Utilitarian Rechargeable Thumb Ring Flashlight

In today's instructable, I'll be documenting my process of making a project I consider to be the boldest of all I've attempted. The documentation shows the entire process and length of the build, right from the conception of the idea, sketch and early design prototypes, adjustments and modifications until the full perfection of the device.

The inspiration behind the build is the need for convenience and comfort while using flashlights when the need arises.

MAIN CHALLENGE AIMED TO BE SOLVED:

The number one challenge I wanted to eliminate with this build is the struggle of technicians working in tight narrow dark spaces (for example, trying to repair a pipe in a very tricky corner under the kitchen cabinet) whereby pointing a flashlight with the second hand blocks your view of what you are working with or the space might even be so narrow it prevents the simultaneous convenient passage of the working hand and the flashlight holding hand.

The idea is to make a a flashlight in a form factor that is so compact and can be worn on the working hand so that the same hand holding the tool shines the light directly at what is being done.

Along the build process, I started to realise other interesting ways of utilizing the device.

The device is also made to be rechargeable, conferring an improved usability on it.

The design, circuitry, assembly and adjustments each as a phase was an interesting one, making it a fun project overall.

Electronics:

TP4056 USB-C charging module with protection (28mm by 17mm, 4mm at its thickest point)

3.7v 800mAh Li-Po battery (of dimensions 32cm by 20cm and 8mm thick)

Connecting wires

1W chip-on-board LED

Tactile switch

1W 2 ohms resistor (I used a 1 ohm resistor but wouldn't recommend it if you can find higher resistor values between 2 to 4 ohms. Above 5 ohms would be too much and the LED would be dim)

Others:

3D print machine and filament

3D print software (I made use of Fusion 360)

Super glue (for attaching the 3D printed case parts together)

Hot glue (for security lights the electrical components in place)

Heat-shrink tubing

Soldering iron and lead

- The Five Fingers Dilemma:

I'm right handed, so it's straightforward for me to know that the flashlight would be worn on my right hand. Now, I'm proposing a ring-like approach to the design: something worn on the finger. But which finger will make the for the most usability and provide the best ergonomics?

The thumb. It's an opposable digit and parallels the other fingers in most of the hand maneuvers and position as in holding a pen, pinching, power grip and the likes. In whatever way the hand is configured, the long axis of the thumb is mostly pointing to whatever is being worked on. The thumb is also well spaced away from the other four digits, so the device being a little cumbersome doesn't always cause discomfort by the closeness between the other fingers.

To get inspiration for the designs, I headed to pinterest to browse through some concept art tech rings that have a form factor that can house the components I'll be using for the build. Got some decent inspiration which I elaborated further on paper to fit into my requirements.

The first design sketch includes the ring body, a rectangular base stack containing the charging module, and another rectangular top stack containing the LED and the switch with a circular passage between the two stacks for the passage of connecting wires.

The second sketch saw me adjusting the top stack to be in a cylindrical in shape and being rotatable up to 90 degree to provide better versatility and be able to point the light around without fatiguing the thumb with frequent movements.

I started the real building process, that is, that of turning the idea into reality by converting the sketches into more detailed and accurate printable 3D representations.

I started with the ring body and the base rectangular stack which was designed as a single unit. The diameter of the ring hole is equal to my thumb's diameter of 23mm, the depth of the base stack is a total of 12mm which equals the total height of the battery (8mm) and charging module (4mm, thickest at the USB-C port). The width of the base stack was determined by the width of the battery which was wider and longer than the charging module at 32mm by 20mm. I made cuts into the walls of the stack to make for a slit to the side for the LED indicator on the charging module and some ventilation and a port for the USB-C terminal of the charging module.

Designing the lid for the base stack: it was a simple rectangular plate with an end that completes and secures the USB-C port and a centered hollow cylindrical extrusion that allows for the passage of the wires and snapping of the top cylindrical stack on it. The top cylindrical stack was designed to accommodate the 1W LED and the tactile switch and a current limiting resistor to the LED. The LED window allows the light rays out and the switch hole allows the switch to be fitted and with spaces around it for ventilation of the compartment. The cylindrical stack is designed with two snap-to-fit holes at the top through which its lid would be attached. The base of the cylindrical stack contains a centrally located circular passage for fitting with the base stack tightly, a washer-like stopper was then designed to hold the two stacks together tightly.

