Energy is becoming ever more expensive, and some fresh ideas are needed. There are already human-powered devices on the market, most of which employ a dynamo to generate power. It is also possible to recover energy from a piezo crystal of the sort found, for example, in the loudspeakers in greetings cards. Making use of this device is relatively straightforward.
Piezo crystals can generate voltages of many tens of volts when given a firm enough prod with a finger to bend the baseplate. The charge moved, however, is relatively small and the crystal is eff ectively a capacitor with a capacitance of only around 20 nF to 50 nF. This means that we need larger scale storage in the form of an electrolytic capacitor.
The piezo crystal can be treated as an alternating current source. We therefore need a rectifi er and a reservoir capacitor. Pressing the metal surface of the transducer ten or twenty times with a fi nger will charge the electrolytic in steps to the point where it has enough charge to drive a LED. The circuit is a ‘charge pump’ in the full sense of the term.
When the button is pressed the electrolytic discharges into the LED, which emits a brief, but bright, flash of light.