Time Delay Circuit

Discussion in 'The Projects Forum' started by Cadague, Mar 29, 2010.

  1. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Hey! I'm a senior at Jefferson High School in Cedar Rapids and I'm working on a time delay circuit that will have a trigger and a reset switch. We plan on creating a circuit that when turned on will generate a tone. When reset, it will wait until triggered. Then, once triggered it will wait 2 minutes then give off an oscillating tone at the frequency of about 1700 hz. We have a circuit using LM555 timers but would like to increase battery life on our circuit. We would like to implement something similar to a watch circuit that draws virtually no power. Our current circuit is drawing about 8mA and we want this number down below 1 mA as we are using a CR2032 battery which has low battery life when driving something with that much current.
     
  2. Wendy

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    Mar 24, 2008
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    There is a class of 555 timers called CMOS timers, the 7555 and the Radio Shack TLC555 are good examples. As an added bonus they will work down to 2 volts.

    Build conventional 555 designs using these timers using high resistances. The other thing is CMOS 555 do not have much drive at all, so transistors will be needed, along with piezo speakers.

    Need help with schematics?
     
  3. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    that would be great if you could give us some schematics. We have some electronics experience from a Digital Electronics course, but not enough background to create a circuit using anything other than the 555 chip. Do you know how much less power consumption the CMOS version of the 555 has?
     
  4. Wendy

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  5. Wendy

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    OK, here are a couple of ideas. I haven't built them, but they are from standard designs, so either should work.

    [​IMG]

    Here is a tool you might find handy in the future.

    http://forum.allaboutcircuits.com/picture.php?albumid=41&pictureid=308

    CMOS 555s can be used almost exactly like 555s, with some differences. If you were to try the first design with a convention 555 it would smoke, since the transistors don't have a base resistor.

    Pizeo speakers can be found on the back of most watches that have sound.
     
  6. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Ok, so if we wanted to run our speaker in a bridge tied load to increase power efficiency and volume even more, we could essentially run an oscillator circuit which is wired opposite.
     
  7. Wendy

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    If you use a conventional speaker you loose pretty much all efficiency, unless it is a high impedance type. It is possible to use two 555s to boost the signal double making a bridge out of it, but CMOS 555 have no drive at these voltages, so you will need a lot more circuitry.

    Or are you talking about using a separate amp?

    There is a reason watches use the speakers they do.
     
  8. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    The speaker we are looking at using is one similar to those found in a greeting card. So it sounds like something like this would not increase efficiency all that much?

    We want to be able to hear the auditory signal from about 200 feet or so at a frequency of 1700 hz. Is this possible with a circuit similar to the one provided?


    We may have to use a separate sound amplifier but I am not sure
     
  9. Wendy

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    For 200 feet you probably will, a watch cell just doesn't have much omph. When I get a chance I put some ideas out that might help (but I suspect the watch cell will be sucked dry quickly).

    On the other side consider the sonalert I suggested. They are cheap and loud, and may do exactly what you want, if not quite at the frequency you want.
     
  10. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Thanks for your help so far. We have been experiencing the problem that our watch cells have been drained fairly quickly too with previous circuits using a conventional speaker taken from a greeting card as this is speaker seems to be the right size and volume, although the volume isn't quite as significant as the frequency of the tone. As long as the tone is piercing enough, it can be heard from quite a distance.


    I will put together the sonalert circuit and see how it functions.
     
  11. Wendy

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    Mar 24, 2008
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    Let me know if I can help in any other way. There are ways of doubling the volume with another CMOS 555, but you probably don't want the extra circuitry. Since sound level is logarithmic the increase may not be as obvious as you'd like.
     
  12. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Ok well thanks so far. I will let you know if there is anything else you can help us with. I think we're getting pretty close to something that will function for what we want, now its just a matter of shrinking it down with surface mount parts and such.

    What is the best way to go about this? Should we look to have another company manufacture a prototype for us considering the parts become quite small.
     
  13. Wendy

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    If you are going to build just one I'd draw the circuit with a sharpie. It would be really easy to etch a board this way. Draw a layout by hand, the draw the PCB by hand.
     
  14. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Sounds good. Thanks for all your help. We'll keep you posted on how everything is going and check in if we have any more questions.

    Thanks again for all your help.
     
  15. Cadague

    Thread Starter New Member

    Mar 29, 2010
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    Ok so we are pretty much down to our final design. We tried a lot of these circuits you had suggested, but in the end, we decided we are just going to use a simple PIC microcontroller. In terms of size and weight for our application, this is sufficient. Additionally, the only additional components needed would be a driver for the speaker and the speaker itself as well as a watch cell.
     
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