Here are some projects I've built, including a ChuMoy Headphone Amp from TangentSoft, and quite a few others.
In the style of Chu Moy
Originally a project from Chu Moy posted on Headwize.com. The design goal is to use a single low-power, low-noise opamp to drive lower impedence headphones with they typical iPod. This version uses the trusty Burr-Brown (now TI) OPA-2134 op-amp. The virtual ground circuit is provided by a TLE2426 "rail-splitter".
Typical enclosure is an Altoids Tin, which I've also done. But here, for everyday use, I put it in a plastic enclosure with a battery door. The first generation iPod Shuffle in the photo has passed on, but the headphone amp keeps working.
RF Activated ON-AIR Sign
A Professional Style On-Air Sign for Amateur Radio
This was from an August 2004 QST article.
A commercial 'On Air' sign can cost well over US$100. This is an easy to build amateur version. You can use any sort of lamp, since it's relay-driven - a 12-volt car lamp, switch a neon sign. I use an array of Red LEDs to illuminate a transparency.
The sign responds to the slightest RF, including an Part-15 AM-Broadcast band transmitter I occasionally use. I've recently had to replace the two transistors that turn on the relay to light the LEDs - the sign had become 'sticky' and didn't want to turn OFF after PTT was released.
Some Other Projects