Introduction: LED Backlit Sign Made From a Free Flashlight

About: Geek, Developer, Maker, Tinkerer, Dad

I was recently asked to make a small (handheld & portable) light-up sign. With a short timeframe and limited budget, I had to use what I had on hand to make it work. Using scrap wood and acrylic, and one of my free Harbor Freight flashlights, I made this light-up sign in a wooden box. Keep reading (or watch the video) to see how I modified the flashlight, built a new battery compartment, and enclosed everything in a wooden box.

Step 1: Acquire Flashlight

For this project, I used a 27-LED flashlight from Harbor Freight. These are often available for free with the appropriate coupon. I've seen flashlights of the same design at other stores as well, but this is the one we'll be using here.

Step 2: Take Apart Flashlight

For our sign we'll be using the larger 24-LED panel from the flashlight. Take apart the flashlight by removing all the screws (it comes apart fairly easily).

Step 3: Remove the Circuit Board and Identify the Positive Voltage Path

First, find the wire that connects to a positive connector in the battery compartment. This is the positive power line. Trace this wire to the circuit board and use a marker to identify the trace on the board. Once you've identified this point, you can cut the wires to disconnect the board from the flashlight's case.

From the positive voltage location on the board, follow it through the resistor on the board and out to the traces that bring positive voltage to each LED. See the photo to see how I marked each positive "line". This will be useful later after we cut apart the circuit board to help identify the positive side of the LEDs.

Step 4: Diffuse the LEDs

Inside our LED sign, we want the light to spread out evenly to avoid "hot spots" on our backlit sign. The easiest way to do this is to sand the round LEDs down flat. The rough flat surface on the tips of each LED will diffuse the light. Since they're all mounted to the circuit board, you can sand them all down at once on a belt sander.

Step 5: Cut Apart the Circuit Board

Because this is such an inexpensive flashlight, the circuit board is a simple one-sided design. You can easily see all the traces on the back and figure out how they connect everything together.

The 24 LEDs are all connected in parallel. That is, all the positive ends of the LEDs are connected together and all the negative ends of the LEDs are connected together. They're arranged in 4 rows of 6 LEDs, with the traces running along those 4 rows. There is also a resistor between the incoming voltage and the LEDs, which steps the voltage/current down to the proper level for the LEDs.

To make a long strip out of this, first cut off the end of the board with the power switch, then cut the board apart between the 4 rows of LEDs. I used my bandsaw, but you could easily use a dremel (rotary) tool or other small saw. You can leave the resistor connected to one of these strips. This will make it easier later because the resistor will already be connected to the circuit.

Step 6: Connect the 4 Strips Together

Earlier, we used a marker to mark the positive trace for each row of LEDs. Now we can use this to identify which lines get connected together. Between each strip, connect the positive ends together and the negative ends together by soldering on wires to the exposed LED solder points. The strip with the resistor on the end goes on the end of the long strip.

I added two long wires to the end of the strip to power it. These will be connected to the battery and switch later.

Once all are connected, you can test them out by connecting the batteries to your circuit, making sure to hook up the positive voltage to the end of the resistor, and the negative to the negative line.

Step 7: Mount the LED Strips for Stability

To keep the LED sub-strips from moving and potentially breaking, secure them to something strong and rigid. In my case, I used hot glue to secure the assembly to a wooden paint stick. Not pretty, but effective.

Step 8: Modifying the Power Switch

The switch that comes with the flashlight is designed to toggled on and off between two circuits (since there are two lights on it). So each "click" goes through this pattern:

  • OFF
  • Circuit #1 ON
  • OFF
  • Circuit #2 ON

For our sign, I just want a click-on, click off operation. To make this work, connect the two switch connectors (two wires on the same side) together. I was able to identify these using my multimeter, but that's a process for another instructable. This makes "circuit #1" and "circuit #2" essentially the same circuit, so the switch will now operate like this:

  • OFF
  • Circuit ON
  • OFF
  • Circuit ON

Step 9: Making the Box Frame

I wanted everything in the box to be removable in case we need to repair, replace, or modify anything. Everything could be inserted or removed from the back.

To do this, I made a basic four-sided rectangle using 1/2" MDF with glued rabbet joints on the corners. Once dry, I glued on a small face frame to the front using 3/4" wide strips of 1/4" MDF. I made the face frame overlap the outside of the box, and then trimmed the edges flush with a flush trim router bit once the glue had dried.

I then cleaned up all the surfaces and glue joints with the belt sander.

Step 10: Making the Back Panel and Battery Compartment

The back panel is made of two small sheets of MDF glued together, and designed to fit snugly into the open back of the sign. The outside piece matches the outside dimensions of the box, and the inside piece fits just inside the box.

Most of the rest of the back panel was glued together using CA Glue (2P-10 from Fastcap) and activator to make the glue set up almost instantly. This glue/activator combo has become one of my favorite things in the shop.

The LED strip was glued in the center of the panel. To fit the batteries, I cut a piece of wood with a small groove to fit a stack of three AAA batteries. Incidentally, upgrading to name-brand batteries made the lights much brighter.

The positive battery connector was salvaged from the flashlight and glued to the end of a piece of wood that rotates around a wood screw. This lets the user swing it open to insert or remove batteries.

The negative battery connector was also salvaged from the flashlight, and glued to another piece of wood. It is set at just the right distance so that the spring is compressed when the positive connector is closed.

Step 11: Installing the Power Switch

I should note that after I cut off the part of the circuit board with the power switch on it, I also removed a resistor that was used for the small 3-LED light on the flashlight. This made the top of the board smooth (aside from the switch itself).

I drilled a hole in the back panel, and then used a chisel to open it up to a roughly 1/2" square to fit the switch. I also chiseled down some of the edges to leave room for the wires coming off the switch. Once the switch panel fit flush against the board, I secured it in place using two screws from the original flashlight (in pre-drilled holes).

Step 12: Connecting the Circuit Together

When I made the LED strip earlier, I soldered on two long (roughly 1 foot) wires. Now that everything is installed, I started cutting these wires down, stripping the ends, and soldering them in place.

The positive wire was run back to the positive battery connector and soldered, and then the loose wire was held in place with a little hot glue.

The negative wire was run from the LEDs to one end of the switch, and then from the other end of the switch to the negative battery connector.

At this point I did a test of the system. With the back panel in place, the positive battery "door" is closed, completing the circuit. I had to add a small spacer to the "door" as it wasn't closing tight enough.

Step 13: Installing the Front Panel

I wanted an opaque white front to my sign. Ideally I would have used a white acrylic, but all I had on hand was clear. To make it more opaque, I took two pieces, and sanded them with a power sander. This made them fairly opaque and diffused the light well.

After inserting the plastic panels, two small pieces of wood were added on the inside sides and screwed in place to hold the plastic up against the front face frame. This will hold them securely and still allow for easy removal if needed.

Step 14: Securing the Back Panel

To secure the back panel in place, two holes were pre-drilled and countersunk, and then held to the box frame with wood screws.

Step 15: Done!

The artwork on the sign you see here was just printed on paper and added to the sign between the sheets of plastic. For the sign I ultimately built, I cut out several letters on the bandsaw and super-glued them to the front of the plastic.

Overall I really enjoyed making this project. I got to make a cool sign and exercise my creativity by using what I had on hand. Thanks for reading (and watching if you watched the video).

Full Spectrum Laser Contest 2016

Participated in the
Full Spectrum Laser Contest 2016

Make a Box Contest

Participated in the
Make a Box Contest