WIRING THE HEAD: We use a green Light Emitting Diode (LED) (see photo below right) to provide illumination for testing.  They are bright, small, run a long time on batteries, make no appreciable heat, and provide a light source nearly in the center of the visual wavelengths. Looking at the photo at left, you can see a battery pack, the back of a miniature toggle switch, and a "sawed-off" green  LED peeking out through the viewing hole. Not visible (hidden by the battery pack) is the current limiting resistor (shown in a photo below left).

OBTAINING PARTS: All the parts we used were purchased at a local Radio Shack retail store. They are all common parts and should be easily available at other electronics parts retailers or by mail order. Please see Page 4 for more information on selecting and obtaining the electrical parts.

PREPARING THE LED: The Jumbo LED we used was 10mm in diameter and about 1.5 times as long. You need to cut much of the diffusing plastic lens off, to leave the LED in the round base with a total thickness of about ", as shown in the photo at right. We did this by carefully holding the LED in locking pliers, making sure the emitter was not on our cut line, and then sawing off the excess lens with a hacksaw, using the plier jaws for guidance. The cut surface was smoothed a bit on sandpaper - it should not be polished, as we want a diffuse light source. Then the leads are bent flat, and using some of the wire that came with our battery holder we soldered the 3" leads to the LED (The red wire is positive and connects to the slightly longer LED lead, which is anode, or positive, connection). The LED is a heat sensitive component, and heat traveling up the leads can damage the device - so solder as quickly and with as little heat as practical.

MOUNTING THE LED: Push the wires though the face board, and then position the LED in the center bottom of the viewing window. An insulating black rubber faucet washer (seen in the photo above right) is then screwed against the LED leads and wires to hold it in place. Finally, some black tape is wrapped around the sides and back of the LED so that only light shines out through the window.

MOUNTING THE OTHER PARTS: Push the miniature toggle switch through the face and fasten in place with the supplied nut; orient such that off is down. Start a screw to hold the resistors but don't tighten - it is easier to fasten the resistors after they have been wired up. Finally, screw down the battery holder.

A Note on Polarity: The LED must be wired with the proper polarity to work. If you reverse the polarity, no harm will occur, but the LED will not light. Make sure the positive lead of the LED (called the anode) is connected to the positive side of the battery (usually a red wire). If your LED doesn't work, try reversing the connections to it.

HOOKING EVERYTHING UP: The wiring is straightforward; you can refer to the schematic diagram at right and the photo above for guidance. Make the 3.3 ohm equivalent resistor by twisting together the leads of three 10 ohm resistors and then soldering them. Solder the negative (black) lead from the battery pack to one end, secure this end under the screw, and then solder the black wire from the LED to the free end. This should leave two unconnected red wires, one from the LED and one from the battery pack. Solder one to each of the two switch terminals (middle and one side if your switch has three terminals).

Insert batteries and toggle the switch on. If the LED does not light:

• Check that all connections are well soldered.
• Check for polarity problems (see box above).
• Check that your batteries are good.

Mount your completed Test Head on the Stage with two screws, offsetting slightly away from the stage's adjustable foot. Congratulations! You have just completed building a fine tester which should give long years of good service.