Op Amp Circuit

Discussion in 'The Projects Forum' started by BobW55, Mar 19, 2015.

  1. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    I am attempting to build a UV sensor for my weather station.
    See thread: http://forum.allaboutcircuits.com/threads/op-amp-zero-off-set.108284/
    Because the amp and sensor I was using was pre-made (Chinese) I have no schematic or specs on the circuit.
    I decided to back to square one.
    I was going to use a 741 op amp, and the sensor portion from the sensor/amp I already have.
    Could some one please have a look at my schematic and let me know if I am on the right track.
    Supply voltage =3 VDC
    Output from UV sensor = 0-1 VDC
    Output from Op Amp= 0 to 2.5VDC; 150 mV per UV Index.
    Here is my rudimentary schematic: [​IMG]

    Thanks
    Bob
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The crummy, very old, very primitive 741 will not work on only a 3V supply. Go read the data sheet.

    Use a modern, CMOS, low-voltage, rail-to-rail-in, rail-to-rail-out opamp.

    I dont understand your amplifier. Why do you have dc blocking capacitors in the feedback path? Are you trying to build a comparator that has a pulse output?
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi Bob,
    Look at the datasheet for the MCP6002 dual OPA, rail to rail output, the 741 OPA is obsolete.

    If the UV sensor gives 0v thru +1V, what level of OPA voltage output are you requiring.?

    E
     
  4. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    Was only going to use a 741 due to having one in my junk drawer. and after seeing my post I note that I have the caps wrong, should go to ground. (noise suppression)
    Looking at the 6002 sheet now.
    The final output I need from this is 0 to 2.5 volts DC.
    Yes the 6002 would be a much better fit for what I need.
    I would just use the sensor IC from this thing and do a Current to Voltage amplifier, but it is surface mounted and I do not think I could get it off and remount it and have it survive.
    So I am stuck with the 0-1 volt pre-amp built into the board it is mounted on.

    Thank you for the advice.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    use a CA3240 op amp
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is the basic circuit. You get to put in bypass caps, pin numbers, specific opamp, etc:

    178.jpg
     
  7. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    Mike,
    Most interesting, thank you.
    What is the program you used to model the circuit?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Read my signature line.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    its LT spice
     
  10. BobTPH

    Active Member

    Jun 5, 2013
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    Almost, but not quite:
    Bob
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    if its 3V working then try a TLC272 dual op amp, or TLC271 single op amp.
     
  12. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    MikeML,
    Downloaded the spice program, very interesting program. Still reading on how to properly run the simulation, but getting there. Thanks for the tip.
    Still searching mouser for parts, pretty much ran out of everything.
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you had 5V or more to power the opamp, you could even use a old LM358. It is rail-to-rail in, and will pull its output pin very close to 0V.

    Here is the .asc file for your prototype. It is a "swept-source .DC repeated-solution". One it runs, you can plot any node voltage or branch current vs the swept input...
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is your circuit again. This time I am using LTSpice to see what happens to your circuit using a real opamp as the supply voltage V(dd) is stepped from 2.5V to 3.5V. Note I am using double stepped voltages, one to simulate the sensor, the other to step the power supply.


    178a.gif
     
  15. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    Thought I would provide some follow up.
    I rebuilt my own amp board using a L7611 Op Amp.
    Unfortunately I must have damaged the UV detector when I went to remove it from the other board. Dang thing was soldered on top and bottom. Normally the UV sensor would put out 0-1 volt from a 3 volt supply. Now the readings just jump all over the place.
    So project is on hold to I find/get another Sensor.
    Thank you all for the help. I will report back when I get a new sensor.

    Bob
     
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