This project started as an instrument for ['http://dorkbot.org/dorkbotcolumbus/Sched.htm#musicevent Sonic Tooth], our local Dorkbot music event. I was looking for something where you could put any sound into the mix.
As an instrument, it's basically a mash-up of an answering machine and a megaphone. It ends up being a small package with a loud punch, so you could use it to play the same sample over and over (and over...). It uses a solid-state recorder with a 120 second capacity (Winbond's ISD25120)
The really cool potential that I didn't explore in this project is the ability to control the recorder via a microcontroller. It looks like the recorder's memory is addressable via a 10-bit address setting. With that, you could have the resulting playback be driven by any number of environmental stimuli. I was looking at using an Amtel ATiny13 to drive it, because I only really needed a 4-bit address space, leaving 4 ADC lines for input. But, that's all to come still.
Step 1: The Design
You could think of this thing as the guts of a digital answering machine hooked to a portable PA system. However, instead of being limited to the outgoing/incoming messages, we have full access to the capabilities of the chip.
The chip is a Winbond ISD 25120p. It records up to 120 seconds of audio at a 4KHz sample rate. It's about the quality of an answering machine (surprise!), but coupled with the amp in the megaphone, it produces a very textured sound.
The design here is the reference design shown on page 33 of the chip's datasheet The only things I really added were a couple of status LED and a 7805 voltage regulator. I drive it off a 9v battery, but you might get longer life out of 4-6 AA cells. YMMV.
The capacitors and resistors above the chip are filters for the mic and speaker, as well as a gain-preset.
I soldered this design together, more as an excuse to practice using a new soldering iron. If you plan on making more than 2 of these things, plan on etching a board. This is a very simple design, and you could probably make a single-sided board. If anyone wants to design one, I'll post it here for you....
Step 2: If It's Too Loud, You're Too....
The output side of this thing is a cheap megaphone from Harbor Freight. Watch their sales, I see it come up every couple months for about $5 US (Normally in the $15-$30 range). The things I really like about this unit are a) the handle comes off easily, and b) the amp board both easy to get at and is completely self-contained. There's no remote switch in the handle like some other designs.
For a modification, I simply de-soldered the on-board microphone (you could reuse it in the design, if you want!) and soldered in a short piece of 24gauge CAT-5 wire. I soldered that to the RCA plug, enlarged the mic hole a bit with a utility knife (note, safety first!), and hot-glued it in place. Again, I'd use a smaller connector format next time.
Also, I'd secure it with cyanoacrylate (thin super-glue - check your hobby store and be sure to get Thin) and baking soda. It's amazingly strong - like concrete. To use it, put down a thin bead of glue, sprinkle in baking soda. When it's dry, repeat. Add a couple layers and it'll never come apart.
Step 3: Second Skin
I put this together in a hurry, so I grabbed a box that I got a replacement iPod shipped in. It actually worked out really well - the circuit board is exactly the right size for the pre-cut foam. I just cut a hole for the 9v battery and a couple holes for switched.
I would have liked to mount the CE switch (lower right), but I ran out of time. As it was, it actually worked better. It made the device easier to handle while moving toward an audio source. In the next design, I'd add a 1/8-inch microphone port to use a PC-style mic, I think.