LED delay so flash is visible

Discussion in 'The Projects Forum' started by oleskiw, Jul 23, 2013.

  1. oleskiw

    Thread Starter New Member

    Jul 23, 2013
    Hi, I'm new to circuits and need some help!

    I have a parallel port sending data to a machine, the data is sent in strobed word format (set data pins, then flash pin1 lo hi lo to store the word). I'd like to put an LED inline on pin1 so that I can see when data is being written. The problem is that the strobe flash is so quick that you can't see the LED light up. Is is possible to have the strobe flash trigger a circuit that will drive an LED for 10-20ms?

    The strobe pin1 is 0v low, 2.3v high, about 20mA, and I have a 5v line coming in to power this and other stuff. I know logic circuits but almost nothing of electronic components, so if you could please describe whats going on and the components that are responsible for the timing values I'd be very greatful.

  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    You should be able to use the strobe to trigger a 555 monostable multi-vibrator, and have the indicator LED connected to the output of the 555. Delete SW1, R4, and C4, connect the strobe to pin 2, and choose R1 and C1 to give the length of pulse you want. Note that the monostable will trigger on the falling edge of the strobe. It should run fine on 5V, but you may need to reduce R3 to about 470 ohms.
    Last edited: Jul 23, 2013