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The "Umbelite"
A solar-powered novelty

see copyright notice. Page created 22-Mar-2005 updated 7-May-2005

This purely decorative folly is a combination of natural materials and technology. I hope my description will inspire others to have a go - do let me know if you come up with more ideas!

The project started when I saw the remains of some "Chinese Lantern" husks (Physalis) in a friend's garden one spring. The ravages of the winter weather had destroyed the fruits and the scarlet paper-like covering, but left intact the delicate framework and its curved stalk. I instantly visualised these as shades for LED lamps! The question was, what should they hang from?

I found the answer in the form of another remnant from the previous year, the top and stem of a large umbellifer - probably hogweed. Like the lanterns, what remained was just a hardened skeleton, but with enough strength in its hollow, woody stem to make a sturdy little lampstand.

I shortened each LED's lead-out pins to about 5mm, and soldered on lengths of thin (32swg) enamelled wire. There's a small gap at the bottom of each lantern, where the "petals" don't quite meet, and it was easy enough to feed the wires up and out through gaps at the stalk end. Having pulled the LED up into the lantern, I twisted the wires together and secured them to the stalk with a small knotted piece of brown thread.

The wires wind along the umbel "strut" forming the actual support for the lanterns, with the stalks just hooked over for show. At the centre, I made pinholes in the hollow stem just below the umbel, so that the wires could be fed down inside. What you thought was a wire spiralling up the outside is in fact the remains of a climber (convolvulus?) that came free with the hogweed!

When I'd assembled the lanterns and stem, I drilled a hole in a rectangular block of wood for a makeshift base, but I kept an eye open for something more rustic. I found the answer whilst walking through an area of recently-coppiced woodland. Scattered everywhere on the ground were wedges of wood, like slices of cheese, cut off to leave a flush surface where branches had been lopped. I staggered home that day with a bag full of these wedges, one of which became the final base for the "Umbelite".

The solar panel was sold as a battery charger, and came with a hinged prop to angle it towards the sun. I removed the prop and fixed the hinged panel to the wooden base, concealing a cavity roughly reamed out to house the battery and electronics.

There's probably a single-chip solution to control the charging and light switching, but here's the circuit I devised. During daylight the solar panel charges the battery via the schottky rectifier (which has a lower voltage drop than a conventional one). When it's dark, a small current from the battery flows through the solar panel to switch on the transistors, which supply about 80mA to the LEDs (20mA each). After a full day's sun the lights stay on for several hours.

If you want to try a similar circuit, make sure the combination of solar panel and battery can't result in overcharging. And a 250mA fuse in the battery leads would be a wise precaution.

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