Atención a la diversidad a lo largo de la historia

speaker 555 timer R1: 1M R2: 100K R3: 1K R4: 100 ohm R5: 10K R6: 10K Q1: 2N3904 NPN Q2: 2N3904 NPN C1: 100 µF C2: 0.01 µF C3: 0.01 µF R1 +9 V R2 R5 R6 R3 R4 Q1 C1 Gnd + C2 Q2 C3

Figure 2-7 Evasive beeping-thing schematic Figure 2-6 Several small high-frequency speakers

timing cycle. In a really large room, you might want a longer beep and a longer cycle, so a 220µF capacitor could be used, and the 100K resistor could be swapped for a 220K resistor. For a smaller room, where it may be easier to locate the device (e.g. a friend’s office), the capacitor could be changed to 47µF and the 100K resistor to a 10K for a very short beep. The best plan is to simply build the unit as is and then fine tune the components until you are happy with its operation. And yes, a variable resistor would be easy to adjust.

Now, where do you hide the beast? Well, since this unit emits hard-to-locate high frequencies, your options are endless. The high-pitched sound will exit through the smallest hole in whatever box you place the parts into. I decided to cram the works into an old wall adapter that has all of the guts removed, including any connection to the AC lines. The little speaker fits nicely into the top of the box, and there was just enough room for the 9-volt battery and small circuit board. Figure 2-8 shows the completed circuit going into the wall wart box.

There was just enough room to get all the parts inside, so I could not install an on-off switch, but

that was OK since the top of the box simply snapped together and I could simply unclip the battery. The unit will run for many days on a full battery, and if you strategically place the beeper, it may take that long for the unsuspecting victim to find it! If you plan to use a wall wart cabinet for the device like I did, ensure that there is no connection between the plug prongs and the AC lines. It is a good idea to remove them completely. Some other good hiding places might be a pop can, lunch box, wall clock, tissue box, or you could even install it into a working appliance. A solid cabinet will need a small hole for the speaker to optimize the distance that the sound will travel. I found that a quarter-inch hole was large enough for the tiny two-inch speaker I used. As I mentioned earlier, you can also use a piezo buzzer instead of a speaker, which would make the unit even smaller and possibly louder owing to the very good high- pitched operation of the piezo element. To use a piezo buzzer in place of the speaker, connect resistor R4 (which used to connect to one of the speaker terminals) directly to the +9 line, where the other speaker terminal used to connect.

Project 2—Evasive Beeping Thing

This interesting project combines a little hardware with some timing electronics to simulate the sound of someone knocking on a door or wall. To create a circuit that sounds like three or four quick knocks, a 555 timer is used as a long delay counter set to stay off for a few minutes and then send out a pulse for about three seconds. This three-second pulse is not much use by itself, so it is fed into a PNP transistor in order to switch on a double-pole, double-throw relay, which is configured in such a way that it turns itself on and off several times per second. The other relay pole is then used to bang a washing machine or photocopier solenoid plunger up and down against the enclosure or wall to simulate the sound of rapping at the door. The operation of the timing hardware will be explained in more detail later, so dig into your scrap bin or

head down to the surplus electronics store and try to find a 5- or 12-volt mechanical solenoid like the ones shown in Figure 2-10.

A mechanical solenoid is nothing more than an electromagnet with a steel plunger placed in the center of the coil so that, when energized, the plunger will pull itself into the coil as far as it can go. Figure 2-10 shows a pair of solenoids taken

from a photocopier and a washing machine with one of the plungers removed from the

electromagnet. The solenoid on the left is rated for 12 volts, and the one on the right is rated for 24 volts, but they can both pull the plunger into the hole when the electromagnet is connected to a single 9-volt battery. At 24 volts, the larger solenoid will snap the plunger into position with great speed