I've been playing with an AM transmitter these days, built with nothing more than a NAND gate, a few resistors, a few caps, one transistor and a tunable coil. The NAND gate is the oscillator, and the transistor with the LC circuit provides modulation, filtering and power gain. The gimmick proved to be quite stable, working for 4+ hours on end without drifting at all as per my Yamaha PLL tuner. Better than a transistor LC oscillator, and certainly WAY better than a 555. I experimented with carrier current transmission (coupling the transmitter output to the neutral of the AC power line) and was able to achieve good coverage around the house. That's quite a feat for the puny 10mW that this thing puts out! Heck, that's even within legal limits for AM.
I no longer have an oscilloscope and my multimeter only goes up to 50MHz, so it wouldn't be wise to try to hit the FM band (plus i have a car modulator i can use for that anyway), but i could aim for shortwave. Trouble is, i had given away my last "world band" radio 2 years ago! Since i was out shopping anyway, i remembered a store at the bus stop which sold cheap crap like watches, clocks and radios.
For $7.69, i got myself a pocket radio with LCD display, world band, AND alarm clock!
They even had radios with a USB/SD card MP3 player built in for $15
, but they were kinda big.
The radio is branded "Interational". Knock-off... Surprisingly, i actually found out who makes them, it's a Chinese company called Kaide/Kchibo, and they even have a website, including PDF manuals.
This radio has "KK-848" written on it as the model, but the manual that came with it (in Engrish of course) was for "KK-9704" which appears to be an earlier model with 7 SW bands, mine has 8. I even found a review of a "KK-9702" here
Surprisingly, it actually sounds decent on headphones, it's got some bass to it. The volume knob goes way up into overdrive, as usual for those Chinese radios. Reception is stable, but the LCD readout is .2MHz off on FM. Just like the last one i had.
Looks like there's hacking to do. I also agree with that reviewer, the band indicator and tune LEDs are way too bright and just waste power. I don't want those shining in my face and eating up my battery, so i'll tone them down. But hey look, there's even a button for LCD backlighting! That's a new one... Unfortunately it's a lame little green LED in the top-left corner, which doesn't really do anything. At least they tried.
I'll be back with pics and hacks.