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Help on low voltage indicator circuit

  1. Jun 2, 2006 #1
    I cant understand where to put the input terminals in this circuit. can you help me with this........

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  2. jcsd
  3. Jun 2, 2006 #2


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    This is my guess.

    It is a light sensitive switch. So the input is light on D2 which is a photodiode. If it is not conducting then there will be high voltage across BZ1 (buzzer?) If it is conducting then there will be a lower voltage across the load at BZ1.

    I believe that photo diode can be either light on or dark on, so I can't tell you which state will be conducting.
  4. Jun 2, 2006 #3
    There is no photo dioded in the circuit. Why would you think that? The voltage that is sensed is the voltage powering the circuit. That voltage is between the arrow on the upper right and the ground. BTW, it is a high voltage indicator. No beep from the beeper until the voltage passese above a threshold.
    Last edited: Jun 2, 2006
  5. Jun 2, 2006 #4


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    Int, I think that you got your arrows backward. I don't know much of anything about electronics, but I've had to read a schematic or two in my time. D2 is an LED; a photodiode is indicated by the arrows pointing toward it rather than away. It looks to me as if the grounds are the negative connection, and positive connects to BZ1 where the top arrow is.
    I'm not sure, but I think that I'm right.
  6. Jun 2, 2006 #5


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    As supernova says, the output asserts (low) and turns on the LED and buzzer when the overall supply voltage reaches a threshold. The threshold is set with potentiometer R2 versus the zener reference voltage from D1.

    This is obviously meant to be a simplistic circuit, but I wanted to point out a common error in this schematic that is often made by people designing with potentiometers. Potentiometers have a minimum wiper current that is required to keep that interface clean and low-resistance. If you connect a pot as shown in the schematic, with the wiper only connecting to the input of an opamp (especially if it is a CMOS opamp with very low input bias current), then over time the wiper contact will become unreliable. If the wiper is adjusted daily or something, then it is less of an issue. But in a circuit like this one, where you set the threshold once and leave it there for years, the operation of the circuit is likely to fail eventually.

    Just an FYI for you design-types.
  7. Jun 2, 2006 #6
    What would you normally do to prevent that? Just load it a bit with a resistor or something?
    Last edited: Jun 2, 2006
  8. Jun 2, 2006 #7


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    Yes. One variation that works well is to put the pot in series with another resistor to ground. Tie the wiper of the pot to the midpoint between the pot and the resistor, and also tie that midpoint into the opamp input. The midpoint tie should be the CCW end of the pot in this application, so that when you turn the pot control clockwise, that shorts out more and more of the pot, and raises the setpoint going into the opamp.
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