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• Ran out of time short term

  Mirrorsails • 04/26/2014 at 12:31 • 0 comments

  My job and schedule have gotten in the way of finishing this project before the end of April.  Funny thing is after the first week in May I'll have lots of spare time.  It will get finished, but I don't want to rush the experience just for the contest.

  I'm hoping to find a piece of light gray slate tile of the correct size and draw the cave painting of the man pointing at the stars.  The painting will be done in charcoal pencil.  Then somehow I need to drill holes through and put LEDs in as the stars.  For each correct note played on the recorder (I had no space pipe laying around) another LED would light.

  I also need to bight the bullet and order some parts.  I got hung up on exactly which op amp would be ideal, rather than which one would work.  I need a dual op amp for the two stage active filter, and I'm thinking of the humble 741 for the stability of the V/2 signal.

  Thanks to everyone for their support so far.

• Single Supply Op Amps

  Mirrorsails • 03/22/2014 at 22:01 • 0 comments

  I had one of the learning experiences last night where you set out to learn one piece of the puzzle, and the answers to that one piece cascade into solving other problems as well. It started with not knowing how to bias an op amp with a single sided supply, and ended up showing me that you can create bandpass filters on op amps with single sided supplies that have output signals at V/2. So I can get everything in one shot!

  While searching I came across an application note from Texas Instruments, A Single-Supply Op-Amp Circuit Collection. Of all the pages and app notes I read, that one was the best I found at not only explaining single sided supplies, but showing how they were applied. On page 17 is a multiple feedback configuration of a bandpass filter.  I seem to have forgotten that it's possible to use the feedback from the op amp to filter.  Originally I was planning on a low pass filter followed by a high pass filter.
  
  

  Another cool thing I came across was Erik Cheever's Single Supply Op Amp Design. I know it's been a long time since I was in school, and as an ME I may not have been as interested in electronics back then, but I never quite got why I might want to use an inverting op amp rather than a non inverting one. This page explained that in an inverting configuration, there is no current going to ground. So it makes a better choice when you are using a single sided source because that current can wind up changing your reference voltage. It was like a light-bulb went off.

  Finally, I can across two online calculators that let you design filters. The first was WEBENCH from TI and the other was Analog Filter Wizard from Analog Devices. They show a lot more depth to the choices between configurations. Who knew there was roll off and ripple to worry about? It also looks like I might need more than one op amp, although I want to stick with one if I can.

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