Bought a new MP3 player

In the process of shopping around for more tubing stuff for the lights, I stopped at Goodwill Fashions (376 Jefferson Rd., in Jefferson Plaza) and found they had an MP3 player for $25 so I picked it up. It's a Memorex MMP8560 with 128MB RAM. With 4 times more RAM than the Digisette, I'll be able to put on more music stuff. However, the interface to get it to start is more complex so I'll probably need to use the computer chip to run it (whereas the old player needed one button to be pressed to start, the new one needs to be powered on first and then started with the play button.)

The good news is that it only draws 40 mA. If I use a linear regulator from 12 volts to get to 1.5 volts (it uses a single AAA alkaline, not a NiMH cell) I'll only have to get rid of around 450 mW of power as heat which was as good as I got with the buck converter anyway. (Remember that the rationale there was to pick a higher-than-audio frequency and the transistor tended to go linear. I might try the buck converter again anyway.)

Frying the stupid MP3 player

I spent about 3 hours working on the circuit to try and get a decent 1.2-volt output. I went through several transistors and some trial-and-error. Once I got to a point where I thought I'd see how things were working, I noted that the indicator light on the MP3 player was just staying on. I tried installing the battery but I got the same thing. Oddly, I can connect it to the computer and read the flash memory just fine. Hopefully it'll "heal" if I leave it disconnected for a while.

Powering the stupid MP3 player

I worked on the audio system some more. I had an unfortunate setback: the LM217 I have is apparently dead, so it won't be the nice 1.2 volt reference. However, the rest of the system works fine and I successfully played audio through the speakers. I figure I can use one of the op-amp's as a voltage regulator. I cut holes in the electronics case to accomodate the phone connectors for speakers.

I used a couple diodes in series and a 4.7K resistor to create a 1.2 volt reference … I'll feed it through an op-amp at unity gain to get a stable source for the MP3 player … maybe with some capacitors if the charging circuit is too noisy.

Unfortunately, I had problems getting 1.2 volts to the MP3 player and eventually had to swap operational amplifiers twice. I finally got it set up and discovered that the MP3 player draws 200 mA. It never dawned on me to check, but that's indeed what it draws when it's running off the battery — it's not too bad at 240 mW, but by using a linear regulator, I have to dump the remaining 2.2 watts as heat — I was hoping to get the whole system (lights included) to operate on less than 5 watts total, so this is a big waste.

I built a twin-T oscillator to feed an op-amp as a comparator to make a pretty efficient pulse-width-modulated power supply. I came up with a circuit that uses 0.002 microfarad capacitors all around, 1K resistors on the resistor-tee, a 470 ohm resistor on the capacitor-tee, and a 4.7K resistor on the collector. It creates about a 0.3V peak-to-peak 40 KHz sine wave. When I wired it up final, I had to do a couple swaps of capacitors (i.e. a ceramic capacitor marked with "223" is 22 * 10^3, not 2.2 * 10^3, so my 2200 picofarad capacitors were really "222") I only had a bunch of "102" (1000 picofarad) capacitors so I had to use 10K resistors on the other tee of the circuit which gave me a nice 40 KHz output again. (The problem was the 2200 pF capacitors were physically quite large and I wanted to pack things into a smaller circuit.)

I got the pulse-width-modulated output to work somewhat, but it pretty much just goes into a linear mode when I try to make it work and again draws 200 mA from the 12-volt supply. I had configured a second op-amp to provide closed-loop feedback to adjust the PWM output, so the voltage was right, but it looks like the op-amps just can't make a 40 KHz square wave and the transistor is running in a linear mode.

I took a quick crack at making a buck DC-DC converter and managed to get it working in short order. The circuit required 40 mA at 12 volts, and I know about 5 mA of that is for the rest of the electronics, so 35 mA at 12 volts is 420 mW — so with the MP3 player using 240 mW, I managed to get to about 50% efficiency. Unfortunately, even 180 mW dissipation on a little transistor is quite a lot so I'll need to get a bigger one with a better heat sink. However, I'm happy that things function!

Starting on the audio system

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.