Build a Mirror Tester
A Foucault or Knife Edge Tester and Ronchi Tester
Page 3: Head Electrical

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. The LED we used in 1998 when we built this tester is no longer available at Radio Shack. Please see Page 4 for more information on selecting and obtaining the electrical parts, especially the LED.  Also note that I used "D" sized batteries in this tester, mainly because it was to be used in a mirror class and would get heavy use. "C" or "AA" sized batteries would be fine for personal use, and would reduce the cost and weight of the tester a small amount.

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 1/4", 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 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.

HOOKING EVERYTHING UP: The wiring is straightforward; you can refer to the schematic diagram at right and the photo above for guidance. Note that the 3.3 Ohm resistor is specific to the LED we used, you may need to use another value: See the calculations below. 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.

USING ANOTHER LED: If you use an LED other than the one we used, you may have to use a different value resistor. This section will tell you how calculate the required value. The resistor is used to control or limit the current flowing in the circuit - too high a current can burn out a LED.

We used a Radio Shack part number 276-215 Green Jumbo Light Emitting Diode with 350mcd brightness and a peak emission wavelength of 565nm. (Note that this part is no longer available - see Page 4) We will use this part's characteristics in our example calculations.

You will need to know the Forward Voltage or Nominal Supply Voltage for your LED, and the Forward Current, which should be listed on the package.

First, calculate the Voltage that will be across the resistor:

V_RESISTOR = V_BATTERY - V_LED= 3.0V - 2.8V = 0.2V

Here we used 3.0V as the fresh battery voltage (1.5V per cell) and the maximum forward voltage of the LED (2.8V) as listed on the package. You could be more conservative and use the typical forward voltage, which is listed as 2.2V.

Second, lookup the forward current of the LED, which will be the desired current through the resistor. (I is used by electrical engineers as the symbol for current).

I_LED = I_RESISTOR = 60mA = 0.060A

Here we again used the maximum value listed on the package. Again, the more conservative approach is to use the typical forward current of 40mA. We converted to Amperes from milliAmperes so that we would get Ohms of resistance in the next step.

Third, we use Ohm's Law (V = I * R) to calculate the desired resistance:

R= V / I = V_RESISTOR / I_RESISTOR = 0.2V / 0.060A = 3.33 ohm

Resistors come in only certain discrete, standard values, and you may have to round up or down to the nearest value - rounding up will slightly lower the current and is the conservative thing to do. Checking an electronics parts catalog, we see that 3.3 ohm resistors are made, as are 3.0 and 3.6 Ohm units. However, Radio Shack does not typically stock resistors smaller than 10 ohm. Since resistors are inexpensive items (about $0.50 for a 5-pack at Radio Shack, and less than a penney in 100 quantity mail order) we choose to use several 10 ohm units in parallel to get our desired value.

Equivalent Resistance of Resistors in Parallel:

R_EQUIVALENT = 1 / [ (1/R₁) + (1/R₂) + (1/R₃) + ... ] = 1 / [ (1/10) + (1/10) + (1/10) ] = 1 / [ 3/10] = 10/3 = 3.33 Ohms

Using more conservative values will decrease the brightness slightly and increase the LED lifetime. Our LED cost less than $3, so we choose to push for maximum output at the risk that it will need to be replaced sooner. Our unit has been used by several club members for over a year with no ill effects noticed, and plenty of brightness for testing even in only slightly darkened rooms.