SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 10 - Schematic for an ultra simple light to sound converter
Figure 10 - Schematic for an ultra simple light to sound converter

The initial experiment using the photoresistor will only require a single NPN transistor, resistor and a battery in order to prove that the laser beam is definitely capable of picking up faint vibrations and changing them into audio. Any generic NPN transistor such as a 2N3904 or 2N2222 will work in this circuit. The battery voltage can be anywhere from 3 volts to 9 volts. If you really want to go basic, then just run a 9 volt battery directly into the photoresistor and out to the headphones, although without the transistor to amplify the signal, the audio will be very faint.



This basic light to sound converter works because any change at the base of the transistor will amplify the current to the headphones. Since the return laser beam will be bouncing around due to the vibration at the speaker, this will cause the headphones to respond to the beam as if it was an audio signal. Although the laser beam is moving, not modulated, the principals at work are exactly the same with the exception that in this configuration, the beam needs to be slightly offset from the center of the photoresistor so that when it moves across the surface, there is a corresponding change in voltage. If the principal at work here was in fact modulation, then a direct hit onto the surface of the photoresistor would be optimal.



Figure 11 - Targeting the photoresistor with the laser beam
Figure 11 - Targeting the photoresistor with the laser beam

Turn on your audio source and adjust the volume so you can barely hear the output on your speaker and then set up the laser and receiver so that the beam strikes the front face of the photoresistor. As soon as the beam hits the photoresistor, the impedance will vary significantly and you will hear a pop and probably a bunch of noise. Play around with the position of the beam to see how the position of the beam on the face of the photoresistor alters the reception of the audio signal. You will see that the optimal position for the beam is just touching the surface of the photocell so that any changes in the beam position from the vibrations will result in the most significant swing in voltage at the output of your headphones. If all you hear is a loud hum, then you probably have too much ambient light in your room. Incandescent light bulbs actually vibrate at 50 or 60 Hertz, and you will hear that in your system as a loud constant hum. As you have probably guessed, the Laser Spy system will not perform very well in the daytime due to ambient light sources competing with your laser beam, but this is fine since real spies usually operate in the darkness!

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