|Battery Operated Light Box|
Light Boxes and Photography
A light box is a very useful photography tool. Whenever you need to isolate a subject using even lighting against a white background, light boxes (or light-tents) are invaluable. Product photography, "scanning" old documents and macro photography are some of the applications that come to mind. For the electronics hobbyist/blogger like me, documenting electronics projects often requires photographing small objects (pcb boards, components, etc), so I wanted to build something I could use around the lab. In a previous article, I showed how I built an AC powered light box using CFL bulbs. This box has served me well, but it's large and often cumbersome to use in the small space in the garage. Furthermore, the fact that it is AC powered means I have to plug it into the wall with power cords and this can be inconvenient. This is when I came-up with the idea of building a second light box that complements my previous build. I set out to build something very compact and portable (though still big enough to photograph many of my projects). To avoid the AC cord's inconvenience, the light-box needed to be battery powered, preferably using standard AA NiMH rechargeable batteries such as these from Amazon.com. The project requires a power efficient design so that the battery lasts at least a couple of hours. I'm happy to report that the design described below meets all these requirements and has been working quite well for me. Let's see how it is done.
Selecting a box
I started this project by looking for an adequate box. Though I initially thought this would be the easiest part of the project, to my surprise, it actually proved more difficult than I imagined. After unsuccessful visits to five different stores (Target, Lowes, Home Depot, Marshalls and Walmart), I finally ended-up where I bought my previous light box: Ikea! They happen to have boxes built with a relatively thick but semi-transparent plastic. This "milky" like plastic is essential for this project since I need to shoot light through it, thus forming a sort of diffuser/softener. Other boxes I found were either completely transparent or completely solid, so they didn't fit the bill. I give you my friends the finalist, the wonderful white Ikea TROFAST storage box (only $2 as of this writing):
The TROFAST white box
With the box selection out of the way, I set out to find the second most critical component: the light source. Because this was to be a battery operated project, I decided early-on to use LEDs because they are some of the most power efficient light sources and can operate at relatively low voltages. I found a good deal on these high-brightness three-LED landscape light strips at the electronics goldmine and bought four of them (one for each of the four box "walls"). It should be possible to use other similar light strips from different manufacturers by following the same principles described below.
The photos below show the top and bottoms views of the LED light strips.
Light Strip - Top
Light Strip - Bottom
These devices are designed to operate from standard backyard/landscaping power supplies. The datasheet claims the strips can run from 15 to 30V DC or 12-15V AC. This is a wide range of voltages, so I was curious to see how they accomplished this. Some reverse engineering was needed so, after some probing, I ended-up with the schematic below:
Light Strip Schematic
Power enters the circuit through a bridge rectifier (MB4S from Vishay). This is what allows the circuit to operate from either AC or DC. I was pleasantly surprised to see a 20V MOV for surge protection at the input. Clearly, this product was designed with some reliability in mind. The rest of the circuit relies on a 78L12 linear regulator and a 35 Ohm current limiting resistor in series with the three LEDs (plus some decoupling capacitors). I was somewhat surprised to see that they didn't choose a current source arrangement based on say, a LM317. It seems the circuit will only regulate the voltage correctly when VDC > 15V presumably accounting for the minimum dropout voltage in the 78L12 plus the drop across the bridge. I noted in the schematics above voltage measurements at several points with a 12V DC input (circled in black) and with a 16V DC input (in red). For input voltages above 15V, the current through the LEDs should be approximately (12-3*3)/35 = 86 mA.
This is a fairly good circuit for it's intended application (AC/DC powered landscape light-strips). However, for a battery operated application like my light box, it is clearly a very inefficient circuit. First of all, the input bridge rectifier wastes I * 1.2V of power. Secondly, the linear regulator wastes I * (dropout voltage) and finally some power is wasted in the 35 Ohm limiting resistor. At the end, I modified all four strips by removing the bridge, the 78L12 and the series resistor (these are components I can reuse for other projects anyways), and leaving only the LEDs and capacitors. In this way, the light strips are reduced to three LEDs in series with capacitors in parallel (these help to reduce ripple). The resulting LED strips will be driven by a boost-converter based current-source to increase efficiency. Let's look at that circuit next.
I first heard about the MC34063 from my favorite EE blog (the eevblog.com from Dave Jones; highly recommended). Dave has an excellent video tutorial on this part posted on his site. I've since used it for a couple of small projects. This circuit is a variant on the typical boost converter topology with a slight modification to turn it into a current source instead of a voltage source (see figure below).
