Little Big Lamp

By Charles Platt
Category: Furniture, Electronics
Difficulty: Moderate

The most popular item I ever built for MAKE just happened to be the simplest: an LED desk lamp. This was in Volume 08, in 2006, when white LEDs were a hot new product. The most powerful ones I could find were 1cm in diameter, rated to deliver 100,000mcd (millicandle). The light wasn’t exactly white — it had a freaky purplish hue. But I liked the weird color, because it showed we were early adopters of cutting-edge illumination!

Recently I started wondering if I could downsize and upgrade the original lamp. So I took a fresh look at those traditional, through-hole, single-component LED "indicators" (as they are properly known).

The 5mm ones are now a lot more powerful than the 1cm type I used in the past. Some of them, known as "5-chip," have 5 light-emitting elements squeezed into one 5mm package, sucking down 100mA of forward current at around 3.3V DC.

They’re still rated at 100,000mcd, but 6 years ago, the ones I used were rated for only 20° of beam spread. Today’s 5-chip LEDs claim a spread of 60°. Does that mean they’re 3 times as bright? No, they should be 9 times as bright, because the light is delivered over a two-dimensional area!

Since I used 72 of the big old ones, and the new ones should be 9 times as bright, I would only need 8 to get the same illumination. But why not go for greater output?

Here we’ll use the traditional-style 5mm LEDs. RadioShack, for instance, offers them as part #276-017. Since they use exactly the same 3.3V DC as the 5-chip variety, you can substitute either in this project without changing the circuit.

Fabrication Choices

I opted for a 12V DC power supply, to make the lamp function in motor homes, where LEDs are ideal to conserve power. For use with 115V AC, you need an adapter that delivers 12V DC at 1 amp. Here we’ll use RadioShack part #273-358.

Instead of adding a series resistor with each LED, the most efficient way to power them from 12V DC is by series-wiring them in threes. This means you need 10V DC for each set. How to get 10V from 12V power? Pulse-width modulation is the way to go. You send a stream of pulses, too rapid for the eye to see, and vary the gaps between them to limit the average current. If you add a potentiometer, this can act as a dimmer. Only a few electronic parts are needed, and they’re listed in the materials and specified in Step 5.

Note that if you use old-style low-power LEDs, your AC adapter can be down-rated to 300mA, which should cost less.

How to build the actual lamp? I decided to use PVC plumbing supplies. For the additional pieces that would hold everything together, I chose 1/8" white ABS plastic, but you can use plywood if you prefer.

Steps

Step #1: Make the lamp head.

The template here (downloadable from the Files section at top) shows the layout of LEDs to fit in the large end of the PVC reducer, which forms the head of the lamp.
You need an adjustable hole cutter to cut a circle of ABS (or plywood) the same size as the template. They're $10 on eBay, but to use it safely, you need a drill press, as shown here. Otherwise, use a handsaw and sand the corners to make a circle.
Tape your template to the circle, and use an awl to poke through the center of each hole. Remove the paper and pilot-drill through each indentation with a 1/16" bit, then drill with a #9 bit. This is the perfect size, and the LEDs will push-fit into the holes, with no glue needed.

3 Responses to Little Big Lamp

Goli Mohammadi on September 20th, 2012 at 9:00 pm said:

Hi sandrorahn,

The equivalent is the ULN2003 Darlington array. Can you find that one in Brazil?

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kb5zxm on September 29th, 2012 at 10:32 pm said:

I would use an old computer battery, or camcorder battery, and charge it with a wall wart trickle charger. Or from a cig,ligher plug but watch the current. The weight of the battery would stabilize the lamp base

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Ian Caister on October 12th, 2012 at 6:30 pm said:

In short: the darlingtons allow the control circuit to switch much higher currents than it would be able to without them.

The darlington array effectively operates as a digital switch.

This design uses a technique called ‘PWM’ or pulse width modulation. Effectively the 555 produces an output that is switching on and off faster than you can see. By varying the potentiometer you change the relative amount of time the output is ‘on’ for and hence adjust the apparent brightness of the LEDs.

The issue is the 555 can only sink a few miliamps of current, much less than what is needed to drive all those LEDs, so instead the 555 drives the darlingtons which take very little current to operate but can sink many times more than the 555. The darlington is basically like a relay.

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