The top plate
2013-08-23 09:50 by Ian
The top plate in the radio box holds the bulk of the custom circuitry for the car computer. Mounted to it, we have the public address (PA) amplifier, the DC/DC for the amplifier, my hand-built filter and audio-level shifter, the main CPU board, the audio board, and a handful of regulators and MOSFETs.
Here is the top plate, after drilling, tapping, and measuring the locations of various components to be mounted...
Level converter and filter PCB:
The level converter PCB is actually comprised of a few circuits that are not related to audio. The bulk of the PCB is taken up by the transformer and resistors that translate the balanced, high power audio from the radio's speaker-out jack into the unbalanced, line-level audio that all the other components in the system require. There is another circuit that performs simple noise-filtering on the car's 12v (13.5v while running) for the sake of the few components in the computer that use 12v and do not have their own filtering circuits. There is an opto-isolator that detects the state of the vehicle's accessory circuit, and a post-filter voltage divider that drops the operating voltage of the vehicle into something that can be safely fed to the Teensy's ADC pins. Full (hand-drawn) schematic follows...
TODO: Schematic not yet uploaded.
Pictures of the completed PCB:
PA Amp and DC/DC:
These two parts are not my design. The PA amp was built as a kit, and the DC/DC was ordered from Ebay as an off-the-shelf component. The PCB on the far-right is the Arduino Pro that is running the TNC software. This piece is discussed on the page for the bottom plate.
The audio PCB itself has a few functional subunits. It has the 4x4 stereo audio switch and the 16-bit register that drives it, the digital potentiometer that is serving as PA volume control, a preamp for the microphone, and all of the transistors that handle the signal-isolation from the Kenwood MC-59 microphone unit for my radio.
TODO: Picture not yet uploaded.
Some... "creativity" was employed to make wiring the switch and the shift-register more compact. The 74HCT4052's that I am using as audio switches were a no-brainer, as most of the pins are mutually common. The stack of 4 ICs on the left is the audio switch, and the statck of two on the right comprise the 16-bit shift register...
Because all the audio components require line-level audio, the microphone in the MC-59 needs a preamp. For this task, I am using the $5 preamp circuit. Here is the schematic and a picture of the completed module next to the MC-59 that will be driving it...
Breadboarding the main PCB and the audio switch circuits: