Over a billion individuals lack access to electric lighting after sunset. Many resort to using hazardous fuels like kerosene, wood, or charcoal when affordable. Although modern solar and battery-powered lighting systems are slowly being adopted, a significant number of people still face light deprivation. Various organizations, including small maker groups, are striving to address this urgent issue, and I am among them.

The Responsibility Falls Heavily on Women.

The challenge of acquiring fuel is substantial. Much time and energy are consumed in obtaining it. For those living in light or energy poverty, achieving basic needs—like cooking or staying warm—requires tedious collection or purchase of fuels. This time could otherwise be spent by adults to establish businesses or by children in school, yet it slips away daily. Globally, girls disproportionately bear this burden, as resource allocation within families often sidelines their education in favor of fuel gathering and household chores.

Using any carbon-based fuel releases not only carbon dioxide but also carbon monoxide, soot, and other harmful byproducts into the home environment. Consequently, women, who typically handle cooking in many cultures, are regularly exposed to toxic fumes as a part of their daily routines. Recent efforts by organizations like Path and the Global Alliance for Clean Cookstoves aim to distribute efficient cookstoves to those in need, helping to create new microindustries worldwide. While this initiative addresses cooking and health challenges, it does not resolve the lack of lighting.

Fuel scarcity translates into elevated costs and rationing. If fuel also serves as a light source, the availability of light diminishes after dark. Access to lighting post-sunset can allow small business owners to extend their working hours or enable children to study after completing chores. Such improvements can significantly impact families and communities.

Solar Solutions Offer Hope.

Women and girls not only endure health risks from toxic fuel fumes but also face dangers like sexual assault while navigating to public latrines or collecting water at night. Public lighting and solar energy systems can address these overlapping concerns. A unified approach could mitigate multiple issues simultaneously.

Several commendable organizations are distributing larger solar systems to communities worldwide. Solar Sister empowers nanoentrepreneurs to sell small solar lighting solutions, while Little Sun has pioneered a buy-one-give-one solar light and mobile charger model. Their website showcases impressive statistics reflecting their positive impact. Numerous small organizations pursue similar goals, and I encourage support for their missions.

However, a significant drawback of these solar systems is that two primary components cannot be produced by the end users. While some maintenance and battery replacement may be feasible, the tools themselves cannot be created locally. The pandemic highlighted vulnerabilities in our supply chains, emphasizing that everything in these devices can be reused—except for the battery.

The Quest for Sustainable Batteries

Most systems rely on lithium-ion batteries. When a battery fails, the lack of a means to replace or recycle it poses a significant challenge. This realization led me to develop a "recipe" for creating a power storage solution that is simple, safe for homes with young children, easily recyclable, and achievable with minimal skills or tools. I am currently drafting a book to share this recipe and am engaging with early testers for the technique.

Forgive the rudimentary battery cell depicted, which has been operational for three years with only occasional electrode replacements and water additions. I never anticipated its longevity, but it’s encouraging to see it function. Its energy density is lower than modern alternatives, yet it generates enough power to illuminate a bright white LED.

Wonderful! The cell currently achieves around 25Wh/liter, capable of powering a standard 5-watt LED light bulb throughout the night with just 2–3 liters of battery capacity, akin to a car battery size. Not optimal, but sufficient! Here’s a more refined version of the cell that scored 25.3Wh/L, while lead-acid batteries typically range from 35–50Wh/L.

After nearly six years of experimentation with this chemistry, I found that success relies on patience, meticulousness, and quality materials. Achieving 5 Wh/Liter is straightforward with basic resources, while reaching 25 Wh/Liter requires more careful planning. This battery chemistry isn't perfect; it's "good enough." It fulfills its objectives, is rechargeable, and can be serviced easily. However, it doesn't match the performance of lithium-ion batteries, which deliver higher current and density. Perhaps in a decade, I can refine the process further to create a battery capable of powering larger appliances, but for now, we can construct a battery for lighting.

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Now, returning to our solar light, consider if we could enhance the efficiency of the LED. It emits a bright white light, consuming substantial power. Is continuous operation necessary? By flashing the LED rapidly, we can achieve a perceived brightness while conserving energy. In my initial experiments, I utilized a "Joule Thief" circuit, which amplifies a low voltage, such as 0.3V, to over 5V, allowing LEDs to function on nearly depleted batteries. The result is a seemingly very bright light that actually flickers at speeds beyond human perception (over 1000 times per second). Fascinating!

