Teacher's Guide: Adaptations of Selected FunFest Demonstrations
Hopper Popper
Make your own hopper poppers from old racquet balls. Locate a very faint seam line around the circumference of the racquet ball. With a utility knife, make an incision on the seam line. Then with scissors, cut the ball in half. Turn one of the halves inside out and carefully trim all the way around the outer edge. Test your cuts often by dropping the 1/2 racquet ball (turned inside out) on the floor or table. When it springs back higher than your dropping point you have a working hopper popper. If the 1/2 racquet ball will not stay in the inside-out configuration, you have trimmed away too much rubber. You must start over with another half-racquet ball. Since this activity involves a significant amount of fine motor control with a very sharp instrument, it cannot be recommended as an activity for younger children.
Dropping Stacked Balls
Use your own collection of balls for experimentation by dropping the balls in pairs, with the lighter ball on top. It is suggested that you use the same ball on the bottom, such as a basketball, to reduce the number of variables. As you use progressively smaller balls for the one on top, you will notice that the top ball bounces higher and higher. You will also notice that the basketball on the bottom bounces progressively higher as the top ball becomes less massive. In this case, the basketball is able to transfer less and less of its energy to the ball on top, and thus retains more of its energy in its upward bounce. The situation of greatest energy transfer efficiency occurs when the top ball is equal in mass to the bottom ball. In the limit of the top ball having no mass at all, the basketball retains very nearly all of its energy for its own bounce. Of course, there is nothing wrong with putting the larger ball on top, since the children can still make meaningful observations from such a situation. This approach, however, tends to produce far less interesting results.
A toy marketed under the name Astro Blaster°, which has several superballs arranged vertically on a rod, can be used to demonstrate an extreme case of this situation. In this device, the lower balls clearly transfer a significant amount of energy to the topmost ball, resulting in a rebound of perhaps three stories high! In the case of this toy, all balls except the top one are prevented from leaving the rod.
Action-Reaction
A very effective demonstration device can be built from a scrap of wood, two pieces of dowel rod, three nails, a rubber band, and a string. Once the device is constructed as shown, burn the string with a match. This insures that you haven't influenced the system with any extraneous external forces.?(You(You must use your judgement about the children's use of tools and matches.) If the cart and nails exert a force on the rubber band causing it to accelerate to the left, then a reaction force from the rubber band will cause the cart to accelerate to the right.
You might experiment further with the device by placing masses of various sizes in contact with the stretched rubber band.
Inertia and Impulse
If you can find a large, massive block (the one shown weighs 50 pounds), you will find that it can be easily supported by a few foam cups. We suggest that you use four to six cups. If you strike the block sharply on the top with an instrument such as a hammer (this strike is often called an impulsive blow), the force of collision is quite large. However, it acts on the block for such a short period of time, that the large amount of inertia of the block precludes any significant acceleration. In other words, the block just won't move much. In this case, the foam cups should survive the hammer strike without damage.
On the other hand, if you lift the block several inches above the cups (Please be careful!) and then drop the block, you will find that although the force of contact of the block against the cups is only slightly greater than 50 pounds, the small mass of the cups does not offer enough inertial resistance to the block to slow it down considerably. In this case, the block is already in motion before contacting the cups, and the cups hardly slow the block's fall at all.
A vertical stack of washers can also be used to nicely demonstrate the principle of inertia. As shown, a spatula may be used to strike the washers. However, the washers must be thick enough and the spatula thin enough so that the edge of the spatula only strikes one washer at a time.
Sliding the striker quickly across the table surface, you should be able to remove the bottom washer from the stack without significantly disturbing the ones above. Books or similar items partially surrounding the washers can prevent them from hitting other objects in the room. As with the pop bottles and dollar bill, if the bottom washer is accelerated rapidly, the frictional force between it and the washer above does not last long enough to produce any significant acceleration of the washers above.
Bed-of-Nails
It is quite straightforward to make your own bed of nails, and construction details are provided below. However, unless this construction could be incorporated in a shop project, it could hardly be considered a hands-on activity for the students. In fact, the construction of even a miniature version of the bed-of-nails can require several hours of hammering. Fortunately, there is an extremely miniaturized version which makes an excellent hands-on activity.
For this, you will need a 3" square of foam insulation and at least 100 straight pins for each group. This type of insulation is extremely common in construction, and is available in nearly all hardware stores in 4' x 8' sheets.
