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Simple LED lighting circuit for low impacters and energy efficient living.

LED technology is advancing rapidly and as they produce good light output for very little power they represent a great opportunity for people wishing to live a low(er) impact lifestyle. However, buying them as pre-assembled units can be expensive and the units often lag behind the latest advances or cluster zillions of LEDs in the same unit, negating the low power characteristics. Break open an LED torch and you'll find little complicated looking circuit boards and it all feels a bit mysterious. It certainly did to me. So how do you go about connecting them up for yourself?

I was fortunate that a 2006 visit to Penrhos from the BFG (Bangor Forest Garden) group included one Frank Bowman of Llanfihangel, a low energy electronics expert and a thoroughly friendly, helpful person. Frank has fitted out his house and yard with LED lighting, powered from a solar panel plus deep cycle battery and is keen to pass on the information and demystify the whole process. He sent me some examples of the circuit he has designed for making up your own LED units and here it is, together with his explanation of how it all works.

Frank Bowman on LED lighting:

Note that LEDs are sensitive to static and should be handled carefully. The circuit design is for 18,000 mcd LEDs, the most powerful available at the moment.

Your batteries generally work between about 12.2 volts and 12.7 volts if you are looking after them and not taking them too low. But, you can be careless (or cruel) and run your batteries lower, and they can go up to a maximum of 14.4 volts when charging, if, for example, you are using a shunt regulator with a windmill. So to be safe, you have to have a circuit where the LEDs won't blow at 14.4volts! And if you simply put one resistor in then you won't get much light out of them between 12.2 volts and 12.7 volts.

The maximum current through the LED has to be less than 30mA and designing it to be a maximum of 30mA at 14.4 volts means that at 12.2 volts the current is 20mA and at 12.7 volts the current is 23mA.

Frank Bowman's circuit design for 18,000mcd LED lighting:

Frank's circuit shown in diagram 3 does the trick and gives good light output between 12.2 volts and 12.7 volts while protecting the LEDs upto 14.4 volts.

This circuit is for 18,000 mcd LEDs. If you want to use other LEDs you can use the same circuit but change R3 to get the maximum 30mA going through them at 14.4 volts. For example, if you use 12,000mcd white LEDs, the 60ohm resistor (R3) needs to be replaced with a 68ohm resistor.

Using this circuit with 18,000mcd LEDs the current consumption will be 25/26mA at 12.7 volts. This means you'd need 40 lights of 3 LEDs each to use 12 watts. Brilliant!

Here's an image of the completed circuit.

And here's the view from the other side:

Notes to LED wiring: diagram 4

The LEDs have a positive and negative terminal wire and must be connected correctly. There are two ways to tell which wire is which.

1. Seen from the top, the LED has a flat edge which marks the negative wire.
2. The LED has different length wires; the shorter one is the negative wire.

Suppliers

Many companies now supply LEDs, including the well known ones like Maplin and Radio Spares. Be aware that there are a plethora of different varieties and you need to be careful to get the right type. The main specifications for the 18,000mcd LEDs are as follows:

Frank previously used LED Supply for the 18,000mcd LEDs but was stung by a large import tax and postage fees. At the moment he uses Lsdiodes who can supply 12,000mcd LEDs so note the change needed to R3 in the circuit.

Frank and I really hope you find this information useful. If you have any comments, queries or suggestions for improvement, please mail me at the address given elsewhere on this web site. Good luck and have fun.

Although its not really possible to do justice to LED lighting in a photo, here's a shot from my place. I've replaced the original bits from the anglepoise light with the circuit and LEDs described above. At close range it provides a very bright working light easily good enough for reading (or assembling simple circuit boards and electronic components). By pointing it out of the window it illuminates our wood store (about 20 feet away) perfectly adequately for collecting an arm load of firewood.

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