Edulight - Design and Build a Solar Reading Lamp
Introduction
The Edulight is a solar LED desk lamp, designed to support reading, homework and other desktop activities. As constructed by Glenergy,
it has enough range of motion in the lamp head to allow some other uses too. In the following sections, we present enough information to build solar LED desk lamps, suitable for reading, from components that are commercially available. We would like to encourage you to improve upon these designs, or to incorporate local components, and to either build lamps for people, or help other people build them for themselves. Of course, you can always buy quality-assured complete Edulights from Glenergy! Buy one now
Design Approach
We have simplified the design so that available 12 V DC components can be used. Suitable solar panels, LED bulbs and batteries can be obtained from a variety of sources. We use solar panels that are small enough to not be able to overcharge the batteries, which allows us to eliminate charge-control circuitry. We use NiMH batteries so that deep discharges will not damage the battery, as would be the case with lead acid batteries. We use a single circuit element (a self-resetting RXE polyswitch) to protect against overcurrent, so that it is not necessary to maintain a supply of fuses.Main Components and Systems
Solar Panel
Nominal 12 V solar panels are used. Voltage at maximum power is in the range of 16 V to 18V. Charging current can be specified from between 80 mA and 250 mA, depending upon the batteries used. We mount a blocking diode on the positive lead of the solar panel to protect the battery from overnight discharge through the solar panel. Of the various solar panel materials available, we have found that polycrystalline Si panels in aluminum frames are a good choice for durability and mounting convenience. However, other technologies may be more suitable for you...We have found that cords connecting the solar panel to the lamp should be at least 3 metres long, and that longer is generally better. We generally use AWG 18 wire.Batteries
We use two packs of five AA batteries, joined with a short wire and all wired in series. This gives a nominal 12 V battery with reasonable flexibility in terms of mounting. Using welded packs instead of consumer-top batteries greatly increases the durability of the batteries, with no mechanical contacts being used. While being less available then consumer-top batteries in developing countries, the use of battery packs also reduces the likelihood of alkaline batteries being used (and charged) in the lamps. We use AA batteries with NiMH chemistries, which can be specified over a wide range of capacities (800 mAh - 2700 mAh are currently available). The packs therefore have energy storage capabilities of from 10 to 32 watt hours (Wh).Of course, there is no reason that other battery technologies could not be employed. There are, for example, sealed lead acid batteries with suitable ranges of capacity and reasonable performance. The advantage of having adopted the 12 Volt standard is that enormous design freedom is made available, along with a broad range of suppliers.
LED Bulbs
The selection of bulb will depend upon a number of factors. In most cases, we recommend a cooler rather than a warmer colour. Cooler meaning richer in blues than in yellows and reds. Light this colour is more suitable for reading than the yellower colours. Brightness is obviously of key importance, and brighter bulbs either cost more, use more energy, or both. The viewing angle is important, as if it is too small then an insuffiently large area will be illuminated while if too large, light will be wasted to peripheral areas rather than being used on the task at hand. We would recommend a bulb with no lesser capabilities than those specified below:| Lumens | 30 |
| Viewing Angle (degrees) | 50 |
| Colour temperature (K) | 5000 |
Our standard bulb has 18 LEDs operating as six sets of three plus a resistor. However, higher intensity and higher efficiency bulbs are now available. While our standard bulb has an efficiency of about 25-30 lumens per Watt, newer high flux LEDs can yield 70 lumens per Watt! So, before selecting bulbs, do see what is available now. Prices obviously vary tremendously along with performance.
The base selected for the bulb should ideally be one that is common in the area of use--especially if it is desired to eventually use more local content. We use the standard Edison screw-in base, but bulbs can be specified and purchased with any of the other common base types (B22 for example).
Lamp Head and Body
We have employed a commonly-available desk lamp, which design we have modified to include a battery compartment, a charging jack, and a switch. The base of the lamp body that you choose should be able to securely hold the batteries that you use. The head should securely hold and protect the LED bulb that you use. We usually use 2.1 mm plugs and jacks to connect the solar panels to the lamps, but any convenient method can be used. Similarly, while we use a rocker switch mounted in the base of the lamp, any switch will do.Protective Circuit Elements
We use an RXE Polyswitch on the positive lead of the battery to protect the battery from overcurrent. This device acts much like a circuit breaker in that if the current gets to high it becomes a very strong resistor and prevents current flow, until the current drops below a threshhold. We use a diode on the positive lead from the solar panel to prevent the battery from draining through the solar panel at night. While you could use a fuse or circuit breaker in place of the RXE, we would recommend that you retain the blocking diode in the circuit. Instead of overcharge protection circuitry, we have employed the administrative control of limiting the size of solar panel depending upon the battery size. We use 1 W maximum output solar panels for batteries up to 1800 mAh and up to 3 W solar panels for larger batteries.Solar Panel Mounting
Within 10 degrees of the equator, we recommend simply mounting the solar panel a few degrees from horizontal, with the high side being furthest from the equator. Horizontal mounting can be used in this zone, but more frequent cleaning would likely be required. Further than 10 degrees from the equator, we recommend mounting at a tilt from horizontal equivalent to the latitude--plus 10 degrees in the winter and minus 10 degrees in the summer. Beyond 30 degrees from the equator, we recommend a 15 degree seasonal adjustment. The panel should be mounted outside.Suppliers
Glenergy can provide the components that you require, or we can help you develop specifications and other suppliers. We can help you evaluate options and select from the goods offered. Depending upon the quantities you require, you may get better pricing from Glenergy than directly from the manufacturers, since we already have ongoing business with them. Conversely, if you find an excellent supplier (price, speed, quality), then we would love to hear about them, and about the reasons for your recommendation!