2019-02-15

Line-Level Audio Switch

Because The Comvette Project includes a TEA5767 FM Radio Module, a mechanism is needed to combine the audio output of the Raspberry Pi and the audio output of the FM radio module into a single audio output that can be fed into the BOSE amplified speaker system.

Early in the development of The Comvette Project, I added a 4 channel audio mixer. While the mixer did work, it was an additional $70 expense and added a bit of noise to the audio stream. So the Mixer was sent to the spare parts drawer and I looked for a simpler method of combining these two audio sources.

What I came up with was a simple DPDT relay switch which connected either the Raspberry Pi or the FM radio module to the BOSE speakers. Since the Raspberry Pi has a lot of GPIO pins available, it was easy to add some code to the Comvette program to activate and deactivate the relay as needed.

The line level switch was built on a 1/2 size proto board. As shown in the diagram below. The Raspberry Pi toggles a GPIO pin which controls a MOSFET module which in turn controls the relay. The proto board has three 1/8" audio jacks, a SPDT relay, and a snubber diode.

The audio jacks labeled "IN #1" and "IN #2" receive the audio output from the Raspberry Pi and the FM Radio module respectively. The audio jack labeled "OUT" sends the selected audio to the in-line volume control potentiometer and then on to the BOSE amplified speakers.

In case you are wondering why I didn't simply combine the Raspberry Pi's audio output and the FM Radio module's audio output together, it's never a good idea to have one audio device drive a signal into the output of another audio device. It degrades the quality of the audio and also reduces the signal levels.

This little line level audio switch board has worked out really well. The audio fidelity is preserved as are the line levels.