Chapter 2
Building an electromagnetic coil gun.
Building an electric motor in 10 minutes.
The single brush motor.
The double brush improvement.
The hanging brush variation.
A 10 minute motor with no magnet.
An electric pendulum in 10 minutes.
Quick and simple laser communicator.
Improved laser communicator.
Building the impossible kaleidoscope.
Building a simple spectroscope.
How to video tape through a microscope.
Going further:
Lasers and holography.

A simple laser communicator.

How would you like to talk over a laser beam? In about 15 minutes you can set up your own laser communication system, using cheap laser pen pointers and a few parts from Radio Shack. For the transmitter you will need:

  1. A laser pen pointer. You can buy one here for about $18.

  2. A battery holder that holds the same number of batteries as the laser pointer (often 3 cells). The batteries can be any size, but they must be the same voltage as the laser batteries. You may need to get one that holds two cells, and another that holds one cell, and wire them together in series. Radio Shack has a decent selection.

  3. A transistor radio. Later we will use a microphone and an amplifier (Radio Shack #33-1067 and #277-1008), but at first we will send your favorite radio station over the laser beam.

  4. An earphone jack that will fit your transistor radio (Radio Shack #42-2434).

  5. One 220 ohm resistor (Radio Shack #271-1313), or two 100 ohm resistors (Radio Shack #271-1311). Actually anything from 180 to 400 ohms will probably work, but I used 200 ohms.

  6. Some clip leads (wires with alligator clips on the ends) to put it all together. At least one of the clip leads should be the type with a long slender point (Radio Shack #270-334), to connect to the inside of the laser pointer. You can substitute regular wire and solder if you like, but the clip leads are fast and simple. Radio Shack has a wide selection of clip leads (such as ##270-378).
For the receiver you will need:

  1. A small solar cell (such as Radio Shack #276-124). You may have to solder your own wires to it if it doesn't come with wires attached.

  2. A microphone jack that will fit the phono input of your stereo (Radio Shack #42-2434 or ##42-2457). Instead of a stereo, you can use the small amplifiers that Radio Shack sells (#277-1008).
It may be had to find a battery holder that holds three batteries. You can use two battery holders (one that holds two batteries, and one that holds a single battery) and connect them in series. Remove any batteries from the laser. Connect a clip lead to the inside of the laser pointer where the battery touched. Usually there is a small spring to which you can attach the clip lead. The other end of the battery usually connects to the case of the laser. Since there are many different styles of laser pointer, you may have to experiment with clip lead placement to get the laser to work with the new external battery pack. You may also have to hold down the laser's push button switch by wrapping a rubber band or some wire around it. Test the connection before you attach the resistors, to make sure the laser works with the new battery pack. If it doesn't light, try reversing the battery. Battery reversal will not harm the laser. Connect the resistor (or two resistors in series) between the battery and the laser. If the laser does not work with the resistors in place, you may have too much resistance, and a smaller resistor should be used. But if too small a resistor is used, the transmitter will not work, and there will be a risk of damaging the laser when the transistor radio is attached to it. Connect the earphone jack to the laser, in parallel with the battery/resistor combination. The schematic of the transmitter looks like this: The receiver is the simplest part. You simply connect the solar cell to the microphone jack, and plug it into the amplifier or stereo phono input. It does not matter which way the wires are connected to the solar cell. Here is the schematic of the receiver:

Setup and testing

Make sure the transistor radio is turned off, and the laser is on. Plug the earphone jack of the laser into the earphone socket of the radio. Connect the solar cell to the amplifier or stereo, and turn the volume up until you hear a hissing noise, then turn it down slightly until the hiss isn't noticeable. The volume control should be fairly high, corresponding to an ear splitting level if it was playing music. Aim the laser across the room so it hits the solar cell. You might hear clicks or pops coming from the stereo or amplifier as the laser beam passes over the solar cell. This indicates that everything is working fine at this point. Now carefully turn on the radio and slowly adjust the volume until you hear the radio station voices or music coming from the amplifier across the room. Do not turn the volume up very far, or you can damage the laser. The radio should be just audible if the earphone jack is pulled out, not loud. If you can't hear the sound from the amplifier across the room, make sure the laser is shining on the solar cell, then try increasing the volume of the amplifier before you increase the volume of the radio. At this point you should be hearing the radio station coming in loud and clear in the amplifier across the room. Put your hand in front of the laser beam to break the connection, and notice that the music stops. Wiggle your fingers in the beam and listen to the music get chopped up by your fingers. Your laser communicator is ready for the next step. To send your voice over the laser beam, you simply replace the transistor radio with a microphone and amplifier. Radio Shack sells small amplifiers (about the same size as the transistor radio) that have sockets for microphones and earphones. You can also use another stereo system, but be very careful with the volume control to prevent damage to the laser. The next section describes an improved laser communicator that takes a little longer to build (about half an hour) but which cannot damage the laser even if the volume is cranked up all the way.

An improved laser communicator.

The improved laser communicator uses a transistor to modulate the light beam. The transistor is a common NPN transistor that costs about 15 cents. The circuit is very simple, and is perfect for a beginner to build on a solderless breadboard. It can also be soldered onto a pre-etched printed circuit board for a sturdier communicatior. For the modulator you will need:

  1. An NPN transistor, such as a Radio Shack #276-1617, or #276-2009.

  2. A 1,000 ohm resistor (labeled 1k in the schematic below). Radio Shack #271-1321.

  3. Two 10,000 ohm resistors (labeled 10k in the schematic below). Radio Shack #271-1335.

  4. A solderless breadboard (Radio Shack #276-175), and/or a multipurpose printed circuit board (Radio Shack #276-148 or #276-159). The solderless breadboard makes the initial design and testing very easy, while the printed circuit board is useful for making a durable finished product that is all soldered together.

  5. The laser, phone jack, and battery case listed in the simple communicator above. The schematic diagram of the modulator is shown below. The laser is connected between the positive side of the battery and the collector of the transistor. The negative side of the battery is connected to the emitter of the transistor. The base of the transistor is connected to the three resistors. The phone jack is plugged into the earphone socket of the transistor radio or the mini amplifier. The finished soldered version of the modulator is shown below: The receiver is the same as for the simple communicator: