When vibration stops, timer starts

Discussion in 'The Projects Forum' started by bluefrog, Oct 29, 2013.

  1. bluefrog

    Thread Starter New Member

    Oct 29, 2013
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    Hi all, thank you for reading my post.

    I'm trying to find a circuit which will help me in my project. I need the circuit to be small (as small as possible) and sense when it is moving (not very sensitive, rather hitting an object or being moved) which will start a simple 2 minute timer once there is no further vibration, sounding a simple buzzer or speaker when the timer reaches zero.

    However, I also hope that everything resets if movement starts again even if the 'alarm' is being sounded.

    I will be running this off 2x 3v batteries


    I can find lots of timers or vibration circuits but sadly not a hybrid like this and I have no idea how to start.

    I know it's poor etiquette to have my first post here as a 'help me' post, so I apologise in advance but time sadly isn't my ally right now lol

    Thank you in advance.

    D :)
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You can constantly set a 2 minute mono stable timer. Each time a vibration is felt by the sensor, the sensor resets the mono stable timer. If no additional vibration is felt, then the monostable runs it course and triggers when the two minutes expire.
     
  3. bluefrog

    Thread Starter New Member

    Oct 29, 2013
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    Ah ok, and just use the shock sensing element as a push switch to restart the circuit? I was thinking of a piezo as a 'non moving parts' shock sensor, but I think that might not be sensitive enough as there will be no direct contact with it and it will have around 2" of foam rubber surrounding it.

    Anybody think of alternatives which don't use reed switches? This circuit is to be fitted into a ball so there may never be a horizontal plane for the switch to be 'off' or 'on' rather just one or the other lol.
     
  4. sirch2

    Well-Known Member

    Jan 21, 2013
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    Sounds like a tough nut to crack. A three axis accelerometer is the first thing that came to mind when you mentioned a ball, but that will not necessarily tell the difference between stopping and starting
     
  5. bluefrog

    Thread Starter New Member

    Oct 29, 2013
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    In theory when the ball has stopped moving (after being thrown and landing) there will be close to zero movement, and certainly no visible movement.

    The movement is impact and being bitten (dog ball lol). So possibly the the piezo will work?? Possibly two in tandem?
     
  6. sirch2

    Well-Known Member

    Jan 21, 2013
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    Accelerometers are an option if the ball is moving then at rest.

    Will the ball always be bitten? If so some sort of capacitive sensor may work - i.e. roll up two layers of aluminium foil with a compressible separator. When its shape changes it's capacitance will change.
     
  7. bluefrog

    Thread Starter New Member

    Oct 29, 2013
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    I was thinking of something along those lines, kind of like a pressure sensitive mat for alarms. However, there's no guarantee that the ball will be bitten so I have to assume it won't - my dogs are fickle creatures lol

    I don't know much about accelerometers, are they essentially (in this application) momentary switches? So; move, switch on, stop switch off?
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You can make a simple spring switch. Stand a very limp spring through a small PCB and the other side of the switch as a rigid wire that makes a ring around the spring. When the pcb has an acceleration, the spring wiggles and triggers the reset.

    These are used on the bouncing balls with LEDs inside of them. Very cheap and easy to install. You just need the right (Thin) spring.
     
  9. absf

    Senior Member

    Dec 29, 2010
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    I once opened up a cheap "step counter pedometer" and it has a wire spring just as GopherT describes. It was made in China so it was around 1-2 USD. If you're able to find them in the dollar-shop, you can always open it up and make use of the spring switch and keep the rest.

    As for using the reed contacts and piezo to detect vibrations, I dont think they are good choices as the reed switch needs to be strike hard before the contact can be closed/opened. Or are you using a magnet next to the reed.

    How to make the thing small so it can fit into a bouncing ball? I think the best is to use a 8-pin SMD pic but it sounds like you've never program a pic before. So the next best solution is to use 555 or 556 timer IC as mentioned earlier.

    BTW did you visit any shops selling Chinese-goods? IIRC I've seen these "bouncing balls with light " things during the chinese new year here. They work exactly like what you've described.

    Allen
     
    Last edited: Oct 30, 2013
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If you cannot find wiggley spring, could use tilt SW, maybe 4, each capacitiive coupled to an NOR gate with one output inverted, same out not inverted, so that wheatrer SW makes or brakes there is always an output. Assuming that NOR out. is normally high, connected to 555 reset, will time out in 2 min if there is no disturbance. If you are in US I have some, about the size of a pea.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How do you turn power off? How long does sounder sound?
    Found them- 4.7 mm dia, 5 mm high exclusive of leads.
    Can you handle surface mount components? Am looking at 3 IC's. Maybe 1 in cubed with standard componets.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I would use a micro power op amp and a transistor to power the LEDs and what ever flashing circuitry is needed.

    The spring switch brings power to a capacitor (quickly) and brings power to the circuit. When the timer is complete with 2 minutes, it triggers a transistor that drains the charged capacitor. Use as small of a capacitor as possible with lowest possible leakage to conserve power.
     
  13. bluefrog

    Thread Starter New Member

    Oct 29, 2013
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    Thank you for the amazing idea's. I'm from the UK where a bouncy ball is traditionally around 1" diameter so please forgive me if I'm getting confused here as the ball is a standard sized tennis ball size (around 2.5" - 3" dia) and made from solid soft foam rubber. There are no lights at all as the ball is opaque and they would not be seen, so a buzzer is going to be used so I can hear where the ball is rather than see it.

    Resetting the 'alarm' is of course a tricky subject, I was literally just thinking of inserting a small audio jack into the circuit to break the power when the plug was inserted. Small and simple and of course it could maintain the power when not in use overnight. Or the reverse, remove the plug to cease power, whichever.

    The circuit will be removable anyhow, so not too much of a problem, just a bit of a pain, if I need to get it out and manually turn it off.

    I like the spring switch idea, used a few making sensors for small collision detection bots. Easy enough to set up, but would the spring be sufficient to ward off false positives if the spring became horizontal?

    Really, I'm so thankful for every idea, as you can tell I'm new to electronics so I need to keep it as simple as possible really, but that's not to say I'm not open to learning new things if there is simple enough guides on it :)
     
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