Starting on the audio systemPosted: 2005-July-15
I received a pair of computer speakers from Jan and Shannon to use on the project. I took them apart, removed the electronics, put some expanding foam in the tuned ports (to help waterproof them) and painted the speaker cones (again for waterproofing.)
I took a look at the parts I have and figure I can just make a daughter-board for the MP3 player which would control its operation, provide power, and contain the audio amplifier. I decided to use a comparator to detect when there's a signal: compare against some small positive voltage and send the output to charge a capacitor through a diode which would discharge with a high R-C time constant. When the capacitor voltage drops below a set level, a second comparator would trigger the play button to be pressed.
I got back to working on the amplifier and started looking at the TDA1520A because I have several lying around — they're a 20-watt mono audio amplifier chip that requires very few external components. Unfortunately, the designers decided to neglect the car-audio market and it's rated to work as low as 15 volts. I figured I'd try it with 12 volts … I wired it all up and found that it just wouldn't function right. Darn.
It dawned on me that I should just use the amplifier from the computer speakers — it's already wired up, it's designed to work with the specific speakers, and it originally ran off 9 VAC so I can get about the same using 12 volts into the AC input … admittedly with some superfluous components, but who cares?
I decided to use telephone jacks to connect the speakers to the main control box as they seem to be easy to make water resistant I sealed up the holes on the speaker cabinets and prepared them for a phone-wire connection.
I took apart the MP3 player and connected wires to the play button and to the data activity light. The activity light pulses around 1 volt whenever the unit is playing, and that should be easier than detecting audio output. I set it up with a connector to plug into a board and I set up a metal bracket to hold the amplifier board, the MP3 player, and the auto-start board. I added DC-blocking capacitors between the MP3 player's output and the amplifier input as the DC levels are likely to be different and the system wasn't designed with a common ground in mind.