High Speed LED Strobe

Projects General Guidance
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You can get a 62.5ns pulse if you reduce OCR1b to 0.

62.5ns 500kHz
62.5ns 500kHz1024×600 10.2 KB

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You went wrong when you've taken on a task you don't understand. You've been told countless times that you can't get a pulse shorter than 62.5 ns on a standard Uno R3 board, yet you continue to post code on the forum that you don't understand a single line of.
The same goes for the fact that the TRANSISTOR you've chosen can't operate at that frequency.

Why ask questions if you're going to ignore the answers?

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You stated that you had a sketch which was never posted properly probably because you never read there forum guidelines. A sketch does not infer a schematic which you were asked for several times. What you want to do would require external hardware which it appears you now have but in the context of your question the processor is not fast enough. I searched on line and got this:

A 0.5 ns monostable (one-shot) pulse is extremely fast this far beyond what normal TTL or CMOS logic like 74HC123 or CD4538 can handle. Those parts typically have rise/fall times in the 3–10 ns range.

At 0.5 ns, you’re working in the microwave domain (about 1 GHz equivalent). You won’t get there with standard logic ICs. Here are practical options:

  1. Use ECL / PECL / CML logic families.

    • Examples: MC10EP195 or MC100EP195 (programmable delay chips).
    • Transition times around 200–400 ps.
    • Requires controlled-impedance PCB layout.

  2. Use a Step Recovery Diode (SRD).

    • Generates sub-nanosecond edges.
    • Commonly used in pulse generators and sampling scopes.

  3. Build an Avalanche Transistor Pulse Generator.

    • Classic and inexpensive way to get ~0.5–1 ns pulses.
    • Uses small NPN transistors such as 2N2369A or 2N3904 run in avalanche mode.
    • Expect rise times of 200–800 ps with careful layout.

  4. Commercial Pulse Generators.

    • Avtech AVR-E3-B: adjustable 250 ps to 5 ns.
    • Picosecond Pulse Labs 10 000 series: typical rise times around 100 ps.

In short:
No standard monostable IC can do 0.5 ns directly. You’ll need ECL-class logic, an avalanche circuit, or a specialized pulse generator. At these speeds, cable impedance, PCB trace length, and terminations become just as important as the device itself.