MC34063 Boost Current Source
I drew my circuit on top of the original boost configuration. The main modification is that the load (in this case the four LED strings in parallel) take the place of R2. The current is simply determined by the value of R1. Here's how it works. Because this is a feedback system, in closed loop operation the switcher will "do what it needs to do" to make the voltages at the input to the comparator identical. Because one of the inputs is tied internally to a 1.25V voltage reference, then the voltage at the other input (pin 5 = V across R1) must also be 1.25V. The current through the comparator input can be neglected, so the current across R1 *and* across the LEDs is simply given by the formula I = 1.2 / R. In effect, this circuit is a simple switch mode current source. In my application, I made R1 about 5 Ohm which results in about 250 mA total current => 25/4 = 62.5 mA per LED strip (note there are four strips in parallel). This is a little below the 85 mA I calculated for normal operation in the original circuit. Reducing the current to 62.5 mA will ensure it is well below the LED maximum rating and also help preserve the battery life. To calculate the remaining components I used an excellent spreadsheet from On Semiconductor that you can find at this link. I ended-up using L1 = 100 uH, Ct = 470 pF and C1 = C2 = 100 uF, RSC =0. Other values are also possible of course. A larger inductor (say 220 uH) will reduce the ripple current, but 100 uH is what I had handy (though the output capacitor C2 also helps reduce reduce significantly). Vin is provided by 4 NiMH AA batteries in series (4.8V nominal voltage). With the circuit ON, I measured a current of about 680 mA drawn from the batteries. Since the eneloop batteries are rated at 2000 mAh, one can expect about 2000/680 ~= 3 hours of battery life which I found acceptable for the application. I naturally added an ON/OFF switch in series with the batteries so that the light-box is only ON when I need to take photos.
Scavenging for components
Here's a tip regarding the MC34063: this is the most commonly used IC in cheap car cell-phone chargers. In fact, I have yet to open one that does not contain a MC34063 inside (and I've opened quite a few!). You can get yourself a dollar-store discontinued model for $1 (or look into your obsolete cell-phone drawer), and 99% of the time you will find a device like this inside. An added benefit of obtaining the part this way is that you can often also reuse the inductor, the Schottky diode, and perhaps some of the other passive components. (I actually didn't reuse the inductor in this project because it had fairly high DCR for my needs, but the IC and diode were fine). Here's a photo of a typical car charger opened-up to reveal the internal components:
A typical car cell-phone charger
Assembling the circuit
After collecting the necessary components, I proceeded to put it all together in a board. The circuit is simple enough that a standard prototype board like the one shown below can be used. The photo also shows the standard 4 AA battery holder (these can be bought at Radio-Shack for example, or you can buy this one from Amazon).
The final circuit in a proto-board
Putting it all together
Most of the remaining work was more of a mechanical nature rather than electronics related. The following figures will give you a good idea of the steps and these can be easily adapted to other boxes/enclosures.
I used white PVC pipes cut in half to mount the LED strips in the box. These are readily available at Home Depot and other stores that carry plumbing supplies. Since these are white, they act as a reflector towards the center of the box thus helping concentrate the light. Note the long screws and bolts used to securely fasten the PVC halves.These were trimmed later on using a Dremmel rotary tool.
Mounting the LED strips
Mounting detail - note the strip is mounted at a slight angle
With the four LED strips mounted, the next step is to connect them together electrically. Speaker wire with white "sleeve" works well for this purpose because it blends well with the white color of the "surroundings". The four strips are connected in parallel. Strictly speaking, this is not an ideal arrangement as you usually don't want to parallel LEDs or diodes. If one strip fails, the current through the other three will increase. However, with the margin we have on the LED maximum current, the situation should not be critical and anyway it will be easily detected/corrected by the user. I also have not found the difference in current through the four LED strips to affect perceived brightness at all, so it seems to work quite well.
Speaker wire used for interconnect
As luck would have it, there's a little plastic feature in the corners that can serve as a guide for the speaker wire. It's simply a matter of drilling an appropriate size hole as shown below:
Drilling the holes
Installing the wire
Final assembly (after soldering the wires)
I used a small plastic box (actually a soap-box believe it or not:) to enclose the final circuit and the rechargeable batteries. Notice the ON/OFF toggle switch to the rght of the photo. Four bolts and nuts secure the box against the side wall:
Circuit and battery enclosure
And here's the final product:
Light Box with LEDs ON
Top View - Photographing a watch
Top View - Photographing a Resistor
As some photographers say, in photography "it's all about the light" (well, not all really, but let's not get too philosophical here:). Having an evenly illuminated object is a great help, especially if you are photographing hand-held as I usually do. To illustrate this point, I photographed a small magnet/souvenir I picked-up on my vacation in the Philippines as shown below. Normally, I also put a small piece of inkjet photo letter paper on top (any photo paper will do, provided it is glossy and reflects light). The paper helps even-out the lighting setup by bouncing light also from the top:
I then took the same photo with the Light OFF (only ambient light; bottom photo) and with the Light ON (top photo). I set the camera for the same aperture; f/8 in both cases and same ISO sensitivity. As you can tell from the photo below, the exposure time went from 1/5s to 1/160s. In photographic terms, this is a five stops exposure difference! (i.e it reduced exposure by half, five times). Notice how the bottom photo is slightly blurry. This is because hand-holding a camera at 1/5s is not easy, even with the len IS (Image Stabilization) feature turned ON . Notice also how the background is clean and that there are no harsh shadows because of the even lighting. This is why light boxes were invented:)
Photo exposure comparison
I've been quite happy with the light box operation. It gets the job done and is very practical for use in my garage "lab". As in any project, there are of course possible improvements. One could add a DC jack to optionally feed an external DC supply and to charge the batteries. The venerable MC34063 is a good DC-DC converter and easily obtainable. However, one can certainly find more efficient (and more expensive) devices these days. A more efficient device will increase the battery life (though not dramatically). An external Low Ron MOSFET switching element would probably also increase efficiency as the MC34063 internal bipolar transistors have relatively high saturation voltage and they contribute significantly to the losses in the DC-DC converter. Whatever you do, have fun building your light box. This is a fun project to build.