This instructable describes how I built a decorative wall lamp. The idea is that of a night city skyline, with some lit windows in buildings. The lamp is realized with a semitransparent blue plexiglass panel with the building silouhettes painted in black. The light comes from 108 “windows” which are lit by leds fixed in holes in the panel. The lamp is powered by a 12V battery or power supply.
Step 1: Materials and Tools
- Semitransparent blue plexiglass panel 50 cm x 22 cm (~ 20” x 9”), 3 mm (0.12”) thick
- white plexiglass panel 50 cm x 22 cm (~ 20” x 9”) , 3 mm (0.12”) thick
- 2 meters aluminium channel 15 mm x 10 mm x 1 mm (~ 0.6” x 0.4” x 0.04”)
- Black paint
- Adhesive paper
- plastic spacers, 7 mm (0.275”) thick
- 6 x M3 x 12 countersunk head bolts
- 108 white 3 mm leds
- 3 NUD4001
- 3 x 2.2 ohm resistors
- 12 V bettry or power supply
- 2.5 mm drill bit
- M3 tap
- soldering iron
- metal saw
Step 2: Some Preliminary Calculations
I have decided the number of lit windows mainly based on electrial characteristics of the kind of leds and current driver I used. The leds are 3 mm white leds, which work with 3.3 V and absorb at maximum about 0.025 A.
In order to have a steady and stable current I used 3 current LED driver NUD4001.
I connected the leds in groups of 3 in series, 12 groups in parallel for each of 3 NUD4001, for a total of 3 * 12 * 3 = 108 leds, see the electrical scheme below.
Following the detailed instruction contained in the datasheet of the NUD4001 (https://www.onsemi.com/pub/Collateral/NUD4001-D.PDF) I calculated the electrical values of my circuit (for each of the 3 NUDS). The datasheet of NUD4001 recommends a total power dissipation lower than 1.13 W, based on heat dissipation of the SO-8 case and pad size:
- Leds current: Iled = 0.025 A * 12 = 0.30 A
- Vsense: 0.7 V (See Figure 2 in NUD datasheet)
- Rext = Vsense / Iled = 0.7 V / 0.24 A = 2.33 ohm
- Vled = 3.3 V + 3.3 V + 3.3 V = 9.9 V
- Vdrop across the NUD4001: Vdrop = Vin – Vsense – Vled = 12 V – 0.7 V – 9.9 V = 1.4 V
- Power dissipation on the NUD4001: P = Vdrop * I out = 1.4 V * 0.30 A = 0.420 W
- Internal power dissipation of NUD4001 (Figure 4 in datasheet, for 12 V): 0.055 W
- Total Power dissipation: Ptot = 0.420 W + 0.055 W = 0.475 W
Since I have no pad here (the NUDs are soldered to wires and other components but “floating in air”), I expect a lower heat dissipation, so I assume (hope) that the value of 0.475 W given by my circuit is low enough not to damage the NUD4001.
The calculation for the resistance gives a value for Rext = 2.33 ohm. Since a 2.33 ohm resistor is not commercially available I used a standard value close to it, 2.2 ohm.
Step 3: Panel Realization
The first step in the panel realization was choosing a nice city skyline to reproduce on the plexiglass panel. After some Internet searching I selected the skyline in Fig. 1.
Then, with an image manipulating software (gimp in my case, but every image manipulation software can do) I have adapted the dimensions so that it could fit on two A4 sheets in landscape orientation. The image was to be used as mask for painting the buildings and drilling the holes for leds, so I “binarized” it to make it pure b&w with very sharp edges and added white spots as reference for windows holes. Finally I splitted the image in two and printed on 2 adhesive paper sheets (Fig2 ,Fig3)
After having carefully cut the prints I attached the white part of the sheets (the sky) on the plexiglass panel, and painted the uncovered portion with black spray paint. After the paint has dried I removed the white adhesive paper and attached the black part of the sheets on the painted buildings. Then, following the white spots I drilled the holes for the leds, and removed the black adhesive paper, taking care not to damage the paint. (I decided to drill the holes after painting because otherwise the paint could clog the holes and make difficult to plug the leds).
Step 4: Solder, Solder, Solder
With the panel painted and drilled I started to place the leds in the holes, with the pins bent and aligned so that I could solder the positive (longer) pin of one led with negative (shorter) pin of the next, in series of 3.
I used wires to close the gaps between the pins when too wide, and between NUD4001 pins and leds.
Also I had to arrange pins and wires so to hide them behind the black paint. In the end it required quite a lot of soldering (and I’m not very good at it). So all seems quite messy, and it is, but in someway it works ;)
Step 5: Close the Box
The last step to finish my led lamp was to fix a second plexiglass panel on the back to protect all the circuitry. Between the 2 panels I put 6 spacers to leave enough room for all the internal components, and I realized a frame to keep all together. I made the spacer from pieces of scrap plexiglass I had laying around, putting together 1 piece 3 mm thick and 1 piece 4 mm thick. I realized the frame from 4 pieces of aluminium channel as long as the panel sides, with the ends at 45°. The channel has an internal width of 13 mm (~ 0.5”), so I made the spacers 7 mm thick (13 mm – 2 x 3 mm for plexiglass panles). Then I glued the spacers on the front panel, 2 on each long side and 1 on the short sides, and laid the back panel. With the frame sides put in place I made 6 holes 2,5 mm of diameter, from the behind, at the spacer positions. Since I wanted the front side of the frame clean and without holes, I stopped drilling when the hole was about 5-6 mm deep, then I removed the aluminium channel and I finished the hole all the way through the plexiglass panels and spacers. Then I threaded the holes with the M3 tap. I also widened and flared the holes in the back side of the aluminium channels for the bolt heads. In the left channel I made a hole on the side for the power cable. Finally I assembled all the panels with spacers and aluminium channels and screwed the bolts. The bolts are 12 mm long, so they go down through the back panel, the spacers and part of the front panel, but don’t reach the front side of the frame channels.
Step 6: See the Light?
After having assembled the led lamp I connected a switch to the power cable and a 12V battery, and switched on the lamp.
At the end I’m rather satisfied by my work, although the leds make a little too much light, especially directly in front (they have a narrow cone of light, so when seen laterally they look less bright). If I don’t misunderstand the NUD4001 datasheet the brightness of the leds could be reduced replacing the 2,2 ohm resistors with higher ones, but for now I'll keep my lamp as it is.