The 100 straight pins should be pushed through the foam until the heads are level with the surface. The pins should be as close to each other as possible. (See the photo.) This should result in a bed-of-pins about 1 square inch in area. At this point, each group should remove any pins that stick out farther than the rest of the group, as uniformity in height is critical. The students should now carefully feel the "surface" they have constructed with a fingertip. Pins placed this close together begin to feel a lot more like a scratchy surface than what one might imagine for a collection of pins.
If you now partially inflate small balloons with water, the students should be able to have their balloons "rest" on their bed-of-pins without puncturing the balloons. You should not inflate the balloons to a size any greater than can be balanced on your bed. This should also be done over a sink, as this density of pin points is only marginally dense enough to support a water balloon without bursting the balloon. You should expect that some of the balloons will burst during experimentation.
Full-Size Construction Details: Construct from plywood at least 1/2 inch thick, 30 inches wide, and 6 feet long. Hand-hold cutouts can be made about 1/3 of the way in from each end and on both sides of the board. The hand-holds make it much easier for one person to carry the bed-of-nails. From 1/4 inch plywood, cut a cover that will fit over the bed-of-nails. Screw eye-screws near each corner, so that with bungee cords, you can easily and quickly secure the 1/4 inch cover over the nails. We suggest laying out a 1 inch grid on the bed with about a three inch border around the perimeter. Driving 6 D common nails at every corner on the grid results in a bed containing several thousand nails. In order that clothes will not be ruined by the nails, a lab coat or other suitable protective cloth should be provided for the victim to wear.
Barrel Crunch
The same experiment can be performed with a common soft-drink can, a hot plate or Bunsen burner, a set of tongs, and a beaker of room temperature water. Place enough water in an open soft-drink can to cover the bottom to a depth of about 1/2". Heat the bottom of the can until steam can be seen escaping from the open can. Continue heating for at least another minute, or until you estimate that steam has replaced nearly all of the air that was originally in the can.
Using the tongs, remove the can from the source of heat and rapidly transfer the can to the beaker of room temperature water, taking care not to spill any boiling water on anyone, placing the can in the water open-end first. If the open end remains beneath the water surface, this will insure that air will not rush into the can to replace the steam. The can should immediately collapse as the steam cools and condenses, creating a partial vacuum.
When younger children are involved, the same phenomenon can be demonstrated by placing very hot water in a plastic container (such as a dishwashing detergent bottle) and heating the air in the container by agitation. The bottle should be about 1/4 full of hot water and must be closed for the purpose of agitation. Open the bottle after agitation to let some pressurized hot air escape, and then close the bottle to prevent any room temperature air from returning. If the bottle is now cooled by running cold tap water over the outside, the bottle will collapse on itself, but not to the degree witnessed by the soft-drink can or the 55 gallon drum seen in the FunFest.
Suction Cups
Inexpensive suction cups can be as simple as pairs of flexible sink stoppers available from grocery or variety stores. It is somewhat clumsy for adult fingers to securely grasp the protrusion in the middle of the stopper, but this also prevents the use of extreme force in attempting to pull the stoppers apart, as shown. It is not difficult for an adult to use enough force to cause the protrusion to separate from the body of the stopper. Single stoppers can also be used on smooth, flat surfaces as suction cups. In either case, the technique for use involves slapping the surfaces together, quickly followed a motion to pull the cup(s) away.
By sealing a plastic bag of appropriate dimension to an appropriate vessel as shown, an effective vacuum can be achieved by lifting the center of the bag. Depending on the weight of the vessel, the formation of a relatively small vacuum will allow the entire apparatus to be lifted by pulling on the plastic bag.
Another simple demonstration which can show the effects of a partial vacuum, can be done with a sheet of newspaper and a hard plastic ruler. Place the newspaper on a table, with the edge of the paper running along the edge of the table. Slide a plastic ruler under the newspaper at the edge of the table, with perhaps 8" of the ruler under the paper. If you lift the ruler slowly, you can demonstrate that it is not difficult to raise the paper with the ruler. However, if you reposition the paper and the ruler, and strike the free end of the ruler sharply, as shown in the photograph, the upwardly moving part of the ruler will attempt to lift the newspaper quickly, and thereby form a partial vacuum between the paper and the table. The atmospheric pressure existing above the newspaper is easily great enough to prevent the formation of a significant vacuum by the motion of the ruler, and the ruler will be broken in the process. This clearly results in the loss of plastic rulers, and appropriate safety precautions should be utilized to insure that no students are cut by sharp edges of the ruler when it breaks.
