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Homework Help: Simple circuit(555 and Capacitor)

  1. May 10, 2010 #1
    1. The problem statement, all variables and given/known data


    Base on this circuit, how would I make the LED stay lit for 2 seconds? My knowledge goes as far as op amp and first-order circuits. I just don't know how to mathematically solve for the right capacitor that will make LED stay lit for 2 seconds because I've never dealt with a 555. Thanks

    2. Relevant equations


    3. The attempt at a solution
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 11, 2010 #2


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    Science Advisor

    There are calculators for doing this which you can access via Google.
    They also give the formulae that they use inside the calculator.

    One that came up first on Google was the following:

    Do you mean to stay lit then go dark, then light up again? Or do you mean just stay lit for 2 seconds and then stay dark?

    If you want long delays, like this, you could probably use a CMOS 555. These have high input impedance so you can use larger resistors and hence smaller capacitors with them.
    The advantage of this is that you can avoid using electrolytic capacitors which are leaky and unreliable in timing applications. Capacitors up to 1 uF are available in non-electrolytic types.

    For example, if C was 1 uF and R1 =10 k and R2 = 3 megOhms the LED would stay on for 2 seconds and go dark for 2 seconds and then repeat this.
    The above calculator accepts 1e-6 Farads 1e4 ohms and 3e6 ohms for these.

    There is a link in this page for a Monostable timer calculator if you only want it to flash once and then go dark.
  4. May 11, 2010 #3
    You might find the 555 timer datasheet useful: http://www.datasheetcatalog.org/datasheet/philips/NE_SA_SE555_C_2.pdf

    Do you want an astable or monostable setup? (That is, flash once or blink). Under typical applications in the above datasheet, they give schematics and accompanying formulae for each scenario. These can be used to calculate the period of oscillation.
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