Electrobalance problems (Right now on the ITR9608-F Optointerruptor)

Discussion in 'The Projects Forum' started by Cemre Tas, Aug 3, 2015.

  1. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    Howdy,

    I'm trying to get this electrobalance to work (Attached schematic image #1) and right now testing out the ITR9608-F opto-interruptor portion of it to figure out why I'm not reading anything. Image #2 is the schematic of the original design I was using but I was just reading a static 47-48uA regardless if I tried to block the light beam so I was assuming it was just reading the current from VCC (5V) to ground through the resistor. Image 3 was just me testing it in general and I am not getting any current output what so ever from this.

    Any suggestions would be great. Also the op-amp I'm using is LM358P but haven't gotten a chance to test that due to the interruptor.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Your meter is 'shorting' the opto transistor. Try connecting it in series with the 10k collecor load.
     
  3. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    I did so so it looks like the attached image, but I am still getting the same current output so there must be something I am missing since it doesn't seem like the interruptor is contributing any current. I have also noticed a small mistake with my original drawing, the 10k load is going to the emitter.
    I have also replaced the 150ohm resistor with a 10k Pot but I can replace that at a whiim.
     
  4. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    I found out what I was doing wrong, it was just really badly connected on my end. Attached is the correct way to set it up and here is a good website I found that helped out:
    http://www.utopiamechanicus.com/article/arduino-photo-interruptor-slotted-optical-switch/

    Right now I am currently messing around with potentiometer values to see if I can bring my 490 value down to a ~250ish range. If I get a satisfying result I'll post a video of it.
     
  5. Cemre Tas

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    Jul 14, 2015
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    I guess I'll use this same thread for another problem I've been having. In the schematic one I have posted the op-amp LM358P and I can't seem to get it to work which I'm sure is another mistake I'm having with wiring. Right now I want to have a simple non-inverting amplifier so I'm using the configuration of the attached picture.

    What am I missing?
     
  6. Alec_t

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    Sep 17, 2013
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    You're missing a power supply on pins 4 and 8! Also, unused input pins of opamps (and logic circuits) shouldn't be left floating. Tie them to ground or the +ve rail via respective resistors (e.g. 10k).
     
  7. Cemre Tas

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    Jul 14, 2015
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    I currently have 1 5v source to play with and have it being put into pin 2 and now directly into pin 8. I also went ahead and put pins: 4-7 to ground through a 10k resistor. However now I'm not receiving any sort of output at all. Pin 3 is also tied to ground so it's an inverting op-amp configuration.

    Before I tied pins 4-7 to ground my output when testing over some 10M resistor was 1.5V when it should be 20V.
     
  8. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    To follow up on this I did a quick circuitslab set up in the attached pic and the results were as pictured, I've attached 15V and ground to the rails but that did not change the outputs. This output does not make any sense since an inverting amplifier gain is:
    A = -(Rf/Ri) so the gain should be 1 and the output should be -5V and not <1V as shown. In practice I am getting 2.5V output without any voltage in the V+ (pin 8) and 1.5V output if I DO connect to the pin.
     
  9. Alec_t

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    Sep 17, 2013
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    Pin 4 is not an unused input pin; it's the negative supply pin and should be connected directly to ground.
    If your supply voltage is 5V you can't get more than 5V out! Most opamps have an ouput voltage at least 1V less than the supply voltage. The datasheet will give the figure.

    Edit: Our posts crossed.
    With a 15V single supply the output can't get below 0V. If you want negative output voltages you need a dual (positive and negative with respect to ground) supply.
     
  10. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    I think my old electrical knowledge is slowly bubbling up but let me see if I understand you correctly.

    In whatever configuration of op-amp I choose, summing amplifier/operational/invert/noninvert I need some sort of input into either the inverting or non-inverting leads which in this case are pins 1 and 2.

    If I connect a single +5V source into the +V pin, which is pin 8 and connect -V (Pin 4) directly to ground I am effectively telling the opamp that the possible output can only range from 0V to 5V, but it's not really 5V because it's 5V - some value that the LM358 takes off (I have the data sheet pulled up but can't remember the name of the thing).

    If I decide to use an inverting configuration the op-amp will output nothing because once again that is below 0. So I will need to replace ground of pin 4 with a -5V source to tell the op amp it can output from a range of (-5V + Voffset) -> (+5V - Voffset).

    If I increase it to -15V and 15V that means it can output an even greater range. Does this seem like a correct understanding?

    So on the very first schematic of this thread I have an opamp I'm using as a transinpedence amplifier, so it converts current to voltage and it lists Vbias. Does the Vbias serve a similar function?
     
  11. Alec_t

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    Both inputs need a defined input voltage (be it ground or supply or some intermediate voltage).
    For an ideal amp, yes. For all practical amps the output voltage won't quite reach either rail (the datasheet specifies the output voltage range).
    Yes.
    Yes.
    Similar to what? The specified LM358 input bias current is about 20nA, which needs a conductive path (via the input signal source) to ground.
     
  12. Cemre Tas

    Thread Starter New Member

    Jul 14, 2015
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    Alright and thanks for your help so far. Basically: http://i.imgur.com/Fp35k5X.png Is what I'm trying to figure out, if I change the current source from the 48uA to 8uA the output on the simulation changes but it does not in physical testing which leads me to believe that something on the op-amp is incorrect in terms of hookups or what not.

    Edit: This is how I am currently hooking up the op-amp in terms of the pin diagram of the LM358:http://i.imgur.com/w8wDP8k.jpg

    What I did notice is that when I do not have the +V connection hooked up to 5V there is a small change whenever I vary the current but that seems to me that the current is entirely bypassing the op-amp entirely since when I short the inverting input and the output (Removing the 100K resistor) the difference is greater.

    When I do connect the +5V to +V then the volt meter jumps up to ~4V and just stays there, varying the current does not change anything so I'm completely stumped on why it seems to ignore the op amp without +V and only wants to take +V voltage and ignore everything else when it is connected.
     
  13. Alec_t

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    Correct. An opamp must have a supply to work properly (just as you need food and a car needs fuel to work properly :)).
    With the non-inverting input at +5V (the 100k variable resistor won't change that significantly) the output will be stuck at nearly 5V.
    Here's a sim showing what I think you are trying to achieve. The yellow trace is output voltage (note how it doesn't quite reach either the +5V rail or the -5V rail) when the input current source (blue trace) goes from 60uA to -60uA.
    CurrentToVoltage.gif
     
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