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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.
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