Balloons in Vacuum Chamber
Marshmallows and balloons can be made to exhibit behavior similar to that seen in the FunFest by using hand-operated vacuum pumps obtainable from scientific supply houses. The price for such pumps will be in the $30 to $50 range.
If you use such a pump, you must also acquire a suitable vessel for evacuation, as well as appropriate connectors, stoppers, and stiff plastic tubing with the proper diameter. The greatest concern here regards a suitable vessel. In developing this adaptation, we found that it was quite easy to cause laboratory flasks to implode. Please do not attempt to perform such activities without a vessel designed to withstand internal vacuum! To get balloons to burst will require very vigorous pumping. Young children will not be able to operate the device rapidly enough to cause balloons to burst.
Marshmallows work well with the hand-operated pump, since there is not the need to pump as vigorously as in the case of the balloons. After the optimum vacuum has been obtained, a rapid release of air back into the chamber will produce the same sort of crinkled marshmallows as seen in the FunFest show. Even if you cannot obtain a high vacuum on the marshmallows with the hand pump, maintenance of a lesser vacuum for an extended period will allow sufficient air to diffuse from the marshmallows to cause them to collapse upon reintroduction of atmospheric pressure.
Pencil Shoot
A much lower energy version of the pencil shoot can be reproduced with soda straws and potatoes. If you can obtain paper straws, so much the better, as this emphasizes the dynamics of the situation, rather than having the students believe the behavior is due to the greater rigidity of the plastic straws.
Have the students hold the straws as shown. Keeping the thumb over the end of the straw seems to be quite important to display the proper behavior. The most likely reason for this is that with a quick strike, the air inside the straw will be pressurized as soon as the striking end enters the potato, and thus will increase the rigidity of the straw. One potato can serve several students in a group, making this a very economical activity.
Vortex Generator
Vortex generators can be built very inexpensively and in nearly any size. The cardboard box model can be easily built by students with almost certain success. One end of the box needs to be completely open. Cut a round hole in the geometric center of the end opposite this. Smaller holes tend to work better than larger holes, and a hole that is too small can always be enlarged if necessary. The open end should now be sealed by taping a trash bag of appropriate dimensions, as shown. This should be done so that there is some tension across the surface of the bag. At this point, the Vortex generator should be functional, and can be used to extinguish candles, move feathers, and other such feats from appreciable distances. As a test of building and aiming skill, Vortex generators from each group could be arranged in a circle, and each group could take turns attempting to extinguish a candle in the center of the circle. Younger children might participate in a similar same game by aiming at a balloon hung from a string in the center of the circle.
A smaller and sturdier version of the Vortex generator can be constructed from a coffee can. After cleaning an empty can, use a can opener to remove the lower surface. Any elastic surface could be used to cover the bottom of the can, and the plastic, slip-on lid from a second can of the same size coffee can works very well. Cut a small hole in the geometric center of the original slip-on lid of the can, and your mini-Vortex generator is complete.
Aside from using liquid nitrogen as in the FunFest show, we know of no simple method to produce reliable "smoke rings" from a Vortex generator. Even if you have liquid nitrogen available, its use cannot be recommended without reservation, as it must be handled with considerable care. For the coffee can generator, however, chalk dust scraped from erasers can be placed inside the can. After doing this, simply shake the can to suspend some of the chalk dust in the air inside the can before producing your "smoke ring". Students will not be able to see these smoke rings easily unless they are quite close to the vortex generator.
For the sake of visibility, elegant Vortex rings can be produced by using food coloring, an eye dropper, and a beaker of water. Merely load the eye dropper with food coloring and release one drop at a time above the clear water surface in the beaker. An eye dropper produces superior results compared to letting drops form from the mouth of the food coloring bottle itself. If the food coloring is refrigerated before use, this inhibits mixing of the food coloring with the water, and the rings will be even more persistent. After the water in the beaker has become too dark from food coloring, refill the beaker with fresh water. Students may experiment with varying the height of release of the drops, but we have found the best rings to be produced when the drops are released only a small distance above the water surface.