However, this method can over-drain a battery, potentially causing damage. It is suitable for harvesting tiny voltages from sources like microbial fuel cells. Yet, the concept remains valuable for our users. If we have a low voltage battery like the inkwell that can be serviced, we can utilize it to power more robust lighting solutions! In my laboratory, I employ a similar "Boost Converter" that connects to a USB port to operate LEDs from my battery. This converter includes basic control circuitry to prevent excessive power drain as levels decrease. Given that the inkwell operates at roughly 1V, I need to boost it to 5V for USB devices or assemble several cells to achieve the necessary voltage.

We have successfully replaced our client’s faulty battery with one she can construct herself or have made locally. But is there room for further enhancement?

Is a full-brightness white LED always necessary? While bright white light is ideal for tasks and studying, it requires significant energy to produce across the spectrum. Internally, these lights generate considerable UV light, filtered through a yellow layer, with some energy wasted as heat. Must we constantly generate such power?

Assessing Requirements for Illumination.

If our client is traversing at night to a public well or pump, she requires light but may not need to see in full color. If we can significantly reduce the energy demand of an LED while maintaining apparent brightness, we can ensure her safety during her walk while using less powerful batteries.

Power Considerations.

Using the inkwell, local communities could create their own lighting systems while waiting for funding or access to more powerful options. Even if communities wish to utilize existing power systems, they can achieve the same effective lighting with fewer batteries. The system must remain adaptable based on available resources.

LED Driver Implementation.

We can integrate a Joule Thief-like circuit, a small microcontroller, crystal oscillator, or a 555 timer circuit to pulse the light at a rate beyond human perception (at least 60Hz). Including a tuning feature would allow adjustments for those sensitive to flickering, especially if the light source is mobile.

The driver’s purpose is to pulse the LED, creating brief, highly efficient bursts of light. When these flashes occur in quick succession, minimal power is consumed, yet users perceive a bright and useful light.

With the right LED, applying a steady DC voltage (around 3.7V) to a single LED may draw approximately 30 milliamps (about 120 milliWatts). Kitchen LED bulbs might equal 15 of these LEDs. By pulsing the LED effectively, we could achieve apparent brightness matching the DC version at only 40 microamps (about 160 microWatts), resulting in a reduction of power usage by roughly 1000 times.

Choosing the Right LED.

The type of LED is crucial. Indium gallium nitride (InGaN) LEDs are highly energy-efficient. However, does our client always need white light? What if we could provide an alternative color that meets basic visibility needs without full-spectrum illumination? Consider older military flashlights, which used red filters to preserve night vision. A more effective option is green light, particularly cyan light in the 505nm range.

Leveraging Biological Sensitivity.

Our eyes contain two types of receptors: rods for brightness and cones for color. In low-light situations, rods dominate, allowing for what is known as scotopic vision, which is most responsive to the blue-green spectrum centered around 505nm. This means that energy expended in this range translates more effectively into visible light.

This green light (similar to that of traffic signals) falls perfectly within the spectrum for optimal scotopic vision. Therefore, every watt of energy allocated to this range is significantly more effective for visibility than energy spread across the entire spectrum. This approach is particularly advantageous for specific tasks, minimizing waste while providing necessary illumination—like having a light in a latrine that can last a decade.

Integrating the System.

Thus, we can select an LED that guarantees the most significant response (505nm cyan), paired with one of the most efficient chemistries (InGaN), and then activate it in a pulsed manner to optimize power usage, utilizing either repurposed or locally crafted batteries.

Existing Solutions Are Available.

Ted Yapo, an innovative maker, has been merging the efficiency of cyan LEDs with the aforementioned pulsed drivers to produce flashlights and other compact lamps that can operate for decades without requiring battery replacements. These solutions, while potentially costly for individuals, become economically viable when produced en masse. Local workshops could easily manufacture these lights with integrated circuitry and distribute them to those in need. Ted’s design features a lithium cell, promising ten years of continuous operation.

Given the lower voltage of the inkwell, I will explore methods that bypass the PIC microcontroller, utilizing 555 timer circuits or crystal/transistor-driven versions to leverage widely accessible knowledge of crystal radios.

This entire discussion stemmed from a serendipitous online meeting with Mr. Yapo, where he noted, "...this work has been a solution in search of a problem from the start." That comment highlighted some missing pieces in my approach to developing a fully accessible and craftable lighting solution for underserved regions.

By sharing our ideas and collaborating, we can illuminate the path out of darkness together.

Stay luminous, friends.

Learn More About the Decade Flashlight.

For additional information on Ted Yapo’s initiatives, follow him on Twitter @tedyapo or visit his Hackaday pages detailing the decade flashlight.

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