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Figure 26 - Making a dual laser alignment block
Figure 26 - Making a dual laser alignment block

If you are serious about your covert spy operations, then you can't go around pointing a visible laser at your target or you will instantly compromise your position at night. Even if you are a mile away, a person looking out the window can easily pinpoint your exact location if you point a laser in their direction, so your only option for truly covert operation is to use a laser beam that is invisible to the human eyes. A 3-5 milliwatt infrared laser module is no more expensive than a typical visible red unit and will work just as well, although you certainly increase the complexity of catching the return beam.

So how do you send an invisible beam across the street and attempt to have it land exactly on the 1mm phototransistor when the beam is completely invisible? Well, you can't, so you will need two lasers - one to do the initial targeting, and the other to do the covert surveillance work. Now, this may sound completely impossible, but I can assure you that it does work, and I have managed to target a window using the infrared laser as well. It is MUCH more difficult, but it can be done if all of the conditions are in your favor. As for the performance of the receiver, it actually works better with the infrared laser as the collimating lens on a real module is usually better than on a pointer and most phototransistors are centered around the infrared band. So if you can aim the thing, it will perform!



The key to using two lasers is to have them pointing in the exact same direction so that you don't have to mess around too much once you have a visible target acquired. You could try fastening the bodies of the two lasers together using some type of clamp, or drill two very accurate holes through a half inch thick piece of aluminum as I have done. The distance between the two lasers is negligible, but the angle of the two lasers must be as precise as possible. Even a degree of error could mean 20 feet of error on a beam that is returning from a 500 foot round trip! You will need to use a drill press to make the holes so that they go through the material at exactly the same angle. If you have an unlimited budget, have a machine shop do this drilling and explain to them that alignment is ultra important.



Figure 27 - Mounting the lasers to a box with switches
Figure 27 - Mounting the lasers to a box with switches

The two lasers will also need to be mounted to a box that will allow room for a battery pack and a switch to turn each laser on or off. Once you have found a way to mount both lasers so that they are pointing in the exact same direction, affix them to a plastic or metal box that can also be attached to a tripod just like the receiver box. Since you will have no control over the position of the target, you will certainly have to move both the laser and the receiver in order to get a lock on the beam. Don't forget to label the infrared lasers switch as you will not be able to see the beam when it is operational!

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