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30.2 NONCONTACT NEAR-OBJECT DETECTION
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FIGURE 30-9 Most PIR sensors are large, transistorlike devices with a fairly common pinout arrangement. This is a block diagram of how the typical PIR sensor works.
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If the sensor incorporates an internal output amplifier and signal conditioner, its output will be suitable for direct connection to a microcontroller or other logic input. A buffer circuit, like that shown in Fig. 30-11, is often recommended to increase input impedance. The circuit uses an op amp in unity gain configuration. If the sensor you are using lacks a preamplifier and signal condition, you can easily add your own with the basic circuit shown in Fig. 30-12. With both the circuits shown in Figs. 30-11 and 30-12, the ideal interface to a robot computer or microcontroller is via an analog-to-digital converter (ADC). Many microcon-
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FIGURE 30-10 The PIR sensor has an infrared window on the top to let in infrared heat radiated by objects. Movement of those objects is what the sensor is made to detect, not just the heat from an object.
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FIGURE 30-11 Use a buffer circuit between the output of the amplified PIR device and the microcontroller or other logic input.
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trollers offer these on chip. If your control circuit lacks a built-in ADC, you can add one using one of the approaches outlined in 14, Computer Peripherals. The output of the PIR sensor will be a voltage between ground and +V. For example s sake, assume the output will be the full 0 to 5 V, though in practice the actual voltage switch will be more restricted (e.g., 2.2 to 4.3 V, depending on the circuitry you use). Assuming a 0 to 5 vdc output, with no movement detected, the output of the sensor will be 2.5 V. As movement is detected, the output will swing first in one direction, then the other. It s important to keep this action in mind; it is caused by the nature of the pyroelectric element inside the sensor. It is also important to keep in mind that a heat source, even directly in front of the sensor, will not be detected if it doesn t move. For a PIR device to work, the heat source must be in motion. When programming your computer or microcontroller, you can look for variances in the voltage that will indicate a rise or fall in the output of the sensor.
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FIGURE 30-12 If the PIR sensor you are using lacks a built-in output amplifier, you can construct one using commonly available op amps.
30.2 NONCONTACT NEAR-OBJECT DETECTION
30.2.6 HACKING A MOTION DETECTOR BOARD
Rather than unsolder the PIR sensor from a motion detector unit, you may be able to hack into the motion detector circuit board to find a suitable output signal. The advantage of this approach is that you don t have to build a new amplifier for the sensor. The disadvantage is that this can be hard to do depending on the make and model of the motion control unit that you use. For best results, use a motion detector unit that is battery powered. This avoids any possibility that the circuit board in the unit also includes components for rectifying and reducing an incoming AC voltage. After disassembling the motion detector unit, connect +5 vdc power to the board. (Note: some PIR boards operate on higher voltages, usually 9 to 12 V. You may need to increase the supply voltage to properly operate the board.) Using a multitester or oscilloscope (the scope is the preferred method), carefully probe various points on the circuit board and observe the reading on the meter or scope. Wave your hand over the sensor and watch the meter or scope. If you re lucky, you ll find two kinds of useful signals:
Digital (on/off) output. The output will normally be LOW and will go HIGH when movement is detected. After a brief period (less than 1 s), the output will go LOW again when movement is no longer detected. With this output you do not need to connect the sensor to an analog-to-digital converter. Analog output. The output, which will vary several volts, is the amplified output of the PIR sensor. With this output you will need to connect the sensor to an analog-to-digital converter (or an analog comparator).
You may also locate a timed output, where the output will stay HIGH for a period of time up to several minutes after movement is detected. This output is not as useful. Fig. 30-13 shows the innards of a hacked motion detector. If the PIR board you are using operates with a 5 vdc supply, you can connect the wire you added directly to a microprocessor or microcontroller input. If the PIR board operates from a higher voltage, use a logic level translation circuit, or connect the wire you added from the PIR board to the coil terminals of a 9 or 12 volt reed relay.
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