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 Click on photo for larger picture 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. 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. Using a disassembled laser pointer. For this project we have removed the laser assembly from a small $10.00 laser pointer. The power supply circuit is the green board attached to the brass laser head. We carry similar laser pointers in our catalog that are easily disassembled for this project. The laser below has voltage spike protection on the circuit board. The one you get may not have this* and so you will want to put a bicolor LED across the transformer like we did in the previous version The power supply circuit came conveniently marked with a plus and a minus next to two holes in the board. We solder the black negative lead from the battery clip to the hole marked minus. We solder one of the 1000 ohm coil leads to the hole marked plus. We solder the red positive lead of the battery clip to the other lead from the 1000 ohm coil The battery clip is attached to a 4.5 volt battery pack (not a 9 volt battery!). Since I didn't have a pack that takes 3 cells* I used one that takes 4 AA batteries* and I replaced one of the four batteries with a straight piece of bare wire. That's it! We have a laser transmitter* in just a few minutes! A new receiver The solar cell receiver has some drawbacks. It is expensive (solar cells are a few dollars each)* and fragile. A cheaper* sturdier alternative is to use a cadmium sulphide photoresistor instead of the silicon photocell. A cadmium sulphide photoresistor is shown below (magnified many times). It does not produce electricity from light the way the solar cell did. Instead* the light that falls on it changes its resistance to electricity. If we connect a battery and a photoresistor together* they can act like the solar cell. As the intensity of the light changes* the amount of electricity output changes in response The new receiver is very simple* and looks like this