HowTo-Color (8) / Hacking Electronics / Simon Monk / 236-3 / Chapter 9
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Hacking Electronics The signal from the microphone is being amplified by a factor of 100. This shows just how weak the signal is in the first place. The “+” input to the op amp is held halfway between GND and 5V (2.5V) by using R3 and R4 as a voltage divider. C1 helps to keep this constant. From the schematic, you can see how you could build the module yourself on, say, stripboard. Op amps like the one used (which is a surface-mounted device) are also available in the eight-pin DIP form. However, a module like this will save you a lot of effort and may even turn out cheaper than buying and building a module from scratch. I realize this is a rather cursory introduction to op amps. These are very useful devices, but unfortunately require more space to explain fully than this book can accommodate. You will find good information on op amps at the Wikipedia site, as well as in books with a more theoretical bent like Practical Electronics for Inventors, Third Edition, by Paul Sherz and Simon Monk, which has a chapter devoted exclusively to op amps. In the next section, we will combine this module with a hacked FM transmitter of the sort used to let you play your MP3 player through your car radio, thus creating an audio “bug.”
How to Make an FM Bug To make an FM transmitter that will broadcast sound picked up from a microphone to a nearby FM radio receiver would require a lot of effort. We are hackers, so we are going to cheat and take apart an FM transmitter and wire it up to a mic module. Figure 9-9 shows the end result of this hack. Figure 9-9 An FM radio bug
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