An LED embedded in a small translucent item (say half the size of a pack of cards) that would flash on briefly if the object was disturbed or tipped over.
I immediately responded that a piezoelectric sensor/generator inside the object wired to an LED would do it.
However, when I tried to demonstrate this concept using a piezo buzzer I liberated from an old phone, I could only get the LED to register a dim blip when I smashed the piezo buzzer with a blunt object.
How would I ensure that a piezo sensor/generator would actually light up the LED adequately without the application of blunt force trauma.
As I mentioned, space would be an issue so no large parts, breadboards, or really complex circuitry.
I'm just trying to figure out the easiest and smallest way to accomplish this simple task.
Thanks a lot for any help you can give me!
3 Answers
A tilt sensor would probably do. All the large component vendors stock varying types of these.
A simple DIY solution might be something like a small spring (from a pen?) placed around a stiff wire so any disturbance causes the spring to contact the wire and close the circuit (connections at bottom of spring and wire like a SPST switch)
The above would obviously need a small battery/solar cell/energy harvesting solution of some sort. If you are trying to do this without a battery then I don't think it's possible with a piezo, as they only produce tiny amounts of current. A shake to charge torch type circuit might be made to work (e.g. magnet slides through coil on tilt)
EDIT - With the requirement of no battery now confirmed, I think I would try the coil idea mentioned above. If all you need is a brief flash then when the thing is tilted and the magnet slides through the coil it should work, You can downsize as necessary (within reason)
Couple of links:
DIY Shake flashlight
Shake Flashlights - how they work
This will likely be pretty difficult to do well with no "intelligence" or energy reserve (e.g. cap charged very slowly with something like a tiny solar cell, discharged through LED on spring sensor closing)
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\$\begingroup\$ Oops, I forgot to mention that the solution has to be fully self-contained. No replacing batteries or charging. That's why I jumped at a piezoelectric solution, it produces it's own power. I know that with this many restrictions any solution might violate one or more of them. I'm really hoping there is a way to make the piezo idea work well because it wouldn't violate any of the restrictions (I think) and seems really elegant. Thanks for the idea though. \$\endgroup\$ Commented Nov 25, 2011 at 8:34
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\$\begingroup\$ @RoryO'Hare: You can edit your original question to clarify these restrictions. \$\endgroup\$– 0x6d64Commented Nov 25, 2011 at 8:44
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\$\begingroup\$ +1 for a small battery. If the circuit is actuated only a few times every day on average, the live span of the thing might only be limited by the battery shelf life. \$\endgroup\$– 0x6d64Commented Nov 25, 2011 at 8:49
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\$\begingroup\$ @RoryO'Hare - Probably be tricky to meet all your requirements. I edited my answer to elaborate on one idea that might work, can't think of much else right now... \$\endgroup\$ Commented Nov 25, 2011 at 9:14
I have done this myself to detect tiny vibrations from anyone touching my bike. It triggers an MCU to text me using a GM862GPS module.
The circuit uses a comparator whose inputs are set to be very close to each other. Then the vibration sensor can disturb the input voltages a little, causing the comparator to trigger.
The diodes are there just to make sure the vibration sensor doesn't generate too high a voltage. It may be possible to get rid of them, but I haven't actually checked.
The trick is to get some feedback oscillation. I once built one of these using a trusty spring from a mechanical pencil. Attach the spring to a pager motor inside a grounded aluminum housing.
When the spring hits the foil it will cause a vibration that will last for an random, but short period of time.
The challenge is calibrating the pager motor so that the spring gets a little extra kit; but shuts off most of time.
