The charging resistor, R1, limits the current to the transistor. You need a modest DC voltage to make the transistor avalanche so I used one of the primary windings on a small AC line transformer as the source for the ~150V DC supply. I built my version of this pulser into an existing timing box I had made before. Avalanch transistor waveform with ~2 ns FWHM Timing pulse returned from open end of a 20 ft. This is one of my favorite little bits of electronics because you get such great performance out of a single common 5¢ transistor that is abused for this purpose! Many bipolar transistors will exhibit avalanche breakdown if the voltage gets high enough on the collector, but you won’t find many that give a much better pulse than the common 2n3904.
When the time comes, this avalanche transistor pulse circuit will come in real handy.
You never know when you might need to measure the speed of light.
