I have a rubidium frequency standard which has an 1pps output pin. It provides 1 uS length pulses of +5V. I tried to hook up this pin to LED through a 100 Ohm resistor and also tried to use a transistor as an amplifier to avoid pin overload, but I cannot see LED blinking. My guesses are - my brain/eyes are not fast enough and LED actually lights up, or, second guess - during that 1 uS frames not enough energy goes through p-n junction.

My question is - how can I drive LED with such pulses?

  • \$\begingroup\$ 1 uS per second means that it's on for one-millionth of the time. You'll need more than a transistor, in order to extend the pulse to cover more time in each second. \$\endgroup\$
    – nanofarad
    May 3, 2016 at 22:42
  • \$\begingroup\$ You need a pulse extender - a circuit that is triggered by the pulse and stays on for a set time. \$\endgroup\$
    – Transistor
    May 3, 2016 at 22:43

1 Answer 1


The LED is indeed blinking on and off with the pulse, just too quickly for your eyes to notice it. The solution is to put a pulse extender circuit in place -- in other words, a monostable multivibrator.

Your garden variety 555 monostable will do the job with an inverter driving it, albeit barely -- you'll need to make sure the input to the 555 is driven hard LOW in order to get close-to-reliable performance out of it, as you are very close to the limits of the IC here.

A more reliable solution would be to use half of an 'HC122 or 'HC123 -- this gets rid of the inverter as you have an active-HIGH trigger available now, and is guaranteed to work worst-case by the datasheet specs of the IC.


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