constant voltage source - for battery operated lamps

Elektor staff are a versatile lot! The present design came into being because one of our designers is a fervent spelaeologist, more popularly known as a caver. Regularly he risks life and limb in all sorts of dark caves, only to emerge hours later into daylight, covered in mud, sweating, and dead-tired, but happy and content. A good, reliable light source is, of course, indispensable for those treks in the dark. Many of the caver"s lamp units in use today are powered by rechargeable (lead-acid or NiCd) batteries. Such batteries are inexpensive over their life — provided they are used often and regularly — and provide a near-constant output voltage. Dry batteries are relatively inexpensive to buy, offer small volume and low weight, and can easily be carried as spares. The last three points are, of course, of inestimable value during caving and in many other applications! Unfortunately, dry batteries have a serious draw-back: their output voltage falls linearly with time, so tha...