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1V vs 1.5V

Discussion in 'Electronic Basics' started by Test, Oct 1, 2008.

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  1. Test

    Test Guest

    I have a motion detector that I want to get info to my computer. I am using LPT
    port to input data. I could not follow what tripped motion. It has a LED which
    lights up when it detects motion. I measured 1V DC when its "off" and 1.5 when
    it's "on".

    What kind of cirquit should I have? On the LPT side a register is on when a
    couple of pins are short cirquited.
  2. Test

    Test Guest


    (English is not my native language btw)
  3. ian field

    ian field Guest

    It seems a bit pointless for such a sensor to not have an output such as a
    set of relay contacts, but if the only output is an indicator LED then the
    simplest possible interface is to replace that LED with the LED section of
    an optocoupler - that gives you an output in the form of a photo-transistor
    which is controlled by the LED, with the advantage that its also
    electrically isolated.

    If you need the LED indication then include the LED in the collector load of
    the photo-transistor.
  4. A simple comparator will do the job. Set it up so that it returns 5 volts
    when past 1.25V and 0 volts when below 1.25V. You can hook it directly to
    the parallel port.
  5. Jamie

    Jamie Guest

    Use an "Optocoupler" in it's place. it'll work perfectly.

    That should give you some idea's"
  6. Test

    Test Guest

    I have a couple optocouplers
    (o I
    could use. But frankly I don't seem to get anything out of them. From the spec it
    looks like I should apply 3V current to pin 1 and 2. In return I should get
    something out of pins 6, 5 or 4. What? Resistance remains full at 4 and 5 no
    matter what I do. Voltage is zero at them.
  7. default

    default Guest

    It takes 2.3 volts or so to light a red led so there's a good chance
    that there's a better place to be taking the output from the motion
    detector - one that offers more of a swing.
  8. ian field

    ian field Guest

    Pins 4 & 5 are not a voltage generator - they are collector & emitter of the
    photo-transistor illuminated by the LED on pins 1 & 2, connect the emitter
    to GND and collector to Vcc via a 4k7 resistor - the output is then on the

    If you've applied 3V to pins 1 & 2 you've most likely destroyed the LED so
    that optocoupler is scrap. The current through the LED must be limited
    usually between 20 - 50mA depending on type.
  9. terryS

    terryS Guest

    Who says Americans can 'speak' English? :) :)

    There was a war of separation (Oops 'Independence') couple of hundred
    years ago just to prove that it was so?

    So citizens of the USA are now classed as 'Aliens' in, for example,
    European countries!

    But the British, who I guess we can assume developed the language and
    have been working on it since long before Chaucer and Shakespeare
    etc. just talk louder when someone doesn't understand them!

    But 'Entente Cordiale' still applies and we remain, as some wit
    observed, "Separated by the one common language (English)".

    Regards to all from a mid-point, Canada! Where incidentally a
    significant part of the population speaks French; another European
    language. And after all if it's technical doesn't matter what language
    it's in. Eh?

    PS. Everybody applying for their passports?
  10. Test

    Test Guest


    Is this what I should:
    | |
    pin 1 -------+ +------ pin 6
    | |
    pin 2 -------+ +------ pin 5 <--- 4.7 K <---- Vcc (3V)
    | |
    pin 3 -------+ +------ pin 4 ------> Ground
    | |

    What should happen in pins 1,2 and 6?

    I am new to electronics. Thank you for your patience.
  11. ian field

    ian field Guest

    Pin 1 is the LED anode & pin 2 is LED cathode - a current between 20 & 50mA
    should be passed from anode to cathode to illuminate the phototransistor and
    switch it on.

    Usually pin 6 is the base terminal of the phototransistor but if the
    opto-coupler is a "circuit pull" from an old PSU it might have no internal
    connection to pin 6, for what you're trying to do that shouldn't matter.
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