Laser Light Show
Assuming that you have access to a laser, there is no reason why you could not build your own version of a visual music pattern generator. For the purposes of demonstrations such as these, one does not need a laser any more powerful than a laser pointer. Laser pointers can be purchased from many electronics stores for well under $50. Other than the laser, all that is required is a small mirror, a dab of glue, and access to a speaker cone driven by an audio amplifier. Probably the most available source of such a speaker would be one of the speakers on a boom box. If you use such a speaker, you would likely need to remove the grille protecting the speaker, and this may require some effort and/or inspiration. If you have a boom box or other audio amplifier (stereo system) that is capable of driving an externally connected speaker, you may wish to do so. It is not difficult to damage a speaker cone with such efforts, so either be careful during construction, or use a speaker that is expendable.
For most room-sized presentations, you will want the maximum motion of the speaker cone possible, so be sure to use the outer section of any speaker that has more than one vibrating surface. (See the next Figure)
For extra protection of the speaker, you could enclose the transmission side of the speaker with an elastic material, as we do with our FunFest apparatus. Whatever your situation, you have the option of attaching more than one mirror to the vibrating surface. This would allow your class to see how variations in vibration occur across the surface of your speaker. You should be able to generate very interesting patterns by using different lasers on different mirrors. Whenever you are using lasers in class, be sure to prevent direct, or reflected laser beams from shining in anyone's eyes. Here, reflected laser beams refers to reflections from polished surfaces, such as a mirror. Diffuse reflections, such as those from a wall or blackboard are harmless. When you attempt such a demonstration, the laser should be immobilized, either with laboratory clamps, or with less formal means, such as enclosing the pointer in a book.
An extremely simple form of this demonstration involves stretching a rubber membrane over one end of a coffee can that has had both ends removed. If a piece of mirror is glued to the membrane as shown, and a laser beam is projected onto the mirror, the same sort of patterns as seen in the FunFest show may be created by having a person make tones with their voice directly in front of the open end of the can. A standard 12 oz. coffee can resonates very nicely within the male baritone range. You could try smaller size cans for resonances at higher tones.
Another novel demonstration involves gluing a small piece of mirror to a person's wrist, as shown. Be sure to use glue or wax that may be easily removed. If the person's forearm and wrist are kept very still, and the laser beam from an immobilized laser reflects from the mirror onto a ceiling or wall, the entire class should be able to "see" the person's pulse.
Electrostatics
Electrostatics experiments work best during the fall, winter, and spring seasons when there is low humidity. Warm summer air tends to hold much more moisture than the cooler air of other seasons. Whenever the humidity is high, static charges will dissipate very rapidly, and electrostatics experiments will be amazingly undramatic.
You can simulate the effect of the student's hair standing on end with the following common objects: empty aluminum soft drink can, crepe paper, tape, PVC (plumbing) pipe, a flat piece of insulation (foam or rubber), and a wool scarf. Cut strips of crepe paper about 1/8" wide, (narrower is even better) and about 4" long. Six strips work nicely, but you could experiment by adding more until the effect diminishes. The more area you have, the more charge it will accept, which could exceed the amount of charge you are able to provide.
Tape one end of the crepe paper to the bottom rim of the can. Set the can on the insulation so that it is insulated from the desk or table. Stroke the PVC pipe vigorously with the wool scarf for several seconds and touch the can. The crepe paper will stand out from the can just as the student's hair stood out. If the crepe paper quickly falls back against the can, you have excessive leakage. You might be able to reduce leakage by drying the can and the crepe paper with a hair dryer.
PVC pipe is sold in 8' lengths and may be purchased from the plumbing section of discount department stores such as K-Mart. You may need to have a store employee cut the pipe for ease of transport in your vehicle. It can easily be cut to appropriate lengths with a hacksaw. We suggest using PVC with diameters from 1/2" to 11/2", and 12" to 24" in length.
Laser PA System
It is recommended that children not be allowed to operate lasers in the classroom, even the very low powered laser pointers. Laser pointers with a typical maximum power of 5 milliwatts can produce retinal lesions when allowed to remain on one area of the retina for a time as short as one-quarter of a second. This makes the operation of a laser pointer by a lecturer reasonably safe, but provides for substantial risk of injury in the hands of a child.For this reason, our adaptation of this demonstration utilizes Light Emitting Diodes (LED's), which produce harmless visible radiation.
