What do these Diodes do?

Discussion in 'The Projects Forum' started by sirch2, Feb 2, 2013.

  1. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    The circuit shown below is for a 40kHz ultrasonic rangefinder. The output of IC2c (pin 8) goes to two diodes which I assume form a basic rectifier across the IC2a op amp inputs. The question is what does IC2a do and what should I expect the output to look like, given that the input is a 40kHz pulse?


    [​IMG]
     
  2. killivolt

    Active Member

    Jan 10, 2010
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    Looks like an Operational Amplifier. IC2A is a summer of IC2D and IC2C and IC2a. I think it controls the voltage & frequency of MA40B8R

    Inverting Adder is what I think you have (I'm most likely wrong). Not an engineer, wish I could help more but, it might bump your Thread. Someone who really knows may see my mistake and say either "I don't have a clue what I'm talking about" or hopefully say I'm correct just confused and
    explain it to both of us.
     
    Last edited: Feb 2, 2013
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    It looks like part of the automatic gain system. As the output of IC2Cmakes AC transitions, and the output is .7 volts more positive than 2.6 volts or 0.7volts more negative than 2.3 volts, the output of IC2A will toggle. (my voltages may be a little off due to the 100k feedback resistor used to provide hysteresis, but you should get the idea) When it toggles, the program in the TINY will alter the outputs on PA0,1&2 to adjust the gain of IC2D and the outputs on PA5,6&7 will adjust the gain of IC2C. At some point of equilibrium, the output of IC2A will cycle with a 50-50 duty cycle representing ideal gain.
     
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  4. killivolt

    Active Member

    Jan 10, 2010
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    So, then (ADC9/in TO/TO) is a comparator and adjust's PA0,1&2 and PA5,6&7 accordingly to reach the ideal gain.

    Then software uses a shift register taken from ADC9/in TO/TO << >> to change the output V Level of PA0,1&2 and PA5,6&7.

    I've been reading about this today I think I'm close but, not sure.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The circuit is an envelope detector. See attachment.
     
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  6. killivolt

    Active Member

    Jan 10, 2010
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    ahhh.

    Thanks, Ron

    Edit: Sorry, sirch2 I didn't mean to tread in your thread. But, I hope I asked some good questions and such.
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
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    Ron, I see the envelope function but how far off was I on the auto gain function? I work with amplifiers that have the transistor of an opto in place of the 3 resistors going back to the TINY. A peak detector and long time constant control the LED current to the opto.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Well, I am no expert when it comes to the entire circuit, for sure. The output of the envelope detector goes to an A/D input of the TINY26, and there TINY26 outputs that control the gain, so I'm pretty sure you are correct.
    In simulation, if the 40kHz amplitude is too low, the envelope detector output does not stay high for the entire duration of the burst. I suspect that the ADC output is monitored to see that the output is high for the duration, and if it is not, the gain is increased until it is, but this is speculative on my part.
    Notice that the envelope detector is the ONLY output of the amplifier. Where is the ultimate output of the entire circuit?
     
  9. killivolt

    Active Member

    Jan 10, 2010
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    I found this, I don't really think it explains it either. But, "Ultra Sonic Range finder"

    It's the same circuit, but at least you know what it's for ?
     
  10. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    Thnaks for the replies
     
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