Question

I have a clear plastic ball with some electronics inside. When bounced, LEDs flash on and off. I suspect that it contains a piezoelectric crystal and I would love to know how it operates.

Asked by: Vincent B. Maier

Answer

Actually the 'flash-when-bounced' ball works on a much simple principle than what you suspected. I happened to have one of these balls at home so I took some photos for you - picture is worth a thousand words. So here it is:

Flash when bounced ball

  1. is the ball from far,

  2. zoom into the electronics inside the ball. One can see the microchip and battery at the bottom and to the left, transparent domed thing is a LED (Light Emitting Diode) at the middle top and the activation spring on the upper right.

  3. zoom into the activation mechanism: basically one end of the contact switch is just a loose spring with another contact being a rigid rod in the middle of the spring.
When you bounce the ball spring starts vibrating and in most cases it touches the middle rod. This establishes the contact that the microchip starts flashing the LEDs.

Other activation mechanisms are also possible. Two metal disks separated by a slight gap and mounted on the flexible holders are another configuration (suggested by Paul Walorski).

Piezoelectric crystal (material that produces potential difference as a result of applied mechanical stress) could be used for this purpose too, however, generally it is not used due to high cost.

Answered by: Anton Skorucak, M.S. Physics, Founder of PhysLink.com

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