[Newbrain] had a small problem. He’d turn off the TV, but would leave the sound system turned on. Admittedly, not a big problem, but an annoyance, none the less. He realized the TV had a USB port that went off when it did, so he decided to build something that would sense when the USB port died and fake a button press into the amplifier.
He posted a few ideas online and, honestly, the discussion was at least as interesting as the final project. The common thread was to use an optoisolator to sense the 5 V from the USB port. After that, everyone considered a variety of ICs and discretes and even did some Spice modeling.
In the end, though, [Newbrain] took the easy way out. An ATtiny 84 is probably overkill, but it easy enough to press into service. With only three other components, he built the whole thing into a narrow 24-pin socket and taped it to the back of the audio unit’s wired remote control.
The seventh post contains the code for the CPU. It isn’t all that difficult or exciting, but the thought process of evaluating FETs and logic ICs against a cheap CPU is entertaining and maybe even instructive.
The amplifier’s wired remote acted like a potentiometer, interestingly enough, so it was a little different than what you would probably find on another piece of gear. We’ve looked at remote hacking several times. Unsurprisingly, the Arduino features in several of them — a small step up from the ATtiny84 used here.