Help designing a Schmitt trigger please

Discussion in 'The Projects Forum' started by eugen39, Mar 2, 2012.

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  1. eugen39

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    Mar 1, 2012
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    Here is what I am trying to achieve: a circuit that will have an output of 0V when the input is 8V and below, and 12V when the input is 12V. My limited knowledge tells me this is a Schmitt trigger but I don't know how to approach it.
    Any help deeply appreciated.
    Eugen
     
  2. Brownout

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    Jan 10, 2012
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    What is the output when 8V<Vin<12?
     
  3. eugen39

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    Mar 1, 2012
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    The input is either 0V, 8V or 12V
     
  4. eugen39

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    Mar 1, 2012
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    Would this make more sense: when 0V < input < 10V then output is 0V and when input > 10V then output is 12V ?
    Thanks
     
  5. Ron H

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    Schmitt triggers have upper and lower threshold voltages. What would you like these to be?
     
  6. eugen39

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    Mar 1, 2012
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    Lower threshold = 8V
    Upper threshold = 12V
    Thanks
     
  7. Ron H

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    You should probably leave a little room for voltage tolerances.
    8.5V and 11.5V might be better.
    What is the impedance of your source?
    Do you have a steady 12V supply available for powering the Schmitt trigger?
     
  8. eugen39

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    Mar 1, 2012
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    I do have 12V available and the circuit will drive a relay with an impedance of about 70ohm
     
  9. Ron H

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    OK, but where does the voltage come from that you are sensing?
     
  10. eugen39

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    My son and I are working on a project on a Lego Mindstorms. We are trying to build an add on and the output from the Lego is 8V in one state and 12V in another state but the motor we are using doesn't work well at 8V. I tried using a 12V relay and an external 12V power source to eliminate the 8V state but the relay is activated by the 8V source too. So basically we need that relay turned on at 12V but stay off at 8V.
     
  11. Ron H

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    Here are two ways to do it. The threshold is about 10V in both of them, with a few hundred millivolts of hysteresis.
    If it is unstable due to noise on the input, you can raise the hysteresis by reducing the value of R5 to a value as low as 200k.
    The relay turns off when the input is high, so you will need a normally closed relay. If you don't have one, we can modify the design.
     
  12. Wendy

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    Mar 24, 2008
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    A simple 555 also make a high drive Schmitt Trigger, inverting of course.

    555 Schmitt Trigger
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Two transistors will make a schmidt trigger, the benefit of course is that one of the transistors can be the one driving the relay. You can easily set it up for 8v and 11v thresholds.

    Total cost 2 transistors and about 4 resistors, and a diode for the relay coil.
     
  14. eugen39

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    Mar 1, 2012
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    I tried it and it didn't work, I probably did something wrong... I used a 556 because Radioshack was out of 555s and I might have messed out the terminals. I will try again today.
    Thanks
     
  15. eugen39

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    Mar 1, 2012
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    Thanks. I will order the parts and put it together. Radioshack doesn't carry all of them. I will let you know how it turned out.
    Thanks again.
     
  16. eugen39

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    Mar 1, 2012
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    I need a little bit more than that, I am a beginner.
    Thanks
     
  17. eugen39

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    Mar 1, 2012
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    What did I do wrong: I connected the input to 2 and 6 pins (and the 8 and 4 pins to +12V and 1 to ground, of course) and the relay to the 3 output and ground but nothing happened. The relay didn't click whether the input was 8V or 12V. At one point I must have shorted another pin because the 555 became very hot.
    http://forum.allaboutcircuits.com/images/icons/icon5.gif
     
  18. MrChips

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    Oct 2, 2009
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    How about using an analog comparator?
     
  19. eugen39

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    Mar 1, 2012
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    That sounds good but please remember that I am a novice so I would need more info.
    Thanks
     
  20. Ron H

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    Check post #11.
     
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