Unfortunately I'm a bit late for the 555 Timer Madness competition, but seeing another entry (555 frequency synthesizer) inspired me to try to build a frequency synthesizer only using 555 timers. As said in that article, the 555 can be used as a VCO by applying a DC voltage to the CV input on pin 5. It is also know, that when in monostable mode, the 555 is non-retriggerable. This allows it to function as a simple frequency divider. To produce a divide by 10, the monostable period is set for a little over nine input pulse periods. The tenth pulse then triggers it again and the output is the input divided by 10. The heart of the 555 timer is a set-reset latch. This can function as a phase comparator. Suddenly we have all the component of a frequency synthesizer. The circuit could definitely do with some optimisation, and there was a fair bit of fiddling around to get it to synch to an input signal, but here is the circuit where IC1 is the VCO, IC2 the divider and IC3 the phase comparator. R6 and C4 are the loop filter. Also included is a photograph of the prototype on my breadboard and a screen grab from my oscilloscope where channel 1 (yellow) is the output and channel 2 (green) the input.
