Hacked Gadgets Forum

November 20, 2013

Inside a Motion Activated Wrist Light

at 4:06 am. Filed under Cool Gadgets

 Inside a Motion Activated Wrist _6108


I find it interesting to see how inexpensive electronics are constructed. My daughter got this Motion Activated Wrist Light at a school event. All of the kids were wearing them so that every time they clapped in rhythm the entire arena was flashing with a pulse of light. She no longer wanted it so I thought it would be interesting to see what made this simple thing work.

The sensitivity is quite good for a disposable item. I am thinking the vibration sensor is something like this Sunrom Model: 4073, you can see by the picture below this sensor has a solid center pin that is surrounded by a spring. This allows the sensor to remain open no matter what orientation it is in and the spring will move back and forth when it experiences a shock allowing it to short to the center shaft. Only drawback to this design is that it won’t be very sensitive when it gets a shock in certain directions however the wrist light seems to be very sensitive no matter how it is bumped.

The construction is very simple, there are two stacked coin cells as the power supply. I didn’t do a full circuit dissection but I think it is quite obvious how it is laid out. When the light is moved the vibration sensor shorts which charges the caps and biases the transistor which drives the LEDs. As the capacitors discharge the base voltage slowly decreased allowing the LEDs to dim slowly until they turn off.



vibration_sensor_2   vibration sensor






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6 Responses to “Inside a Motion Activated Wrist Light”

  1. winstontan Says:

    Nice non-destructive teardown. Simplicity always trumps complexity!

  2. Tony Says:

    I’ve seen that pin & spring arrangement in cheap stuff a few time, but never enclosed. Enclosed would cut down on any corrosion issues.

    I guess having that part enclosed makes automated assembly easier, so for part vs assembly cost it works for them.

    At $1 for singles it’s hardly a cheap part (considering the item).

  3. George Johnson Says:

    I took apart one of those “voice boxes” from one of my dogs “fling toys”. (you shoot them like a sling shot, and then they make a sound based on what animal it is. Dogs LOVE those things) She had pretty much destroyed it already, and I wanted to see what was in there.
    They had the same type of sensor, but it was just out in the open. Very cheap, but it worked, for a while. (everything else was useless, need to get to them before they take such a beating. Such is the life of a dog toy)

  4. Inside a Motion Activated Wrist Light #WearableWednesday « adafruit industries blog Says:

    […] Inside a motion activated wrist light on Hacked Gadgets: […]

  5. Louis Lebblanc Says:

    Do you know if that kind of sensor has another name? I was looking for something like it for a watch I’m building but I couldn’t come up with a search term that would turn it up. Typically vibration sensor shows up piezos or accelerometers. And “spring vibration sensor” doesn’t seem to show up a whole lot, a few ebay and alibaba links… They might just be a fairly uncommon part…

  6. x Says:

    “…it won’t be very sensitive when it gets a shock in certain directions…”

    Side-to-side we can assume is no problem, the spring simply bends slightly and bumps against the center pin.
    However, end-to-end is also no problem because the end of the spring is weighted by the unsprung turns and if you look at the plastic end-cap having a circular deflection ring, coupled with the fact that the last coil is slightly smaller, it appears that an axial displacement would result in the end of the spring being deflected towards the center pin again.
    A clever mechanical solution to a mechanical problem…

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