For this activity, two circuits must be constructed, a receiver circuit ant a transmitter circuit. Each of these circuits is shown below. However, a couple of details of construction must be discussed. If you use Solar Cells from Digikey (contact info at the end of this page), construction is not at all difficult. If, however, you use Solar Cells from Radio Shack, the greatest difficulty lies in preparing the solar cell for use. The solar cells available from Radio Shack are very brittle, and are easily broken. For this reason, we suggest that you use double-sticky foam tape to secure them to the bottom of a 35 mm film canister. The canister should also have a hole drilled in the side near the bottom for the electrical leads.
The Receiver Circuit
If you order Solar Cells from Digikey, the Receiver is extremely easy to build. The Solar Cells from Digikey are encased in plastic (or maybe glass), already have the leads connected to the cell, and are extremely durable. All you need to do is drill a couple of convenient holes in the bottom of the film cannister and use a little double sticky tape to fasten the solar cell to the bottom of the cannister.
Solar Cell before and after mounting in the film cannister.
If you decide to use Solar Cells from Radio Shack, before mounting or soldering the electrical leads to the solar cell, you will probably first need to break the cell to a size that will fit in the canister. A fairly reliable means of doing this involves placing the part of the cell you wish to use inside a large book. Make sure you include at least part of the shiny edge of the front face of the cell in the part you wish to keep. This edge is the only part of the front face where you will be able to solder one of the electrical leads. (See Figure)
If you are careful, you may be able to get two or more useable pieces of solar cell from the original piece. Even if you wind up with a fairly small piece of solar cell, it may work just fine. The figure below shows the appearance of an appropriate piece of solar cell inside a film canister.
When you solder the electrical leads to the cell, use the most flexible wire available (usually very thin wire, 24 gauge or higher). As mentioned, solder can only be applied to the shiny strip on the front surface, but can be applied anywhere on the back.
If the wires are at all stiff, bending the wires after the cell is mounted in the canister can cause the cell to break. After the wires have been routed through the hole in the canister, a dab of epoxy in the hole helps to isolate the cell from stresses and strains. Placing clear tape over the open end of the hole will prevent inquisitive fingers from touching (and likely breaking) the cell inside the canister. For the purposes of this project, you need not pay any attention to the sense of positive and negative voltages produced by the solar cell.
The Transmitter Circuit
Before connecting the audio source, the transmitter circuit consists of a single loop of four elements--two resistors, one LED, and one battery. Soldering always provides the best connections, but is not necessary. The circuit shown here has some of the connections formed by twisting together the wire ends. If this approach is used, you might need to squeeze some of the twists with pliers to make the connections more reliable. In order to make the LED light, you must observe its polarity. Looking at the bottom of the LED, you should be able to observe a flattened section near one of its wires. The wire nearest the flat section is the negative side of the LED. In the picture, this wire is connected to one of the resistors. The other wire from the LED is connected directly to the positive pole of the battery. If your LED doesn't light, try reversing the battery connections. Be sure not to hook your LED directly across the battery, however, as this will cause the LED to burn out. One of the primary purposes of the resistors is to insure that the LED does not carry too much current.
Some of the 1/8" mini plugs are shown with the plastic covers removed. This makes the connections visible to the students and hopefully less mysterious. It is probably a good idea to solder the connections to the mini plugs and alligator clips. However, the ready-made wires with alligator clips available from Radio Shack can eliminate much of the need for soldering.
When connecting the audio source, put the 1/8" mini plug into the headphone output of your device. The two alligator clips should then be connected "across" either of the resistors, as shown. The reason is that some audio sources (boom boxes, whatever) provide very little resistance across the output terminals, which can effectively short out the resistor. Since two resistors are used in the circuit, even if one of them is shorted out, the other will provide sufficient resistance to prevent the LED from burning out. Of course, this assumes you are using either the stated Radio Shack LED's or LED's with similar current carrying capacities. The resistors specified can carry currents appropriate for the LED's without becoming overly warm. Physically smaller resistors with the same resistance values may be used, but they may become hot enough to cause mild burns to anyone touching them.
Operation
With the LED illuminated and an audio signal present across one of the resistors, students should be able to place the open end of the film canister near the LED and have their amplifier broadcast the audio. It is generally a good idea to start with low volume settings for both the audio source and amplifier (the device connected to the solar cell) until you determine appropriate settings.
