Trouble with Schematic Diagram - Distance measurement using radio waves
A &amp; C are essentially set/reset latches.
1C13A  &amp; IC13B form a cross coupled set-reset latch triggered via pins 1 and 6 (active low).

C6 is discharged at power on and holds IC13B pinn 6 low via D3 while R3 charges C6. Once charged the "low" is removed.
This is a 'power on reset.
S1 on sets IC12A input high so 1C13A low so RS latch toggles.
This affects LED1 and "lots of things" via IC12C.

The circuit at "C" behaves in much the same manner.



"B" is essentially a many bit binary divider chain which can have a 2^N divider output selected by 'jumper'.

A 4040 datasheet here is a 12 stage binary divider/counter. All divide stage outputs are accessible externally. The three 4040s = IC14 IC15 IC16 are chained in series to provide a 36 stage binary divider.
The two adjacent sideways "D" symbols are jumpers or switches which allow a desired divide output to be passed on to IC12F thereby resetting the RS latch in section "C".