SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 12 - The test system is basic, but fully functional
Figure 12 - The test system is basic, but fully functional

If you have your speaker and laser setup on the same workbench, then the process of targeting your photoresistor was probably only a 10 second job. Now, try to place your speaker at the other side of your room and see how long it takes to get the beam back to the target! I found that the distance across a room made the alignment significantly more difficult and even the deflection on the floor as I walked around made huge changes in the position of the beam. You will also have noticed that any slight vibrations of your desk or speaker stand resulted in all sorts of wild and wacky sound effects coming through your headphones. At one point, I was able to hear my own voice due to the thin surface of one of my tables vibrating the speaker stand.

These initial tests become very important so that you understand how well the Laser Spy system works, but at the same time how incredible finicky it will be to align at any real distance. Even though your test rig consists of nothing more than a cheap laser pointer and three semiconductors, it is actually a fully operational unit that could actually listen to a conversation a mile away if you could somehow capture the return beam. Seriously, this $2 unit is not much inferior to those "professional" Laser Spy devices you can find on the Internet being sold for hundreds or thousands of dollars! All they have to offer is some built in audio filtering and a more stable alignment hardware base. If you feed the output from your test rig into a real-time computer filtering software and mount everything to a solid base, you would have a system as capable as any available. Scary, huh? But we can actually improve the sensitivity of the receiver, so let's do that.



Figure 13 - A phototransistor is faster and more sensitive to light
Figure 13 - A phototransistor is faster and more sensitive to light

The photoresistor was able to convert the movement of the laser into a voltage change, but it is actually not the most optimal part for the job due to being slow to respond and having such a large reception surface. A phototransistor is a much better light receiver because it acts like and amplifier, has a much faster response time and offers a smaller reception area to help catch the very slight changes in motion of the return laser beam. A phototransistor is just a transistor in a clear case that has a photosensitive area feeding its base. In other words, it acts like a standard small signal transistor but has no pin connected to its base.

Any NPN phototransistor will work in this project, and they come in a variety of styles and shapes. The most common variety will actually look identical to a clear LED, but may have a flat top to help focus the light onto the photosensitive base. This clear flat top style will be your best bet. If you like to salvage your parts, then you can look inside an old roller ball computer mouse, as they will usually have several small phototransistors inside. Another source is any appliance that includes an infrared remote control.

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