Build an Astronomer's LED Flashlight

Astronomers use dim red lights during observing session to prevent the eye's pupil from constricting. Light Emitting Diodes (LED) are often used to create astronomer flashlight for a couple of reasons. 1. A LED uses very little power and therefore batteries last long. 2. A red LED will emit only red light therefore guaranteeing low eye sensitivity.

A professionally edited version of the following document appeared in Astronomy magazine March 1993 page 60.

My astronomer's light cost me less than $10.00 and does a nice job. It is a red LED type which I built with little difficulty. The text and drawings should allow most anyone to replicate my astronomer's light.

The "bulb" is actually a red LED and a 47 ohm resistor mounted in the original bulb's base. The resistor limits the current with 2 fresh batteries to 18 milliamps and with dead batteries to about 10 milliamps. The brightness varies as the batteries discharge, but the variance is not enough to be annoying.

1. Crush the light bulb's globe with a pliers and use a screwdriver to clean out the base (use leather gloves, and watch for broken glass).
2. Unsolder the bulb base to "open" the hole in the base of the bulb.
3. Cut the LED's cathode lead (short one) and bend into a U shape.
4. Cut the LED's anode lead (long one) to length.
5. Cut and bend the resistor lead.
6. While constructing attempt to get the LED light source at the same point as the original bulb for a good focus.
7. Solder the anode lead to the resistor.
8. Solder the resistor into the bulb hole.
9. Solder the cathode lead to the base notch.

TIP 1 - there is an interaction between the LED, bulb hole, and bulb notch. Play with it a little.
TIP 2 - The resistor and diode can take only a limited amount of heat, solder quickly!

This will work on any two-battery-powered (AA, C, or D) flashlight. I find the easiest type to modify are ones with a bayonet (not screw in) bulb as depicted in the drawing.

Parts list:
Flashlight with batteries and bulb
Radio Shack LED                276-087
Radio Shack resistor           271-1105 or RSU-11344611      

Too bright?, use a larger ohm resistor (68 ohm works well). You may use any value 47 ohm and higher.

Good Luck! Yours, Ron Wagner

© copyright 1992, 1997 Ron Wagner. The above article may be reproduced, in its entirety, for personal or classroom use only. This notice must accompany any reproduction.