The Metamorphosis of an Old Flat Bed Scanner

This is what a flat bed scanner looks like (in case you shouldn't know).

Since I got my color laser printer/scanner (I reported about its resurrection almost a year ago), I had to release my 20-odd years old Acer flat bed scanner into retirement, although it still worked ok. It was cheap even at the time of purchase, and its resolution was rather modest. Scanning was nerve-rackingly slow at only 300 dpi resolution.

Instead of sending it to the eternal scanner hunting grounds I thought about re-purposing it as a light table - a tool that I had on my shopping list for years but didn't allow myself to afford it.

To check what was inside, the first thing I did was, of course, opening the scanner. This largely depends on the type of unit you have available but is more or less self-explaining. On mine there are two latches at the bottom of the case on one end (accessible through 2 deep, rectangular holes), and on the other end a sheet metal cover attached with 2 Phillips screws. I pried open the latches with a screwdriver and could then lift off the upper part of the case; the other end of the case is just hinged/hooked to some more latches. I also removed the metal cover by undoing the two screws.

Basically a flat bed scanner is a box with a precision optical glass window at its top and a flap to cover this window and flatten whatever you are scanning. The flap is usually easy to be removed. Inside there are some electronics, some mechanical components as well as some optical elements and a tiny CCFL (cold cathode fluorescent lamp) with its high-voltage driver module. Modern scanners rather use LEDs instead of a CCFL, by the way. Most of the control electronics is located on a PCB mounted on the metal cover.

I removed everything that was no more required. I want a light table that more or less consists only of the box and a glass window on top of it, plus a light source and the power supply. This was a rather straightforward procedure. Basic tools (screwdrivers, pliers, soldering iron) are sufficient for this step.

I could harvest the following parts (for later re-use in different projects):

• 1 tiny CCFL (to be handled with great care - it is a very delicate glass tube containing toxic mercury! If you will not re-use it, please dispose of it carefully and according to your local regulations!)
• 1 small driver PCB for the CCFL (caution, high output voltage!)
• 3 oblong, precision glass mirrors
• 1 optical lens
• 1 small PCB containing the optical sensor and a light barrier
• 1 43 cm steel rod
• 2 bronze sleeve bearings
• 1 stepper motor
• 1 toothed gear
• 1 toothed drive belt
• some pulleys

From the large PCB I extracted the following components:

• 2 TO220 voltage regulators (7805 and 7812)
• 1 inductor
• 12 small electrolytic capacitors
• 1 quartz crystal
• 1 japanese transistor
• 2 small heat sinks
• 2 connectors
• 1 USB socket
• some mounting hardware

Some of the harvested components are shown in the picture above.

In addition, there is a 16 VDC/900 mA wall wart that I successfully managed to re-use as a power supply for the finished light box.

I have no further use for the large PCB, so I decided to let it be installed with the power switch and the PSU socket soldered on it. This is the easiest way to have the switch and the socket mechanically fixed at the correct location within the housing. The PCB also contains several SMD components that are beyond my horizon, but I just left them there, without being used or under power. I had to cut a few PCB tracks, then attached two wires (red +/black –) for connecting the socket/switch to the light source. The metal mount for the PCB is re-used as well and attached with its own two screws.

I thought about what kind of light source I should use: Some incandescent bulbs? Some (compact) fluorescent lamps? I'm sure that with all of these the light distribution would have been less than optimum, apart from the fact that they all generate a certain amount of heat. I decided to try an LED strip attached to the side walls of the case's bottom part. This kind of LED strips can easily be found at different places such as Amazon or ebay. LEDs have a very long life span compared to incandescent bulbs and are much more energy efficient (i.e., you get a lot more brightness for your buck).

Required materials:

• A cheap, auto-adhesive, warm-white 5 m LED strip specified for 12 VDC
• Some sturdy white cardboard
• Some plastic adhesive
• Hot glue gun
• Some red and black wire
• Screw terminal block
• Some zip ties
• Some tracing paper (see 'Update' below)
• Some self-adhesive aluminum foil

As shown in the pictures, the inner side of the case bottom is somewhat bumpy. To be able to install the LED strip in an easy way I fixed some cardboard strips cut to measure inside the case bottom; I first attached them provisionally with some drops of plastic glue; once that was dry, I generously added hot glue in order to permanently attach the cardboard within the case.

The LED strip I selected features a total of 300 LEDs on a length of 5 m. They are arranged in 100 groups, and each of these groups consists of 3 warm-white LEDs connected in series and a current limiting resistor of 150 Ohms, also in series with the LED group. All the groups are connected in parallel. The whole strip is specified for a 12 VDC supply voltage and a total current consumption of max. 2 A.

The label of the scanner's original wall wart indicates an output voltage of 16 VDC at 900 mA; its no-load output voltage is about 22 VDC (about 20 VDC under load). So I decided to use 2 sections of 46 LED groups each, connecting them in series (and thus requiring twice the voltage, but only half the current).

Dividing the LED strip into shorter sections is no problem, as long as the LED groups remain intact. I cut 2 sections of 46 LED groups each. The remaining, short section of 8 groups (0.4 m) was left over and was put aside for a possible future project. In the 2nd picture you can see that one strip with 46 LED groups nicely fits twice around inside the cardboard divisions within the case.

Soldering wires to the LED strips can be a little tricky, but with a clean, narrow tip on your soldering iron this shouldn't be a real problem. Take care when attaching the self-adhesive LED strips to the cardboard! It's very difficult to remove them without damaging them once they stick.

Even if the two LED strips are supplied with only 10 V (instead of 12 V) each, the overall brightness seems ok to me, and operating the LEDs with a little bit less than nominal voltage increases their life span to almost eternity :-). After power on, it looks now as shown in the 3rd picture.

I covered the inside of the glass screen in the upper case part by a piece of (opaque) tracing paper in order to get a uniform and diffuse illumination. However, as shown in the pictures above, the illumination is not perfectly uniform but somewhat brighter at the edges. To remedy this I attached some reflecting, self-adhesive aluminum foil to the central section of the case bottom; this helps distributing the light a little more evenly. (Please consider that the two pictures were not taken at the same time of day, so the brightness is somewhat different...)

The flap shown open in the 4th picture is useful for dust protection when the light table is not in use. While using it, the flap can be removed easily because it is only plugged-in. The light colour is different from pics 1 and 3 since this picture was taken with a different camera.

Update 2022-11-10: I replaced the opaque tracing paper that I only had available in DIN A4 size (and of which I had to attach two sheets together) by translucent baking parchment being large enough to cover the whole window.

Although I was somewhat reluctant to hollow out and repurpose/reuse a scanner that worked ok but not really well, and very slow, at that, in fact I would have had it either standing around and gathering dust or letting it go. I'm sure that this project saved quite a bit of material from being sent to the recyclers or the landfill. And I got something useful for the amount of three times nothing, apart from having various parts added to my junkbox for free.