UART chip.

The switch’s operation is really quite simple, merely requiring [Phil] to type in the desired audio channel into the terminal. The ATiny and a small relay do the rest, directing the audio to the proper output.

Pico-Sized Ham Radio

There are plenty of hobbies around with huge price tags, and ham radio can certainly be one of them. Experienced hams might have radios that cost thousands of dollars, with huge, steerable antennas on masts that can be similarly priced. But there’s also a side to the hobby that throws all of this out of the window in favor of the simplest, lowest-cost radios and antennas that still can get the job done. Software-defined radio (SDR) turned this practice up to 11 as well, and this radio module uses almost nothing more than a microcontroller to get on the air.

The design uses the capabilities of the Raspberry Pi Pico to handle almost all of the radio’s capabilities. The RF oscillator is driven by one of the Pico’s programmable I/O (PIO) pins, which takes some load off of the processor. For AM and SSB, where amplitude needs to be controlled as well, a PWM signal is generated on another PIO which is then mixed with the RF oscillator using an analog multiplexer. The design also includes a microphone with a preamplifier which can be fed into a third PIO; alternatively it can receive audio from a computer via the USB interface. More processor resources are needed when generating phase-modulated signals like RF, but the Pico is still quite capable of doing all of these tasks without jitter larger than a clock cycle.

Of course this only outputs a signal with a few milliwatts of power, so for making any useful radio contacts with this circuit an amplifier is almost certainly needed. With the heavy lifting done by the Pico, though, the amplifier doesn’t need to be complicated or expensive. While the design is simple and low-cost, it’s not the simplest radio possible. This transmitter sends out radio waves using only a single transistor but you will be limited to Morse code only.

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Hackaday Podcast 146: Dueling Trackballs, Next Level BEAM Robot, Take Control Of Your Bench, And Green Programming

Postpone your holiday shopping and spend some quality time with editors Mike Szczys and Elliot Williams as they sift through the week in Hackaday. Which programming language is the greenest? How many trackballs can a mouse possibly have? And can a Bluetooth dongle run DOOM? Join us to find out!


Take a look at the links below if you want to follow along, and as always, tell us what you think about this episode in the comments!

Direct download (52 MB)

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How To Modify Your Car Stereo For Bluetooth Or Aux-In

If you’re an automotive enthusiast of taste, you can’t stand the idea of fitting a janky aftermarket stereo into your nice, clean ride. Flashy, modern head units can spoil the look of a car’s interior, particularly if the car is a retro, classic, or vintage ride.

Thus, we’re going to look at how to modify your existing stock car stereo to accept an auxiliary cable input or even a Bluetooth module. This way, you can pump in the latest tunes from your smartphone without a fuss, while still maintaining an all-original look on the dash.


A simple Bluetooth module designed for wiring into car audio systems. There are two wires for 12 V power from the vehicle, and the audio signal is sent out over the RCA plugs. The RCA plugs can be cut off and the module hard wired inside your stereo if you have room. Cutting off the plastic case can help too.

Depending on your choice of audio player, you may prefer a 3.5 mm aux jack, or you might want to go with Bluetooth audio if your smartphone no longer has a headphone port. Whichever way you go, the process of modifying the stereo is largely the same. To achieve your goal, you need to find a way of injecting the audio signal into the head unit’s amplifier stage, while making sure no other audio sources are getting sent there as well.

Whether that audio source is a 3.5 mm jack or a Bluetooth module doesn’t matter. The only difference is, in the latter case, you’ll want to buy a Bluetooth module and hardwire it in to the auxiliary input you create, while also splicing the module into the stereo’s power supply. In the case of a simple headphone jack input, you simply need to wire up an aux cord or 3.5 mm jack somewhere you can get to it, and call it done.

This guide won’t cover every stereo under the sun, of course. Edge cases exist and depending on the minute specifics of how your original car radio works, these exact methods may or may not work for you. However, this guide is intended to get you thinking conceptually about how such mods are done, so that you can investigate the hardware in front of you and make your own decisions about how to integrate an external audio input that suits your usage case. Continue reading “How To Modify Your Car Stereo For Bluetooth Or Aux-In”