Op Amp - Floating after High or Low?

Discussion in 'The Projects Forum' started by Scucci, Jun 19, 2011.

  1. Scucci

    Thread Starter New Member

    Jun 19, 2011
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    I had this working last night... I so HATE having to ask this question... but since a teardown and putting everything back on the breadboard, I can't get it to work again.

    I had a signal coming from the 4th op amp on an LM324 triggering the circuit to oscillate, then float. I should've written down what I had working because I've been tinkering all day and I can't get it back.

    I would move an object over the photodiode, it'd trigger the op amp to go high, that was routed to the spot shown in the attachment and everything worked. I've been try'ng to get it put back together how I had it since last night, no luck.

    The signal has to go high (was high last night), then float.. if it stays high (+12V) only one half of the LEDs light, if it's low (-12V) only the other half lights, if it's 0V then none of them light.

    Any ideas?
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Your LED driver should not be wired like that. Take a look at the attachment.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Your LEDs could be smoked, there is no current limiting I can see between the -12VDC and +12VDC. They may be saved by the fact you have 5 per 12V side. However, if these LEDs drop 1.5V they would be toast, 5 * 1.5V = 7.5V with no current regulation. Even 2.2V Vf (Red LEDs) is iffy, since 2.2V * 5 = 11V. Resistors in series with an LED chain are not optional.

    You also have a problem with IC1D. It is extremely poor design to leave unused op amps and gates just laying about. Instead, connect pin 14 to pin 13, then connect pin 12 to ground.

    LEDs, 555s, Flashers, and Light Chasers
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    None of us ever have the "I built it once, but I can't build it again" syndrome. :p Last time this happened to me, I swore that the next time I built a prototype circuit and it was running, I'd take nice detailed photos of the layout from a number of different directions. 20/20 hindsight...
     
  5. Scucci

    Thread Starter New Member

    Jun 19, 2011
    2
    0
    I -think- I figured out a little of what I did. Looks like I might've put the wire to the "trigger" on the breadboard in the wrong place. Instead of putting it to the output of the last op-amp, I might've put it to one of the inputs, which was getting some signal from the photo diode... but I still can't figure out how having a higher resistance made the photo diode more sensitive the first time around. I'm not good with op-amps apparently.

    The LEDs seem to be handling everything fine (without a resistor with them), but since they're only on for a few seconds at a time... I can't even begin to imagine how much time it would take to notice any color shift in them (what I use to see damage) so it'll probably be best put something inline with them. Also one side gets a bit more juice than the other side, another "LED's worth", so the numbers aren't even and that pisses me off (for lack of a better term). So, if you were going to put in a resistor... that's the side to do it one... IIRC it was on the "-12V" side. So, between the 390ohm resistor and them would probably be the place.

    After playing around with it a little though, I've come to the conclusion that there is only so much I can do with it as it is. I want to put the whole shebang (several shebangs) under frosted glass and still have it react to motion or having something resting over it. As it is now, there's no good way to do that (that I can figure out) to have it be sensitive enough to trigger from motion, and not trigger from just having the glass over it. Saving space is important because I want to get some PCB's made and using as little space as possible is nice because as the size of the PCB goes up, so does the cost. :(

    I'm pretty much settled on using a microprocessor programmed to handle everything now. As much as I hate some of the "Arduino" stuff (using a $30 board to make a light chaser when $3 in ICs and parts will do the same.) I think that's probably going to be the easiest route in the long run. That will give me exact control over the sensitivity of the photo diode, and solid control over timing and the pattern of the LEDs. So, I guess I'm going with Arduino since I can use it to easily test everything... then use the board to program the controllers and get them off the arduino board itself.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Dump the Ariduino and go Picaxe. You won't regret it.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Them's fighten words stranger!
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    My favorite western classic is "High Noon". So how do you want it, twenty paces or quick draw? :D
     
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