vb.net read barcode from camera FIGURE 34-2 The guts of a smoke detector. in Software

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FIGURE 34-2 The guts of a smoke detector.
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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. 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. Then clip off the wires to the battery compartment, noting their polarity. Connect the circuit to a +5 V 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 V, which can complicate your robot s power supply and interfacing needs. Remember to note the voltage when the piezo disc is active. It should not be more than +5 V; 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. 34-3. Here, the output of the smoke alarm circuit is clamped at no more than 5.1 V, thanks to the 5.1-V zener diode. There is no effective way to measure the peak voltage by using a volt-ohm meter, because the output of the smoke alarm detector is often an oscillating signal. 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-V zener circuit as shown in Fig. 34-3. While this may be unnecessary in some instances, it will help protect your digital interface from possible damage caused by overvoltage from the smoke alarm circuit board.
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34.3.2 INTERFACING THE ALARM TO A COMPUTER
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Assuming that the board works with +5 V applied, your hacking is basically over, and you can proceed to interface the alarm with a computer port or microcontroller. By way of example, we ll assume that a simple microcontroller that periodically polls the input pin is connected to the smoke alarm circuit board. The program checks the pin several times each second. When the pin goes HIGH, the smoke alarm has been triggered. If your microcontroller supports interrupts (ideally with latched inputs that can be polled), a better scheme is to connect the smoke alarm circuit board to an interrupt pin.
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FIGURE 34-3 Use a 5.1 V zener diode to ensure that the smoke alarm output does not drive the computer/microcontroller input above 5 V.
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34.3 SMOKE DETECTION
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Then write your software so that if the interrupt pin input state changes, a special I smell smoke routine is run. The benefit of an interrupt or latched input over polling is that the latter requires your program to constantly branch off to check the condition of the input pin. With an interrupt or latched input, your software program can effectively be ignorant of any smoke detector functionality. If and when the signal pin is active because the smoke alarm circuit was tripped, a special software routine takes over, commanding the robot to do something else. Rather than connect the output of the smoke alarm circuit board directly to the input pin, use a buffer to protect the microcontroller or computer against possible damage. You can construct a buffer using logic circuits (either TTL or CMOS) or with an op amp wired for unity-gain (with unity-gain, the op amp doesn t amplify anything). Note also that the smoke alarm circuit board derives its power from the robot s main +5 V power supply and not from the microcontroller. Alternatively, you can use an opto-isolator. The opto-isolator bridges the gap between the detector and the robot. You do not need to condition the output of the opto-isolator if you are connecting it to a computer or microprocessor port or to a microcontroller.
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