The resulting macros were not spectacular, with shutter speeds of 1/30s at ISO400, but it did prove that the concept was viable. So, armed with a new soldering iron and a supply of LEDs (6690 cd) and resistors from RS (68 Ohm) I set about seeing how many LEDs I could squeeze onto the ring; the magic number was 20. The circuit is relatively simple (10 parallel arrays of a series of 2 LEDs and a resistor), until you try to wire it up on tracked circuit board (involving a lot of breaking the tracks to avoid the inevitable short circuit.
The resulting light was much brighter than the original, which is unsurprising given the lights are 5 times brighter and there 3 times as many.
When the LEDs were aligned to point at a subject about 8 to 10cm in front of the lens, in a totally darkened room I managed a macro shot with a shutter speed of 1/100s at an ISO of 100 (albeit at a shallow depth of field of f/3).
So there we have it - a ring light which can easily evenly illuminate a macro subject, and unlike a flash can be left on while you set up. The set up still needs a few tweaks, and I need to work out the light temperature for white balance correction, but it is just about there. As a final touch I dismantled an old tripod and glued the mount to a PP3 battery holder (with build in switch). Combining this with a simple connector, both parts (light and battery) can be screwed on separately and then electrically connected which saves a tangle of wires.
All in all the mount was fairly cheap - of the parts I had to buy, the LEDs came in at £20 (including 5 spares), the resistors £1, the circuit board was an offcut from work, and the battery holder/connector about £4 - so roughly £25.
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