Once the circuits are operational, there are a number of things the students can investigate. First, the LED will not be capable of powering the solar cell with as great a range as the laser in the FunFest show. This is because the light from the LED spreads out over an increasingly large area as one goes further from the LED. The students could use this observation to determine the effective range of their optical networks. They can also experiment to find the "shape" of the region of space around the LED where the solar cell can pick up the signal. This region should loosely resemble a cone. To increase the range of their LED's, students may place simple magnifying lenses in between the LED and the solar cell. They might also place mirrors at appropriate positions between the LED and solar cell to send the audio information in different directions.
A very interesting activity can be created by using an infra-red LED, rather than the LED's that create visible light. Our experience indicates that students prefer to work with the visible light LED's, perhaps because they can easily tell that they are "on", but infra-red LED's can reinforce the concept that light that cannot be seen may still be useful. Using the same polarity convention as for the visible light LED's, connect your infra-red LED. Even in a darkened room, the LED will not produce any visible radiation as long as you do not exceed its nominal current carrying capacity. With the infra-red LED, it is quite possible to have a greater range than for the visible variety, and infra-red radiation may be reflected by nearly any flat surface, including a student's hand! You may also use your magnifying lenses for the infra-red radiation just as you did for the visible light LED's.
If you have any optical fiber (light pipes) available, you might be able to use these to bend the light signal around corners before being intercepted by the solar cell. This would tie in nicely to a discussion on how this concept can be integrated into future communications technology. Finally, if you have any television or VCR remote controls available, be sure to let the students experience what these "sound like" when pointed at the solar cells. These devices use infra-red LED's to communicate their information. You should also hold the solar cells near all sorts of light sources, such as incandescent and fluorescent bulbs, as well as television or computer monitors that are on.
Sources of Equipment
The Astro Blaster and other physics toys may be ordered from Fascinations Toys & Gifts, Inc., of Seattle, WA. When ordered in quantities of 24, the Astro Blaster is only $2.00 each. Fascinations may be reached at 1-800-544-0810.
The 50 pound block is made by cutting 3/4" particle board into 2 ft by 2 ft squares and stacking them until you have about 50 pounds. Epoxy the squares together and bolt through a 12" x 12" x 1/4" aluminum plate for stability. Countersink the holes on the bottom so that you will have a smooth surface on the bottom of the block.
The glass container is a cover for a residential electric meter. These are available from the local electric company.
The first choice for a latex or neoprene cover for an external speaker would be to use a latex glove. If a glove is too small, check your yellow pages under rubber products. Ask for 1/16" neoprene sheeting. If you find no such listing, you can order 1/16" neoprene sheeting from the McMaster-Carr company, at any of the following addresses:
600 County Line Rd, Elmhurst, IL 60126
9601 John St., Santa Fe Springs, CA 90670
Monmouth Junction Rd., Dayton, NJ 08810
Neoprene sheeting is cut from 1 yard wide rolls, and sells by the linear foot. From McMaster- Carr catalog number 90, the catalog number is 8568K11, and the price is $7.62 per linear foot.
Laser pointers can be purchased for under $50 from Radio Shack. The external speaker shown is from Radio Shack. Small mirror pieces may be formed from pieces of mirror tile, which may be purchased from discount department stores, such as K-Mart. The laser used in the FunFest show is a .5 mW He-Ne made by Uniphase. It can be ordered from Edmund Scientific Co., Cat #E34,891 for $215.00 The address is: Edmund Scientific Co., 101 E. Gloucester Pike, Barrington, NJ 08007-1380.
Laser PA System
Mini Amplifier 277-1008 11.99
Jumbo LED's, Various Colors 276-205,206,086,214,215,216; $2.39 to $3.99
Infra-Red LED 276-143 1.69
1/8" Jack Set 274-283 3.59
Alligator Cables 278-1156 3.99
100 Ohm, 1W Resistor 271-152 .49
Audio Cable 42-2420 2.49
9 Volt Cap Leads 270-324 1.89
(From Radio Shack--These are much harder to work with!) Solar Cell 276-124 $4.19
Solar Cell (called Photovoltaic Detector in Digikey catalog) Part # PDB-V107-ND, priced from $2.00 to $1.60 each, depending upon quantity ordered. Order from (be sure to call & ask for current prices & availability):
Digikey
701 Books Ave South
Theif River Falls, MN. 56701
800-344-4539
