Ultra simple uber help

Discussion in 'The Projects Forum' started by dingo, Apr 8, 2011.

  1. dingo

    Thread Starter Member

    Mar 11, 2011
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    Hi Everyone,
    I've been messing around with TINA simulations but to no degree of success. I've also posted a similar thread but I guess the thread got carried away to a different discussion.

    I don't know anything about the 555 timer.

    Here is what I'm looking to do.

    When circuit board is energized with 9v, then wait 10 seconds and an LED lights up.

    Can anyone help me with the schematic? Do I need a 555 timer? Please help. I'm just a beginner.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    There are a number of ways to do this and some involve the 555. But you could also do it with an RC tank (to give the time delay, while the big capacitor slowly charges thru the high ohms resistor), and a comparator to switch on the LED when the voltage on the cap has reached a preset reference value. The comparator and LED could be powered off the 9v source.

    Only issue is a way to discharge the cap when the power is off. How often will the on/off cycling of the LED occur?
     
  3. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    What is the circuit going to be used for? Arduino would be very easy way to do this but might be a little more pricey then what some of the more knowledgeable guys recommend.
     
  4. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0
    thank you for reply, wayneh.

    What do you mean by that? Sorry i don't know what cycling mean.

    The LED should only light up once, then never again UNTIL I manually energize it again.

    I hope i'm not confusing you.
     
  5. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0

    Hello mbxs3,
    The LED will actually be use as a reed relay to turn on a device. But right now I just want to make it simple and just light up a LED.

    I'm just looking for a very inexpensive design.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    I was just concerned about the state of the voltage on the cap once the power is removed. If it doesn't drain off quickly, it will still have a voltage the next time it gets turned on and will take a different amount of time to charge up. The voltage will be different each time you hit the switch and thus the delay will be different each time.

    I think this is easily solved once the circuit gets drawn up, if you choose to go this way.
     
  7. dingo

    Thread Starter Member

    Mar 11, 2011
    34
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    thanks for clearifying to me about the cap. The circuit is usually only able to energize after about 10mins. Will this be enough time to discharge the cap? or is there is a way to make the discharge faster?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    That'll be fine. I'll see if I can draw you something soon.
     
  9. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0
    Thanks a lot!!! I'll be looking forward to it wayneh!!
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Here's a first draft. I've just guessed at the resistor and capacitor values to get "close". Certainly do the calcs or do some experimenting before finalizing this circuit. Make changes depending on what parts you have.

    I've left in a MOSFET controlled by the comparator, and this makes the circuit more versatile. If you really just want to light a single small LED, you could eliminate the MOSFET and use the comparator to sink up to ~5mA current thru the LED. You'd have to reverse the logic on the comparator. Now, "on" turns on the LEDs by the 3.3K resistor pulling up the MOSFET's gate and opening a path to ground thru the MOSFET. Using the comparator only, "off" at the comparator output opens a path to ground thru the comparator. (The comparator cannot source current to light the LED.)
    Picture 1.png
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    You want the LED to go out after a while?

    This is a starting point, but it does have the problems wayneh mentions...

    [​IMG]

    Ct will be discharged after power up time out, and C1 needs to be very small, say 0.01µF. R1 can be 10KΩ.
     
    Last edited: Apr 8, 2011
  12. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0
    Wow thanks Wayneh and Bill for taking your time out of your busy schedule to help build a schematic for me.
    @wayneh, I will give your design a test as soon as I get home!

    @Bill, what are the values of the Resistors, Capacitor to have a delay of 10 seconds before the LED lights up?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Pick a large capacitance, such as 1000µF, then run it through the equation. If the resistor seems reasonable then use it (or the closes value).

    T = 1.1 R C, 10min = 600 seconds

    600 sec = 1.1 1000µF R
    R = 540KΩ

    Reverse check...
    1.1 1000µF 540KΩ ≈ 594 sec

    RC circuits like this are notoriously inaccurate, but this would probably work.

    The image is a lightly modified stock from my library, which is why it didn't take long to draw.

    You didn't answer the question about the LED.
     
  14. dingo

    Thread Starter Member

    Mar 11, 2011
    34
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    Hi Bill!
    Thanks again for taking your time to help me. Yes I would like to LED to die out within a certain amount time as well.

    So in summary.

    Board is energized. Wait 10 seconds. Then LED lights up. After 4 seconds, then the LED dies. End Cycle.

    thank you
     
  15. wayneh

    Expert

    Sep 9, 2010
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    That's completely different. The comparator circuit won't work and you'll need something fancier using the 555.

    There are other ways - you could start charging a second cap for the turn-off delay and use another of the 4 comparators on the LM339 to turn the LED off once the 2nd cap is charged. But I'm no longer sure that's much simpler than just using the 555.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    A second 555 would be required to turn the LED off. This is because you have two time delays, for for the power up indicator, and the second for LED duration. Want a schematic?
     
  17. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0
    Bill,
    I used your example and made the simulation in TINA6. When I have all the resistors at 1k, the led just flicker once immediately when the board is powered.

    Please let me know what I'm doing wrong. I'm sure I did.
    I'm also not sure of what the capacitor with the curved line under stands for. What kind of capacitor is this?


    Wayneh,
    I've tried your example in TINA6, but I'm lost at the LM339 and the big Circle with DGS. What are they?
     
  18. dingo

    Thread Starter Member

    Mar 11, 2011
    34
    0
    I was looking around while I was waiting for you replies and I found this inside of a large PDF. I tried to simulate this in TINA6 but it just doesn't work on me. Should I just forget about simulations?
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Try using the parts I listed in post #13 for Rt and Ct. Those are the timing components, and you changed them. Notably the R2 (which is Rt on my schematic) from 540KΩ to 10KΩ. What do you expect to happen? R1 and C1 are extremely not critical. Use the math I gave you, T = 1.1 R C = 1.1 10KΩ 100µF = 1.1 seconds. I don't use simulators, but the 555 schematic is accurate. I have heard repeatedly that the 555 doesn't simulate well.

    When I get a chance I sketch up a second schematic.

    A simple calculator can do more than a simulator when dealing with equations. I have two on my computer hutch.
     
    Last edited: Apr 9, 2011
  20. wayneh

    Expert

    Sep 9, 2010
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    That's a MOSFET, a transistor for switching larger currents than the comparator is capable of. I left it in the diagram because you mentioned moving on from the LED to something else. Almost anything else other than an LED will require a larger current. A MOSFET makes an excellent on/off switch.
     
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