Pulse counter control why?

Discussion in 'General Electronics Chat' started by BReeves, Nov 5, 2013.

  1. BReeves

    Thread Starter Member

    Nov 24, 2012
    I'm building a replacement control board and my curiosity has me wondering. The existing unit has an event counter controlled by an old Dallas micro controller that to my thinking is using reverse logic. Please refer to the attached schematic.

    Looks to me like they are holding the opto on then turning it off to count. Can anyone think of any logical reason to have the opto LED on then make it go off to fire the counter?

    My controller will probably fire the LED in the optoisolator directly when it needs to advance the counter. Makes no sense to me to hold an output high when all you really need is a short pulse. Any ideas on why an engineer would design it the way it is? I'm just a self taught technician and wondering if I might be missing the reason it works the way it does.

    Last edited: Nov 5, 2013
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    Yes, it is maybe a bit unusual, but the circuit does result in "normal" logic between the input and the counter relay.
    When the signal from the MCU is low, Q1 is Off, the opto is On through the 1k resistor.
    With the the OPTO On, transistor Q2 is turned Off, so the relay is de-energized.
    When the input signal is high, the relay is energized as expected.

    Maybe it was done this way as a kind of fail-safe ?
    You could always re-wire the opto to work more conventionally.
    Last edited: Nov 5, 2013
  3. BReeves

    Thread Starter Member

    Nov 24, 2012
    Thanks for the reply,

    I would wonder why they would want to leave an LED lit and turn it off to pulse the counter. My controller will only pulse the led when it needs to count.

    This brings up another question in another part of the controller. One piece of hardware it controls needs 5 volts applied to something (no schematic) to be enabled which is always, unless something goes wrong and it needs to be disabled. This 5 volts needs to be isolated and I will use an opto to control it.

    Guess what I'm asking (wondering) if it's perfectly OK to leave the LED in the opto on all the time. Do LED's not have a life span or is it so long that I will be dead before it quits working?
  4. crutschow


    Mar 14, 2008
    If operated within their ratings the LED lifetime is likely to be many years of continuous use.
  5. BReeves

    Thread Starter Member

    Nov 24, 2012
    Thanks, my old age and mind set of learning most of what I know about electronics before transistors were common starts showing sometimes :)
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    I've had a red LED run continuously for ~10 years (88,000 hours), by which time its brightness was roughly half that when new.