(I initially designed and printed another type of snap-to-fit lid, fourth and fifth image, but it didn't come out well as the dimensions were too tiny to be printed so I then designed the stopper design)

The top lid of the cylindrical stack was made with a thunderbolt design extrusion at the top and two projecting tips for the snap-to-fit joints on its underside.

Attached are the STL files for 3D printing and the STEP files if need be for any modification of yours as in the diameter of the ring hole customized to your size, the size of the base compartment to suit the battery size you are working with.

So I sent the STL files to be 3D printed, the diameter of the printing bit was 0.4mm, and I used PETG filaments as the device would for sure be exposed to outdoor conditions, impact and heat generated over time which may cause PLA filament-printed cases to degrade over time. The downside of using PETG is cleaning up the printed structure of all the "spaghetti".

I printed the ring body and base compartment unit with a black filament which ended up looking nice and mature, almost concealing its plastic nature. The lid was printed in red and the top stack was printed in black.

NOTE: I actually did want to print the extruded thunderbolt design on the top lid in red but somehow forgot to do that. Print yours in red, it will nicely accentuate the design.

This mostly involves assembly of components of the circuit spread across two compartments.

The battery terminals are soldered to the respective terminals indicated on the charging module: the negative of the battery to the negative battery terminal indicated on the charging module and the positive of the battery to the positive battery terminal indicated on the charging module.

The output terminals to the LED circuit (positive and negative) are joined to connecting wires. It is advisable to use black/blue and red wires to properly indicate the terminals to avoid wrong connections, I have used the wires from jumper wires I have lying around, strictly keeping in mind which is positive and which is negative and by cutting the negative shorter than the positive.

In the top stack, the LED, resistor and the switch are all soldered in series and the output wires to be joined with those from the charging module.

WIth the battery going in first, the battery and the charging module are placed into the base compartments and the charging module being a little smaller than the compartment was held in place along its edges by hot glue. In the top stack, the LED fitted to the LED window and held to it at its terminals by hot glue. The switch pushed through the hole and the wires joined to those coming from the base stack then the connections were insulated by heat-shrink tubing.

The lid of the base stack is glued in place and the the top stack fitted into the base stack and pinned together with the stopper, the lid of the top stack is then fitted in place. Now we have the parts come together to form a single unit. The top stack can be rotated on the base stack, just be sensitive to when the wires begin to get taut and twisted, for that reason I only recommend up to 90 degrees rotation on each side.

The rechargeable flashlight ring is worn on the thumb and can be seen in the images above demonstrating its use cases: You can easily fill your hands with loads or occupy both hands since the flashlight is worn on the thumb.

You can easily light up book pages while holding the two halves of the book apart with your two hands.

You can work with one hand in places where two hands (working with one hand and pointing flashlight with the other hand) would cause obstruction to your view.

To charge the device, plug in a USB-C charging cable through the port, the charging indicator would show a red glow through the charging indicator slit by the side. The glow changes to blue once the battery is full. TP4056 module typically charges at 5v 1A (5W).

It however is important to remember that the essence of this device is for it to be wearable and not so burdensome so the components had to be small, which doesn't exclude the battery. The relatively small capacity of the battery means it won't last an entire day of use and so it isn't going to make traditional bulkier flashlights obsolete as they each have their own use cases.

To calculate the average duration for which the LED would stay on:

Battery Voltage = 3.7V

LED Forward Voltage = ~3.0V

Resistor = 2Ω

Resistor drops = 3.7V - 3.0V = 0.7V, Current through the circuit would then be 0.35A (V = IR)

LED Power = 3.0V × 0.35A = 1.05W

Resistor Power = 0.7V × 0.35A = 0.245W

Total Power Draw = 1.3W; Battery Life = 0.8Ah/0.35A = 2.29 hours ≈ 2h17min

Overall, I'm really happy I could bring the project idea into reality as I expected it to be, and I've been getting a lot of questions from people seeing the flashlight ring on me. 😄