Flexlight: a Solder-free Coin Cell LED Flashlight

My goal for this project was to create a simple battery-powered LED flashlight with minimal parts and no soldering needed. You can print the parts in a few hours and assemble it in about 10 minutes, which makes it great for an (adult supervised) afternoon project. The very simple electric circuit in this flashlight is inspired by LED throwies, which you can learn more about here.

The Flexlight is a 5-piece design (3 purchased, 2 printed) that leverages the elasticity of the flashlight cover and LED lead to act as a normally-open switch, which can be pinched by the user to temporarily turn on the flashlight. At the time of writing this Instructable, I've designed and tested two versions of the flashlight - one uses a CR1225 coin cell and the other uses a CR2032 coin cell. The material cost for this flashlight is < $2 USD.

In addition to the .STL files for these two versions (Thingiverse link here), I also share the downloadable parametric Fusion 360 models so you can modify/adapt the design further to suit your needs. I'm excited to learn from your improvements on these designs!

Smaller Flexlight design (uses CR1225 cell): https://a360.co/2H2ZO2F
Larger Flexlight design (uses CR2032 cell): https://a360.co/35z9ADb

Safety

Small Parts: The purchased components are relatively small and pose a swallow hazard, do not leave small children unsupervised with these parts.
Light: The light produced by the flashlight can be intense, do not shine the lit LED directly in your or others' eyes.
Battery: Do not short the positive and negative terminals of the battery, as this can damage it, heat it up, and/or cause it to discharge its contents.

Supplies

Tools:
- 3D printer (or access to one) - for reference, I printed these pieces in PLA on an Ender 3 Pro
- Access to a computer with Ultimaker Cura (Link)
- Flush cutter and pliers
- Small Phillips screwdriver (for #2-56 screw)
Materials:
- Your printing material of choice
- 1x 3V Lithium cell battery (CR1225 or CR2032, depending on the Flexlight model)
- 1x 5mm Clear LED with >= 3V forward voltage (White, Purple or Blue recommended)
- 1x #2-56 x 1/4" long machine screw

Step 1: Printing Your Flexlight Parts

Grab your preferred version 'base' and 'cover' .STL files from Thingiverse (or make your own version from the F360 files!) and open up Cura. These parts will be printed in two separate sessions, as discussed below.

Flexlight Base

The base is relatively straightforward and takes less than 2 hours to print. I recommend the following settings:

Print in the "upright" orientation as shown in the images, to ensure the battery retention feature does not break across a layer line during battery installation/removal.
Print with support placement > Everywhere
'Standard Quality' with infill 20% and layer height 0.2mm should be sufficient, but higher quality is OK too.

When printing is complete, use your flush cutters and pliers to remove the support material.

Flexlight Cover

The cover is designed to leverage a special mode in Cura that allows us to print it in one continuous spiral, and takes less than 1 hour to print. I recommend the following settings:

Print in the "upright" orientation as shown in the images.
Print with Special Modes > Spiralize Outer Contour
'Standard Quality' with layer height 0.2mm should be sufficient, but higher quality is OK too.

Why Spiralize Outer Contour? The primary advantage of this feature is that we can print our thin cover without layer seams, which are present in typical prints and would detract from the look and feel of the cover.

Step 2: Assembling Your Flexlight

With your printed and purchased parts in hand, it's time to build!

Slide the LED into the printed base and rotate it so the cathode (negative side, typically the shorter lead) is resting on the battery retention feature. Bend the anode (positive side, typically the longer lead) down through the cutout in the base.
Move the cathode to just below the battery retention feature and insert the coin cell (negative side up) under the battery retention feature and cathode. Push the battery until it clicks into place in the base. Confirm that the cathode is now being sandwiched between the battery retention feature and the negative face of the battery.
- Just after installation, the anode should not be touching the positive side of the battery. If it is, bend it a little further away so it is nominally not touching.
Do a quick test by pushing the anode to the positive side of the battery and ensure that the LED lights up. When you release the anode, it should spring back and the LED should turn off. If the LED does not light up, check the following and correct as needed:
- Confirm that the anode and cathode of the LED weren't swapped during installation.
- Confirm that your battery is not depleted.
- Confirm that your LED is not damaged.
- Look for shorts between the LED leads.
Gently slide the cover onto the base, ensuring that the anode lead is not being bent out of place as you slide the cover on. Fasten the cover to the base on the end opposite the LED, using the #2-56 screw.

All done! Now, you can squeeze the cover near the dimple regions to turn the Flexlight on, and simply let go to turn it off! If your quick test in Step 3 was OK, but now the Flexlight doesn't work as expected, remove the cover and:

Confirm that the anode wasn't bent out of place.
Look for shorts between the LED leads.

I hope you enjoy using the Flexlight for your illumination needs, whether this involves stashing several around your living space for power outage / emergency response, building them for fun / education, or something else entirely. I'm also excited to see what changes/improvements you make to this design - like reducing part count, modifying it for use with a different size LED, or improving the ergonomics. Thank you for reading!

Step 3: [Optional] Electrical Information

Out of curiosity, I measured the voltage across and current through the LED during use for each of these designs and share these values here.

The internal resistance of the coin cell battery and the relatively high forward voltage of our white or blue LED allows us to get away with a simple electrical circuit in the Flexlight. However, the current sourced from the battery is still an order of magnitude higher than the typical value listed for full rated battery life (~2-3mA measured vs. ~0.2 mA typical rated). Adding a resistor (~ < 100 ohms) in series would be a good approach for improving battery life of the Flexlight, but would result in a dimmer light.