Page 144 - 15 Dangerously Mad Projects for the Evil Genius
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122      15 Dangerously Mad Projects for the Evil Genius



                  Signal








                                                     Average







                                                                                            Time




                                                                     Unrectified
                                                                       Rectified





         Figure 9-25   Rectifying the signal


        By rectifying it, we chop off the negative bit,      The output will be the input multiplied by the
        making our average signal positive in proportion   gain. The gain is set by the degree of negative
        to how big the waves are.                          feedback and can be calculated as 1   R3 / R2.
           This signal is oscillating at over a billion times   Since R3 is 1M , and R2 is 1K, the gain is 1001.
        a second, so we do not need to consider smoothing  Op amps are really designed to be operated with a
        it with a capacitor since the rest of the electronics  split supply. For example,  9V, GND, and –9V. To
        won’t be fast enough to see anything more than an  do this would require two batteries, which we do
        average anyway.                                    not really want. So, instead we use the arrangement
                                                           of R5, R6, and C2 to provide a reference voltage of
           The next step is to amplify that weak signal and
                                                           about 4.5V, halfway between GND and 9V.
        we use an amplifier IC to do this. The type of
        amplifier we use is called an operational amplifier,
        or “op amp” for short. This kind of amplifier has
                                                           Summary
        two inputs, one positive and one negative. It
        amplifies the difference between these two inputs.  Now the Evil Genius has a means of bugging
        However, to make the amplifier stable, you always  people, and also a means of detecting when he’s
        operate it with feedback. This passes a portion of  been bugged (albeit only if the bug is of the cell
        the amplified signal back to the input, letting you  phone variety).
        control exactly how much it amplifies (its gain).    In the next project, we will make a system for
           In the arrangement we’ve used here, which is    sending sound over a laser beam. No radio waves
        called “non-inverting,” we employ the negative     means no way of eavesdropping with a radio
        input just for feedback and pass the signal we     receiver, and so the Evil Genius can once again
        want to amplify into the positive input.           communicate securely.
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