If your introduction to digital electronics came more years ago than you’d care to mention, the chances are you did so with 5V TTL ... on this circuit that use diodes instead of a resistor ...

Notes: The 280nm LED has a forward voltage drop of between 5V and 7.5V depending on the voltage bin. This is higher that I expected when I designed the circuit, and it will ... 500mA , which is set by ...