The BEAM Circuits Collection is a
The SC (Schmitt Comparator) Head
A simple little design by Martin
Keen and Wilf Rigter
I'll eventually post an explanation, but in the meantime,
here it is:
Meanwhile, a slightly refined version (with flashing
LEDs to indicate "lock") was posted
a few weeks later:
For more information...
The discussion of this circuit starts here,
and pretty much wraps up with Wilf's presentation
of V4 of the circuit here.