Yet another 555 timer question...

Discussion in 'The Projects Forum' started by starkman, May 10, 2011.

  1. starkman

    Thread Starter New Member

    May 10, 2011
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    Or maybe a 556 if that interests people.

    I've been reading lots on the internet and have seen a number of things that are close to what i might want, but don't quite seem to fit the bill... and i since i'm a measure twice, cut once sorta guy i hate frying stuff or doing things wrong.

    What i'm trying to develop is a mock grenade for airsoft games.

    The idea is that a switch is pushed, there is a 5 second delay. Then a piezo element will make a good loud screechy sound for about another 5 seconds and then shut off. without out any sort of reset until the switch is pushed again since i don't want it going off after the initial time for the rest of the scenario.

    I get how to set up a 5 second initial delay with a cap/resistor combo/555 combo.

    From here it gets more sketchy to me... Can i dump from the first 555 (on pin 3 i believe) to both a speaker and a secondary cap/resistor/555 combo that will time things out for another 5 seconds and then ??? trigger back from the second to the reset (pin 4) on the first???

    so to recap, i basically need a 5 second delay, 5 seconds on (of noise), and then a shut down. I figure i'll use a 9v battery for a power source.

    Should i use a 556 instead.

    Any help and circuit layouts would be highly appreciated. My eyes hurt from all the diagrams i've already looked at and my little boy wants his white board back. :)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This one is pretty easy. You need two timers, like you are thinking. You can use qty2 555's or a 556, it doesn't matter on that front. I have some work posters that may help on that front in my albums...

    [​IMG][​IMG]

    I have quite a few 555 articles and resources on my blog...

    Bill's Index

    Give me a little while and I will draw something up that will probably work. Basically you need two monostables, one feeding the next. Ever get a protoboard? They are very handy making the prototypes for something like this.

    I don't use a chalk board, I use Paint with a package of templates I've created called PaintCAD. The two posters are a small part of that.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Try this out...

    [​IMG]
     
  4. starkman

    Thread Starter New Member

    May 10, 2011
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    Thanks Bill, i will!

    Probably going to be not all that far from a fry's electronics later today or tomorrow so i'll pop in there and pick up the parts that i don't have.

    A couple questions so i can improve my understanding if you don't mind and i'm sure some of my terminology will be off, but i hope i make myself clear.

    i notice the speaker element is tied to pin 3 of the second 555 and not the first 555. My initial gut feeling is that shouldn't it be between them... or are you holding the second one high (or open) and supplying it (the speaker) power during the countdown of the second one which would then shut down after the second 5 count and take the power with it.

    Are there certain caps that i shouldn't be using. I seem to recall somewhere that one or another type wasn't good for timing applications.

    what does the r1,r2,c1 section do?

    What are c7, c8 caps there for?

    Also if i wanted to use a pot to create a variable initial delay time (maybe something like 2 - 10 seconds) what would you suggest and where would it go? I'm thinking as a replacement for r3 with a 100k or so resistor in series?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    The first timer is set for 5 seconds, when it times out it triggers the second timer, which is also set for 5 seconds. While the second timer is high the alarm goes off. It is extremely simple.

    The formula is also extremely simple, t = 1.1 * R3 * C2 ... or ... t = 1.1 * R5 * C5

    If R3 ever goes below 10Ω the 555 will blow out. Once triggered the button is no longer active, until the first timer times out.

    A full explanation of these kinds of timers is here, complete with theory of operation. It will answer your questions about the other parts.

    555 Monostable

    Now for something different. It occurs to me you probably want an arming circuit, so the grenade can only be used once. Either a key or magnet or something else that a person isn't likely to carry on their person. Before the contest you arm it, when you use it it is dead until it is reset.

    What part of the country are you?
     
  6. starkman

    Thread Starter New Member

    May 10, 2011
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    Thanks again. After some more reading i think most of my initial questions are answered well enough. I'm going to pick up a breadboard and parts and build things out today or tomorrow.

    Oh, and it looks like the caps thing i was asking about have to do with current leakage on electrolytic caps that can cause timing errors when longer delays are desired, but i'm guessing i'm ok since i'm only looking at 5 second intervals.

    I'd considered some sort of pull pin or arming device besides just a switch but i've been trying to keep things as simple as possible for the initial design till i get a good feel for everything. The airsoft sport is honor based on taking hits and so i doubt someone would re-use a grounded weapon... but it would be kinda cool and make things more realistic and i certainly wouldn't say no to any further advice.

    I live in phoenix, arizona... the fry's comment probably narrowed it down a fair bit.

    I'm also working on a different project with a piezo shock sensor to do an impact based grenade as well. I've found two diagrams that look like it would fit the bill for the trigger. And with what you've provided for me, i should be able to tie it together with yours by substituting it for the power on delay part with a bit of experimentation... after i get this one and the concepts behind it nailed down

    http://electroschematics.com/6023/vibration-sensor-circuit/
    http://romux.com/projects/electronics/shock-alarm-circuit
     
  7. starkman

    Thread Starter New Member

    May 10, 2011
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    Bill,

    I picked up the parts (except the piezo - duh... will be picking that up today) and dredged my mind for what i learned 20 odd years ago when i actually was studying electrical engineering and promptly made a big mess on my breadboard.

    Then i picked up a bread board simulator and laid this out. Does it seem to look about right to you. I know i used a bit more wire/jumpers than i have to by placing some items side by side in the columns. I mainly did that to try to help me keep reference (and because it looks better in the simulator) and i'll get rid of the single column jumpers as i build it.

    Thanks,
    Craig
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Uhh, don't have time for a complete review, but...

    On both 555s you have pins 6 and 7 going to ground, instead of between the resistor and the cap.
     
  9. starkman

    Thread Starter New Member

    May 10, 2011
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    Good call, totally missed that. Looks cleaner now to boot.

    Here's an updated drawing. take a look when you've got time. And again thanks for the time and effort to help me out.

    And a quick question. does it matter if c2, c5, and c7 are polarized capacitors or can they be the non polarized? I do notice in your schematic that you do differentiate the two types. If they can't be non polarized can i get a quick explanation. I do like to understand why things are the way they are.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    It doesn't really matter if they are polarized or not, but generally polarized are a lot smaller. I don't have time right now to study the layout, I'll be back later.
     
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