A simple 555-based AM transmitter.
Here’s a quick video of the circuit in action, transmitting Lullaby/Колыбельная by Аіда Николайчук:
There are quite a few circuits (see links below) that use a humble 555 timer chip as a medium-wave transmitter. Here’s my variant. It doesn’t get much simpler than this!
The 555 is used to generate a carrier wave, with the pot adjusting the frequency between the extremes of 64.8kHz and 648kHz. Since R1 is comparatively small compared to R2, the duty cycle is kept close to 50%.
The timer output (pin 3) drives the base of an NPN transistor which switches a signal to nowhere .. or at least just down a length of wire acting as the antenna. I used a BC547, but any small-signal NPN would work.
The audio input modulates the output by wiggling the base voltage around a 10nF capacitor link to ground.
Since the circuit is essentially modulating a square wave, the carrier is not particularly selective and the result is quite a broadband signal. There’s no trouble picking up the signal on an AM radio above and below the natural and harmonic frequencies. Well, we get a signal, but lots of noise also … but it does work!