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Homework Help: Set time delay for a comparator Op-amp circuit

  1. May 29, 2012 #1
    1. The problem statement, all variables and given/known data
    [See attached diagram]

    I'm trying to design a circuit that connects to the input terminals of the noninverting input of this comparator circuit such that:
    -The input to my circuit consists of a SPST switch and a 9 volt battery
    -When the switch is thrown on, there would be a 10 second delay before the LED turns on
    -When the switch is thrown off, there would be a 4 second delay before the LED turns off

    2. Relevant equations

    Op-amps, RC circuit time constants

    3. The attempt at a solution

    So obviously I'm trying to use some configuration of capacitors and resistors to achieve the proper time constant to fit the specifications of the question. However, I've been messing around with this for awhile now to no avail.

    The noninverting input can be set to anything between 1.5-9V.

    I think it needs to involve a 330uF capacitor.

    I'm not sure how I'm going to achieve two different times using the same component values (and thus same time constant?)

    I tried connecting a 28k resistor in series with the voltage source (& switch) to the non-inverting input, with the 330uF capacitor connected between the input and ground, and the inverting voltage set to 6.1v but this configuration of components is not giving me the desired response.

    Attached Files:

    Last edited: May 29, 2012
  2. jcsd
  3. May 29, 2012 #2
    A simple RC circuit like the one below should get the job done.


    The voltage Vc across the capacitor would sit across the Vin terminals.

    Simply configure the RC combination such that after [itex]5\tau[/itex] you've obtained the desired delay.

    [itex]\tau = RC[/itex]

    You need to determine when the output of your comparator is going to be nonzero. (i.e. when the voltage at the positive input terminal exceeds the voltage at the negative input terminal)

    You should be able to come up with a similar type of arrangement for the shut off delay, except that when the power is removed, another capacitor will dump previously stored charge across the diode and resistor combination such that the diode is turned off after 4 seconds.
  4. May 29, 2012 #3
    I got it. After many solid hours debugging, turns out one of the supply rails was floating. Oops.

    Thanks for the help!
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