Flowers that open with Flexinol wire
For our Demola project, we needed a tree with flowers that open by electronic control. The purpose was just to illustrate the concept of a physical tree that would react to energy saving and other good deeds.
The paper flowers are driven by shape-memory alloy wire that changes shape when heated up by electricity. There are basically two modes of operation: contraction and shape-change. The contraction force is much stronger (on the order of 10-100 grams), but it requires a restoring force such as a spring or another piece of wire. Instead the much lighter shape-change effect can be used for bending paper, though returning to the original position is still a problem.
A 100-400 mA adjustable current supply is perfect for driving 0.006" Flexinol wire. Such a supply is easiest to construct from a bipolar transistor, the base of which is fed through a potentiometer from constant voltage. The reason why a constant current supply is needed is that the resistance of the wire drops as it heats - therefore it would quickly overheat if the current was not limited.
Further parts are just some random microcontroller board and FET switches, left over from other projects. All is controlled through bluetooth and it gets power from li-ion battery, so there aren't any wires going to the tree. The electronics are hidden in the cardboard box that also acts as a base.
The implementation is not quite are neat as this one. My flowers don't close by themselves. Furthermore, the construction would not be strong enough for the intended placement in a school. Nevertheless, it demonstrated the concept and filled its purpose; and it was a nice way to get familiar with the Flexinol wire.
– Petteri Aimonen on 29.6.2012