Schmidt Trigger not Outputting Correct Value on Power Up - why?

Discussion in 'The Projects Forum' started by Mach3Maelstrom, Apr 11, 2013.

  1. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
    10
    0
    I'm building this circuit:
    [​IMG]

    I want the Schmidt trigger (first op amp) to begin by outputting +5V, creating a negative voltage ramp at the other op amp. When I power up the device, I have a 100R resistor in parallel with the capacitor so the ramp stays at 0V until I hit a switch.

    My challenge: the Schmidt trigger will sometimes output +5V and sometimes -5V -- I need to make it only +5V on power-up.

    I know it's stable in either direction and eventually switches when the voltage ramp goes high (or low) enough, but I need it to consistently output +5V every time I turn power on.

    Please let me know if you have any suggestions. I've been wracking my brains for a solution. :)
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Connect a capacitor from the +5 to the + input of the first trigger...then a resistor to ground.

    If that interferes with your input signal, use the principle of a switch-on capacitor with a transistor to inject the stimulus somewhere else.
     
  3. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
    10
    0
    Like this?
    [​IMG]

    According to LTspice, it still outputs a -5V, but I haven't tested it out on a bench yet. I'll do that shortly, I just want to ensure I understood you correctly and also why these adjustments work.

    Also, for clarification: Won't the +5V/capacitor network mess with the system if OUT begins to go positive?
     
  4. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Gee, I didn't see all those components that were not in your first post.
    Yes, C2 will change how the output of U2 affects the circuit. Some op-amps even refuse to work into a pure capacitance. This will have to be re-evaluated.
     
  5. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
    10
    0
    Hi #12: Sorry for the confusion I may have caused. You can't use pots in LTspice (at least not smoothly), so I eliminated the pots (which were configured as variable resistors, anyway) and replaced them with simple resistors. Also, you must use those big, bulky voltage sources in LTspice, whereas in EAGLE, it's just a small +5, -5 indicator. Again, sorry if my second pic was confusing.

    In essence, replace R1 as an 8M pot and R6 as a 15K pot in the picture above and that's the system I have. Those voltage sources are also +5V & -5V... I just adjusted them for a quick "what if" experiment and forgot to change them back.

    I've been able to get the first op amp to consistently output +5V in the past, but when I re-wired it it suddenly "stopped working"...
     
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