Here's an interesting circuit that
creates a clock pulse for a 4017 from a flashing LED. The flashing
LED takes almost no current between flashes and thus the clock line
is low via the 1k to 22k resistor. When the LED flashes, the voltage
on the clock line is about 2v -3v below the rail voltage (depending
on the value of the resistor) and this is sufficient for the chip to
see a HIGH.
