Anyone made an optical interface for their Fluke Scopemeter?

Discussion in 'General Electronics Chat' started by bluebrakes, Feb 29, 2012.

  1. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    I've been following a few websites into making my own homemade optical connector but have found that mine is a bit sporadic if it works or not.


    [​IMG]

    Original Link
    http://omapalvelin.homedns.org/electr/fluke-cable/


    I've tried using the above circuit on a laptop with a dedicated COM port (which so far vaguely works) and on a much more up to date laptop with a USB to RS232 converter.

    I'm about to try the CP2120 USB version but it would good if anyone has any experiences to share on this...
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    Did you measure the voltages accross the two capacitors?
    The voltage might be to low for a correct working of the circuit.
    You could try to use an extern ± 12 Volts powersupply.

    Bertus
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Do you have an oscilloscope so you can observe the waveforms?

    Some hysteresis on IC1A may help. Connect a resistor (experimentally test value, perhaps 50kΩ to start) between IC1A-1 and IC1A-3.

    You might also try reducing R2 to 5k ohms or so.
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Have you measured the voltage on pin 2 of IC1? It should be somewhere near 0V, but tweaking it might get better results for you. If you could view pin 3 with a scope, you'd know better where to set pin 2 voltage.
     
  5. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    thanks guys... Did some measurements this evening:

    Between ground and...

    Pin 1 : -6.8V
    Pin 2 : -3.7v
    Pin 3 : -6.7v

    When there is a signal (TV remote)...

    Pin 1 : -6.8v to 2.2v (a big question mark hangs over whether this is enough to change state to high).
    Pin 2 : -3.8v

    Pin 8 : 6.6v
    Pin 4 : -7v

    Probes measuring between pin 4 and 8 : 13volts

    I'm wondering if the voltage output is enough to trigger a high state in the computer's port. Shouldn't this be at least 3 volts?
     
  6. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    I've just tried a 4.7 and then dropped to a 2.2k resistor and it seems to be alot better in establishing a connection. Equally, when there is a signal, pin 1 peaks at 4.7v (low at 5v).
     
  7. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    It kind of works, although connection keeps dropping and won't work with a USB to RS232 adapter (prolific or CM340).

    but here is a screen grab when it does work... (one I downloaded earlier in one of the attempts it half worked)
     
  8. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    i just tried a 50K hysteresis resistor between 1 and 3... sadly just the same results.
     
  9. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    I managed to get another screen grab, this time of a remote control being fired into the IR photo transistor.

    I'm measuring between pin 1 and 4. Any help?

    So far i've tried a lower value R2 and a hysteresis resistor between 1 and 3.
     
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  10. Quantumechanic

    New Member

    May 6, 2012
    1
    0
    I built the circuit at http://omapalvelin.homedns.org/electr/fluke-cable/ several years ago and found that it was very unreliable, the main problem being with laptops and USB/Serial adapters, they just dont produce enough voltage on the serial interface.

    Although RS232 will work at the 5V level, in practice the voltages produced by the Diodes and Capacitors fall far short of this and not able to power the comparator etc.

    So I solved the problem by building this one from YouTube http://youtu.be/9fvNy7Vitys if you read the description there is a link to the circuit diagram.

    I tested this on several laptops and the notoriously useless Belkin USB -serial adapter without any issues.
     
  11. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    Thanks for the reply. I too have solved the problem, although I kinda cheated to some extent. I purchased a CP2102 USB to TTL converter off of ebay and then made this basic converter circuit with it...


    [​IMG]

    [​IMG]

    [​IMG]

    It works really well and finally I can use speeds of up to 9600K on the 99B flukemeter.
     
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