SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 14 - Choosing the best phototransistor for your receiver
Figure 14 - Choosing the best phototransistor for your receiver

Since laser light is so bright compared to any other type of light, the actual wavelength and lens type of the phototransistor is not all that important. Some phototransistors are more sensitive to infrared light than visible light, but they will still work just fine with any color of laser beam. If you are choosing a new phototransistor for this project, then look at the datasheet to determine the lens style, optimal wavelength, and make sure it is an NPN type not PNP. There are hundreds of manufacturers and part numbers, so just go to an online electronics supplier like Digikey and enter "NPN phototransistor" to start your search.

If you have a huge selection to choose from, try to find one that has the best sensitivity to the wavelength of your laser, but keep in mind that for full covert operation of the Laser Spy, you will probably want to switch to infrared light, which falls between 800nm and 1000nm. Red lasers usually have a wavelength of around 650nm, green lasers have a wavelength of 530nm, and the newer blue lasers have a wavelength of around 470nm. Again, don't be overly critical of the phototransistor wavelength chart as any light will create a response, especially laser light due to its brightness. My phototransistor was rated for optimal performance in the infrared region but worked perfect with the visible red laser as well as the green laser I tested.



Figure 15 - The Laser Spy light to sound receiver schematic
Figure 15 - The Laser Spy light to sound receiver schematic

The second version of the light to sound circuit is a bit more technical, but is still very basic as far as schematics go. The phototransistor is setup as an amplifier that feeds its output directly into the input if an LM386 audio amplifier IC in order to drive the headphones to a decent volume level. Oddly, the level of the output is never the issue; it's always the laser alignment that causes the most effort and I have yet to turn up the volume control more than about 10 percent of the way. Over priced rigs from so called "Spy Stores" will usually include some type of vocal pass filter or noise reduction circuit in the receiver as well, but in reality, these are pointless when you can simply feed the unfiltered output from this device into a computer sound card and process the audio using software that is going to be many times more effective than any filtering hardware. Your best bet is the recording and storage of the raw audio for post work on a computer later, so fancy front end filter hardware is really a waste of time.

The LM386 is a very common audio amplifier IC that requires very few external components to operate. It has more than enough output power to drive a set of headphones to a very loud level, so the variable resistor that controls the volume level is often set very low when in use. You could certainly feed the output from the phototransistor to any audio amplifier or even a computer sound card as well. If you do plan to include some type of advance real-time audio filtering, then take the audio from the output of the phototransistor so that no extra noise is included in the signal as it is amplified. Many audio editing programs include support for real-time filters and audio processing, so for very little money you can create a real-time post processing system that will rival anything that was available to even the most well funded spy agencies just a few years ago. There are audio processing programs available for your OC that can restore the faintest conversation from the noisiest background.

Back Home Last Next
You are Viewing... Page 8 of 18
Lucid Science Electronics from the Fringe AtomicZombie Hack-a-day SparkFun