The post explains a simple low cost solar pocket LED light circuit which could be used by the needy and, underprivileged section of the society for illuminating their houses at night cheaply. The idea was requested by Mr. R.K. Rao
The Circuit Request
I want to make a SOLAR pocket LED light using a 9cmx5cmx3cm transparent plastic box [available in the market for Rs.3/-] using a one watt LED/20mA LEDS powered by a 4v 1A rechargeable sealed lead-acid battery[SUNCA/VICTARI]&also with a provision for charging with a cell phone charger [where grid current is available].
The battery should be replaceable when dead after use for 2/3 years/prescribed life by the rural/tribal user.
This is meant for use by tribal/rural children to light up a book; there are better led lights in the market for around Rs.500 [d.light],for Rs.200 [Thrive].
These lights are good except that they have a mini solar panel and a bright LED with a life of ten years if not more ,but with a rechargeable battery without a provision for its replacement when dead after two or three years of use.It is a waste of resource and unethical.
The project i am envisaging is one in which the battery can be replaced , be locally available at low cost.The price of the light should not exceed Rs.100/150.
It will be marketed on not for profit basis through NGOs in tribal areas and ultimately supply kits to tribal/rural youth to make them in the village.
I along with a colleague have made some lights with 7V EW high power batteries and 2x20mA pirahna Leds and tested them-they lasted for over 30 hours of continuous lighting adequate to light up a book from half-meter distance; and another with a 4v sunce battery and 1watt 350A LED giving enough light for cooking in a hut.
Can you suggest a circuit with a one AA/AAA rechargeable battery,mini solar panel to fit on the box cover of 9x5cm and a DC-DC booster and 20mA leds. If you want me to come over to your place for discussions i can.
You can see the lights we have made in google photos at https://goo.gl/photos/QyYU1v5Kaag8T1WWA Thanking you,
The Circuit Design
As per the request the solar pocket LED light circuits needs to be compact, work with a single 1.5AAA cell using a DC-DC converter and equipped with a self regulating solar charger circuit.
The circuit diagram shown below probably satisfies all the above specifications and yet stays within the affordable limit.
The design is a basic joule thief circuit using a single penlight cell, a BJT and an inductor for powering any standard 3.3V LED.
In the design a 1 watt LeD is shown although a smaller 30mA high bright LED could be used.
The solar LED circuit is capable squeezing out the last drop of "joule" or the charge from the cell and hence the name joule thief, which also implies that the LED would keep illuminated until there's virtually nothing left inside the cell. However the cell here being a rechargeable type is not recommended to be discharged below 1V.
The 1.5V battery charger in the design is built using another low power BJT configured in its emitter follower configuration, which allows it to produce an emitter voltage output that's exactly equal to the potential at its base, set by the 1K preset. This must be precisely set such that the emitter produces not more than 1.8V with a DC input of above 3V.
The DC input source is a solar panel which may be capable of producing an excess of 3V during optimal sunlight, and allow the charger to charge the battery with a maximum of 1.8V output.
Once this level is reached the emitter follower simply inhibits any further charging of the cell thus preventing any possibility of an over charge.
The inductor for the pocket solar LED light circuit consists of a small ferrite ring transformer having 20:20 turns which could be appropriately altered and optimized for enabling the most favorable voltage for the connected LED which may last even until the voltage has fallen below 1.2V.