Optical Switch Troubles

Discussion in 'General Electronics Chat' started by pntrbl, Oct 22, 2009.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I have an optical switch (Optek OPB831W55Z) hooked up but I'm not getting the expected results. It's switching when the slot comes by, but only down to 6v from the 12v I'm running it on.

    It's entirely possible I'm interpreting the datasheet wrong, but about all I know to watch out for is max current ratings. The rest of the sheet looks kinda greeky to an amateur like me.

    The diode side has 50mA as absolute max so I selected 470ohm for a current limmiting resistor. The phototransistor is rated up to 30mA so I went with a 330ohm resistor there.

    Maybe I selected the wrong aperture? The 55 in the P/N indicates .050 apertures and I'm running an .060 slot thru it. I'm looking at it in a stationary position at the moment tho. The slot should be lined up for full shine.

    But only 6v.

    It occured to me maybe the current limiting resistors are built in, altho there's nothing in the datasheet to suggest so .....

    Who knows? But I would be very grateful if one of you wizards could point me to the error of my ways.

    Thanx

    SP
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    No datasheet posted.

    Making a wild guess, I'd say your collector resistor was too low in value.

    Without more information, you'll just be wasting our time.
     
  3. hobbyist

    Distinguished Member

    Aug 10, 2008
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    I'm just throwing a guess out here.

    Since no one else is answering right now I'll try to see if I can help somehow.

    I don't have much experiance knowledge about the kind of switch your using, but,
    it sounds like an interupter?

    Does this set up use the photo transistor as an inverter, and are you trying to drop the votage at the collector down lower than 6 volts?
    By using the 330 ohm resistor as a collector resistor?

    If so then 1. the input from the LED may not be bright enough to give the right amount of base current.

    or 2. the collector resistor (if its the 330 ohm resistor) maybe too low, and the transistor is switching into it's linear region instead of saturating.

    This probably isn't much help, unless you can give specific details of how the transistor is used as (inverter) or what, and where is the 330 ohm resistor at (the base or collector, or emitter of transistor)?
     
  4. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I wish I knew how to link the datsheet, but I don't. Dummy on me.

    The 330 ohm is on the collector. The emitter is on Gnd. NPN. I believe the phototransistor would be functioning as an inverter in that I'm expecting it to turn on and take the collctor "low", maybe .7v? I don't really know what the word inverter means in this particular circumstance so that's pure conjecture on my part. Somehow I think googling "inverter" isn't gonna help either. Too much info .....

    On the diode side I have a 470 ohm between 12v and the Anode. Cathode to Gnd.

    There are no suggested schematics in this particular datasheet. Those help a schmuck like me a lot, but it is what it is.

    SP
     
  5. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Jeez, I'm sorry! Turn those resistors around! The 330 is on the diode and the 470 is on the transistor.

    Such as it is, my thought process was keeping the max current for either under the max current ratings listed in the sheet.

    SP
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Alright you want 0.7 v. at the collector, I'm not sure it can get that low,
    but calculating then, 0.7v. at the collector would leave 11.3v. across the 470 ohm collector resistor. That's a current of 24mA.

    The base current depending of the hfe of this transistor has to be substantial enough to get that collector current.

    Like I said the LED may not be bright enough to get this result.

    Suggestion:

    Experiment.

    Use a LED resistor (current limiting) that is allowable maximum current.
    Then shine this on the photo trans. t6hen start raising the collectoer resistor, to find out how low you can bring this collector voltage. You may not get it as low as your expectimng, but then youll know how low it will go and then you can design the rest of your circuit with this voltage value as a reference, to work off of.

    Then you;ll know what component values to use.
     
  7. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    The max current for the phototransistor is listed as 30mA. 24Ma should be good.

    I'm gonna man up and eliminate the 330 ohm on the diode. I put that on thinking to limit the current to below it's 50mA max, but maybe all I'm doing is killing the light.

    What's the worst that could happen? A little smoke? LOL! I got some spares just in case ....

    SP
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Take it slow,
    All LED's need a limiting resistor.

    Don't use up ALL your spares just work it logically,

    the intensity of the light may stoill not be a key factor, it has more to do with the transistor current amplification HFE.

    Follow what I suggested in my last post. Work with the transistor resistor, to see how low this output is capable of going.

    You maqy have to amplify this photo output with another inverter with a higher gain, transistor.

    From my experiance photo transistors are good sensors, but they themselves need to have there outputs amplified.

    Depending on the kind of photo trans. they are.
     
    Last edited: Oct 22, 2009
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Make the pullup resistor from the collector of the photo transistor about 10K, and try again. The current transfer ratio for most of these interrupters is < 0.4, so 20mA in means about 5mA out. With a 12V supply, the lowest value of collector resistor would be 12V/5mA = 2.4K, so try something higher than that.
     
  10. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Thanx guys. I got a 1K on the collector resistor now and it's coming down to 2V. More experimentation coming up ....

    SP
     
  11. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Got a 4.7K up now and it's down to .25V! My troubles are over. :D

    I don't really need to know, so if no one wants to bother that's OK, but how on earth is it getting below .7v? Isn't there a diode junction in there?

    SP
     
  12. hobbyist

    Distinguished Member

    Aug 10, 2008
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    The 0.7v. your thinking of is the diode Vbe, the emitter and collector are not connected as a diode junction with each other.

    The diode junctions are from base to emiotter and base to collector.
     
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