Things to make - LED reading light

Things to make - LED reading light


Led reading light

This project makes a flashlight that puts out just enough light to read by. It might be useful on camping trips, for reading road maps while riding in a car at night or for someone outdoors at night pursuing amateur astronomy. Its advantage over a manufactured flashlight is that this design gets more than 50 hours operation from a set of batteries. It uses a led which lasts practically forever instead of an incandescent bulb which typically last about 20 hours.   
It may be that this project is of interest only to those who already have some of the required tools and materials. Otherwise the project might be too expensive. In particular, some people may already have a soldering iron, solder, black electrical tape and a small amount of hook up wire. Also needed from a discarded ink pen, is a large ink pen cap with a metal clip. You will need to purchase an extra bright led. This is Radio Shack part number 276-205. It outputs 6000 mcd of light and draws 20 ma of current from the batteries. The price is about $2.00. Two 1.5 volt AA alkaline cells are a good choice for batteries. Standard(non alkaline) batteries will last maybe one third as long. C or D cells will increase the hours of operation 3 and 6 times respectively but you may find their larger sizes objectionable. A 56 ohm resistor is needed. Either 1/4 or 1/2 watt will do. The color code for this resistor in case you need to pick one out from used parts, is green, blue and black. The ink pen cap with metal clip will function as an on/off switch. The resistor is to limit current so the led will not be destroyed. The Radio Shack led package will tell you that about 1.9 volts and 20 ma is needed for the led. Ohms Law(voltage=current times resistance) can be used to calculate the resistor's value. Just rearrange the Ohms Law terms to get resistance=voltage/current or ohms=volts/amps. 20 ma is .020 amps. For the voltage use 1.1 volts since you start with a total of 3 volts(2 X 1.5) from the batteries and 1.9 is used by the led. That leaves 1.1 left to be across the resistor. So the equation becomes ohms=1.1/.02. The math comes out to about 50 ohms but the closests common resistor value is 56.

Use a safety pin to poke two holes in the pen cap on either side of the clip. The holes should be exactly to the side of where the clip end contacts the cap. This is at the free end of the clip. Then strip the insulation off of a piece of hook up wire and weave it through the holes as in the lower diagram. Bring the wire all the way around the cap and twist it together. The wire should be tightly wrapped around the cap. At least one end of this wire needs to be long enough to reach a battery end. See the photo at the bottom of this page for where the batteries will be. The clip and wire make a  closed switch that will keep the light turned on without having to hold the switch closed. Then to turn off the switch, slip a piece of thin card board or some other material between the clip end and the wire.

Make a hole on each side of the cap near the closed end of the cap. This can be done by heating a nail on a stove, holding the nail with pliers and letting the hot nail melt a hole. The holes should be slightly larger than the hook up wire that you soldered to the led and resistor.

Select two pieces of insulation covered copper hook up wire, each a couple of inches longer than the pen cap. Put an end of each lead into the open end of the cap manuevring the ends through the melted holes.

Solder the resistor to the long positive lead of the led. Solder the leads coming from the open end of the cap to the free end of the resistor and the free led lead. Make sure you remember which lead goes to the resistor. That lead must go to a positive battery terminal. Put tape on any bare wire of the led/resistor/lead wire assembly that might touch and short circuit.

Carefully pull the leads farther through the melted holes until the led is up againt the open end of the pen cap.After that, wrap black electrical tape tighly around the end of the cap so about 1/4 inch of the tape's width is on the led. Go around 6 or so times keeping the tape stretched as you go so that it holds the led securely.

Tape the two batteries together side to side with a positive end next to a negative end. Place the cap along side the batteries and tape it securely in place running the tape under the metal clip and going around at least several times. Solder the lead coming from the resistor to the positive battery end that is near the closed end of the cap. The wire coming from the switch can now be soldered to the negative battery terminal that is near the closed end of the pen cap.The wire from the other led lead goes to the metal clip. Cut it to length allowing about 3/4 inch for wrapping around the base of the clip. Remove the insulation from that 3/4 inch portion before wrapping. Put a piece of paper or thin cardboard between the switch contacts. Connect the other two battery terminals to each other with a short piece of wire soldered to the two terminals.

If the light will only be used inside a vehicle, the batteries can be elimnated. Two leads coming out of the pen cap could go to a plug connector made to get power from the vehicles cigarette lighter. Or run the wire from the positive led lead to a terminal on the vehicle's fuse box. The other negative lead would be attached to metal that is connected to the vehilce's body. The resistor value must be changed to use the higher voltage from the vehicle. Roughly 2 volts will be across the led which leaves about 10 volts across the resistor. Ohms Laws gives resistance=voltage\current=10\.020=500 ohms. The common closest value is 470. Don't worry about running the vehicle's battery down. It would take at least a few days of constant operation to do that.

The finished led reading light photo should eventually appear at the bottom of this page.