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switch on at threshold and hysterisis circuit

Discussion in 'Electronic Basics' started by M B, Mar 11, 2005.

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  1. M B

    M B Guest


    I want to switch on an npn transistor when a sensed voltage exceeds a
    certain threshold, say 4v, but I don't want it to switch off again until the
    sensed voltage drops a bit below that, say 3v - because the signal will not
    be clean.

    I suppose I need a schmitt trigger, but I have no supply voltage for the
    circit - only the sensed voltage itself which is found across a 10k load
    resistor. Is what I want possible? Any help appreciated.


    Matt B
  2. Use a micro power comparator, like the LT1716. Also, use a 2.5V
    micropower reference. Then, set up your input so it powers the
    comparator and reference, and a voltage divider. The divider should get
    to 2.5V when the input rail gets to 3.5V. The noninverting input goes to
    the reference through a resistor. There is a resistor of approximately 5
    to 10 times the value from the noninverting input to the output (that's
    your hysteresis). The inverting input gets the divider. The output goes
    to your circuit you want to power.

    As the input is rising, the divider will be below the reference (which
    will track the rail up to 2.5V). Once the rail gets to 4V, the divider
    will be above 2.5 + the hysteresis, and the comparator will start
    sinking current.

    Note that you shouldn't pull more than about 40uA from the circuit, or
    it'll drag down the voltage across the 10k input resistor. Thus, make
    sure your reference isn't going to suck up alot of power, and make the
    divider very high resistance. When the circuit it on, it'll pull current
    from the input out to ground through the comparator.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  3. When the NPN is switched on, where will its conducting current come
    from if you have no supply voltage? From the sensing circuit? What is
    the end objective?
  4. M B

    M B Guest

    Hi Terry,

    I have a sealed "off the shelf" timer which contains a litium battery. It is
    looking for closed contacts but seems to work fine across the collector and
    emmiter of a switched on npn transistor.

    Matt B
  5. M B

    M B Guest

    Thanks for that Robert,

    It sounds like a very elegant solution.


    Matt B
  6. Jamie

    Jamie Guest

    a shocky diode perhaps?
  7. Harold Ryan

    Harold Ryan Guest

    Hysteresis is a great thing. As a matter of fact, I use a low pass filter
    along with hystersis to eliminate most noise on the test stands for GM and
    Chrysler. Hysteresis in this case means positive feedback. A NPN function
    like an inverter. You will have to add another NPN in order to have positive
    feedback. Use a feedback resistor that connects the 2nd NPN to the input
    Just adding a RC filter to the input can make big trouble because the input
    signal will now remain at the threshold voltage for a longer time.

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