in the summer of 2016 I needed to tackle sequential circuits. So started the real fun ( hair pulling ). On a single flip-flop level I started with the basic R-S to J-K to the D. However when I tried to make these circuits toggle then I began to understand the problems of signal levels, timing and edge vs. level triggering. No matter how many searches on the internet I did, I could not get things to toggle or at least toggle without random errors. I had built a clock using transistors but was not sure of the signal purity ( The only test equipment I have is a analog vom and a transistor tester ). I made the call to allow one chip for this project and that is a trusted 555 timer chip. Did that help, No way. Finally I came across a circuit that converts the square wave output of my clock to a very short pulse. Now my flip-flops started to toggle the way I wanted.
Below is my program counter connected directly to the clock board on the left. There are four toggle D flip-flops on the right and note the pulser boards to the left of each one. Without them I would not have any hair left.