i need help

Discussion in 'The Projects Forum' started by SonOfAnarchy ZA, Jul 13, 2011.

  1. SonOfAnarchy ZA

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    Jul 13, 2011
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  2. SgtWookie

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    Do you want the input signal to stay at 5v once it gets there?
    What is it that you are trying to do; what is your application?
     
  3. SonOfAnarchy ZA

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    yes id like the 5v to stay there. the circuit is there just to turn on a row of led in a sequence and then stay on
     
  4. praondevou

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    Since I guess u want a linear increasing voltage, you could use an LM258 which works with a single supply. Configure it as an integrator. With the proper input configuration the ouput will increase from 0 (low-level output voltage of this IC is typ. 5mV) to 5V. The LM 3914 input signal voltage is actual not so problematic as long as it stays under the max rating. see datasheet.
     
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  5. SonOfAnarchy ZA

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    how would i configure it to a integrator?
     
  6. praondevou

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    First: What's the voltage of the power supply you will be using?
     
  7. SonOfAnarchy ZA

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    im using a 5 volt suppply but can also be a 12 volts if i connect it before the regulator
     
  8. SgtWookie

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    Here's one way to do it using a few common components; see the attached.
    Q1 and Q2 are wired as a current mirror.
    The 12v supply current through Q1 and R1 determines the charge current for C1.
    The values of R1 and C1 determine the time it takes to charge C1 up to 5v.
    D1 clamps C1's voltage to ~5.1v.
    D2 provides a discharge path when the incoming 12v is turned off.

    You can substitute many other small-signal PNP transistors for the 2N3906's shown.
    You can substitute many other 5.1v Zener diodes.
    You can also substitute many other diodes for the 1N4148, but a low forward-voltage Schottky diode like a 1N5817 would be the best.
     
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  9. SonOfAnarchy ZA

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    thanks man i will try it out tomorrow let you know how it goes
     
  10. #12

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    Discrete current mirrors suck horribly. You need resistance in the emitter circuits to make them track well. A few hundred ohms should help.
     
  11. SgtWookie

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    That's what I get for posting stuff before I had any coffee. :rolleyes: ;)
     
  12. praondevou

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    I have to admit I liked Wookies circuit more because it's discret. But I would like to know if the attached circuit would cause any problems under real-world conditions? (bypass caps are missing, ok)

    Or would it work too? I didn't build it physically, so I couldn't say. For the single-supply opamp I'd use the LM258
     
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  13. praondevou

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    I repeat my question from the previous post. Any of the gurus here has an answer?
     
  14. SonOfAnarchy ZA

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    THANKS MAN that circuit worked like a bomb!!!!
     
  15. SgtWookie

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    A bypass cap across the supply pins is always a good idea.
    One item that's missing is a method to quickly discharge the 47uF timing cap. Without that, the charge may be retained for a period of time and cause all of the LEDs to be turned on if the power is cycled on/off/on within a fairly short period of time. That's the reason I added the discharge diode above the Zener in my circuit. It's not absolutely necessary, but it takes care of the rapid cycle problem.

    Just about any voltage feedback single-supply or RRIO opamp that's stable at the gain used would work. LM358's are commercial opamps and cheaper than the LM258's, but the LM258 would work.
     
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  16. SgtWookie

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    Glad it worked well for you. :)

    Like #12 mentioned, adding a pair of resistors in the emitter paths of Q1 and Q2 would improve the circuit. Since the current flow is so low, you could use a pair of 10k resistors.
     
  17. praondevou

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    Thanks for the info .;)
     
  18. SgtWookie

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    Hello Mike,
    Perhaps you did not read the entire thread before you replied, but the problem was resolved by the end of the first page.
     
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