Pulse width modulation using the control input of the 555 timer and restoring the original wave through filters. Here the PWM frequency is around 100Khz and the restored signal is not very accurate but since the AM spectrum is 5-10x higher the signal will be reproduced more accurately. You can actually use the 555 timer and the PWM as an AM transmitter. Filtering and detecting are quite similar between these two modulation schemes so you shouldn't run into trouble. Moreover The detection/restoration schemes are quite similar between PWM and AM. So if you max out the 555 timer you should cover half the AM spectrum (550Khz-1.5Mhz) and transmit this. Using a normal AM radio you should be able to detect it and Restore it. This circuit holds one more perk over AM. AM uses the amplitude of the carrier to transmitt information thus making the signal susceptible to electromagnetic interference and deteriorating the quality but this little gizmo encodes the information in the width of a pulse making it closer to FM than AM where this problem doesn't occur. Here's a link to a similar AM transmitter via PWM circuit i found in youtube. http://youtu.be/Co7JIoJivl4
