Time Delay circuit using ONLY R and C possible?

Discussion in 'General Electronics Chat' started by aris, May 7, 2010.

  1. aris

    Thread Starter Member

    May 5, 2010
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    Hi there,

    I am interested in building a circuit that would provide a time delay.
    I have found online various circuits that use Resistors and Capacitors along with transistors or 555 timers.
    Would it be possible to build a circuit for a ! minute delay using purely resistors and capacitors??

    Thank you in advance!!
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Absolutely. It would be inaccurate and a power hog. Better to use a counter IC and a crystal osc.

    60k resistor and 1000uF cap will give you a ~60sec time constant

    The cost for an accurate enough resistor and capacitor would be much more than the cost to build an accurate timer.
     
    Last edited: May 7, 2010
  3. aris

    Thread Starter Member

    May 5, 2010
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    0
    Is it only a matter of accuracy of the time delay if I use only R and C? what do you mean by "power hog"? I am not interested in getting a highly accurate delay. Could you please provide a circuitry for this? the power supply will be 12V.
    Thank you for your help in total retched. I appreciate it a lot!!!
     
    Last edited: May 7, 2010
  4. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    depends on what the load draw of your indended use for this delay.
    The more current demanded is going to need larger value components to provide the desired delay.

    What are you trying to delay?
     
  5. aris

    Thread Starter Member

    May 5, 2010
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    0
    to give a full idea of what I want to do:
    I have a 12V power supply (12V-7.0A max) which I want to feed through an XOR gate and then the output of that to this delay circuit who's output will be fed to an inverter from high to low (0V).
    could you please help me to sort out and the schematic for something like that?

    Thanks all of you guys for helping me!
     
  6. aris

    Thread Starter Member

    May 5, 2010
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    here is a block diagram view of what I want to do
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached.

    I'm using XNOR gates instead of XOR, as they include both the XOR function and can be used as inverters.

    R1 can be a 1MEG trim pot so that you can adjust the pulse time.

    Since the input is not a Schmitt trigger, the exact timing will be somewhat up in the air; the output may even oscillate while the input is between a valid logic low and a valid logic high.
     
  8. aris

    Thread Starter Member

    May 5, 2010
    45
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    could I have a transistor based inverter? would that make things more stable?
    thanks for your reply SgtWookie!!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I thought you wanted to keep your parts count low?

    There are four XOR or XNOR gates in a single IC. You might as well use them.

    Unused CMOS inputs must have their inputs wired to Vdd or GND to prevent them from oscillating at high frequencies.

    You will need an 0.1uF capacitor across the Vdd/GND supply inputs.
     
  10. aris

    Thread Starter Member

    May 5, 2010
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    0
    Yes but you say that that the output of this circuit will oscillate that's why I am asking if the use of transistors would give a more stable output?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    If you use CD4093B Schmitt-trigger quad NAND gates, you won't have to worry about it.

    I don't know why you need to use the XOR function. Is there a particular reason for that?
     
  12. aris

    Thread Starter Member

    May 5, 2010
    45
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    the full circuit is as follows in a block diagram form.

    What I wan t is an alternative to a DPDT relay. I have a constant 12V supply and then I get triggering only when I get 0V. The initial inputs to the XOR will be 12V from the voltage supply and 12V from rs232 port. The triggering happens to the following circuit when I have 0V. thus I want 0V output and then this 0V through the inverter place at the feedback route make that 12V (high) so when this reaches the XOR gate gives a 0V (low) output and thus the RC circuit is not charging and so there is no triggering to the output. If you have any idea I can build something like this please share it with me!!!
    I have to say that I really appreciate your help!!!

    Thank you!
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    That is not standard RS-232 level. You need an interface IC, like a MAX232.

    The conventions for RS-232 allow the signals to go the full swing from -12v to 12v, or a 24v swing. This is beyond the limitations of 4000 series CMOS, which is generally limited to 16v maximum; in some cases 18v maximum.
     
  14. aris

    Thread Starter Member

    May 5, 2010
    45
    0
    I see..so can you provide me with your thought on how this circuit should be in order to avoid flactuations over the output please!!

    thank you!!!!!!!!
     
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