Bluetooth Speaker
A low-cost Bluetooth speaker was designed in KiCad with components predominately sourced from American companies. The speaker was designed to look like a vintage radio and features an optional audio filter that can be selected by the user in order to allow the current audio signal to sound like an old-fashioned transistor radio.
The audio filter was initially modeled in MATLAB, and further designed in LTSpice. In order to accommodate the op-amp which was used in the audio filter, a voltage inverter needed to be included in this project. Finally, the filter was implicated using an analog switch. When the switch is turned off, a regular unfiltered audio signal will pass through. When the filter is turned on the audio signal is diverted through the audio filter prior to being passed to the audio amplifier.
The volume for this speaker is controlled by an analog potentiometer unlike the majority of Bluetooth speakers on the market. This goes along with the vintage radio idea in which the user turns a knob to adjust the volume rather than pushing a button.
Audio Filter Design
A low-pass filter that was designed can be seen here, as well as the audio switch. The low pass filter smooths out the audio signal (seen in image 3). The filter selection logic is quite simple, selecting the powered MOSFET with an inverting gate.