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You can either buy a new smoke detector module for your robot or scavenge one from a commercial smoke alarm unit. The latter tends to be considerably cheaper you can buy quality smoke alarms for as little as $7 to $10. In this section, I ll discuss hacking a commercial smoke alarm, specifically a Kidde model 0915K, so it can be directly connected to a robot s computer port or microcontroller. Of course, smoke alarms are not all designed the same, but the basic construction is similar to that described here. You should have relatively little trouble hacking most any smoke detector you happen to use. However, you should limit your hacking attempts to those smoke alarms that use traditional 9-volt batteries. Certain smoke alarm models, particularly older ones, require you to use AC power or specialized batteries (such as 22-volt mercury cells). These are harder to salvage and, besides, their age makes them less suitable for sensitive smoke detection. Start by checking the alarm for proper operation. If it doesn t have one already, insert a fresh battery into the battery compartment. Put plugs in your ears (or cover up the audio transducer hole on the alarm). Press the Test button on the alarm; if it is properly functioning the alarm should emit a loud, piercing tone. If everything checks okay, remove the battery, and disassemble the alarm. Less expensive models will not have screws but will likely use a snap-on construction. Use a small flat-headed screwdriver to unsnap the snaps.
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Inside the smoke detector will be a circuit board, like the one in Fig. 39.2, that consists of the drive electronics and the smoke detector chamber. Either mounted on the board or located elsewhere will be the piezo disc used to make the loud tone. Remove the circuit board, being careful you don t damage it. Examine the board for obvious hack points, and note the wiring to the piezo disc. More than likely, there will be either two or three wires going to the disc:
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I Two wires to the piezo disc: the wires will provide ground and
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V power. This design is typical when you are using all-in-one piezo disc buzzers, in which the disc itself contains the electronics to produce the signal for audible tones. I Three wires to the piezo disc: the wires will provide ground, V power, and a signal that causes the disc to oscillate with an audible tone. Using a volt-ohm meter or an oscilloscope, find the wire that serves as ground. (It is probably colored black or brown, but if no obvious color coding is used, examine the circuit board and determine where the wires are attached.) Connect the other test lead to the remaining wire. Or if the disc has three wires, connect the test lead to one of the remaining wires. Replace the battery in the battery compartment, and depress the Test button on the alarm. Watch for a change in voltage. For a two-wire disc you should see the voltage change as the tone is produced. For a three-wire disc, try each wire to determine which produces the higher voltage; that is the one you wish to use. If you are using an oscilloscope, find the wire that produces a clean on/off pulse.
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FIGURE 39.2 The guts of a smoke detector.
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Once you have determined the functions of the wires to the piezo disc, clip off the disc and save it for some other project. Retest the alarm s circuit board to make sure you can still read the voltage changes with your volt-ohm meter or oscilloscope. Then clip off the wires to the battery compartment, noting their polarity. Connect the circuit to a 5 vdc power supply. Depress the Test button again. Ideally, the circuit will still function with the lower voltage. If it does not, you ll need to operate the smoke alarm circuit board with 9 vdc, which can complicate your robot s power supply and interfacing needs. If you have an oscilloscope note the voltage. It should not be more than 5 volts. If it is, that means the circuit board contains circuitry for increasing the drive voltage to the piezo disc. You don t want this when you are interfacing the board to a computer port or microcontroller, so you ll need to limit the voltage by using a circuit such as that shown in Fig. 39.3. Here, the output of the smoke alarm circuit is clamped at no more than 5.1 volts, thanks to the 5.1-volt zener diode. Because the output of the smoke alarm detector is often an oscillating signal, there is no effective way to measure the peak voltage by using a volt-ohm meter. The meter will only show an average of the voltage provided by the circuit. If you are limited to using only a voltohm meter for your testing, for safety s sake add the 5.1-volt zener circuit as shown in Fig. 39.4. While this may be unnecessary in some instances, it will help protect your digital interface from possible damage caused by over-voltage from the smoke alarm circuit board.
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