Halloween is almost here, so I have to get ready for all the little characters who will show up at my doorstep looking for a handout. What does Halloween have to do with sensors? Logistics. Some trick-or-treaters ring my doorbell, others rap on the door, and a few just wait quietly in hopes I will notice them. I can hear the bell from anywhere in the house, but I need to hang out by my front door to hear the knocking or see the silent standers. So a way to detect the urchins coming up my walkway would relieve me of sentry duty.
Detecting people is a classic problem in sensing. There aren't any Star Trek–style "life-form" sensors out there, but many technologies can determine someone's approach by measuring some physical property:
- 1. Mechanical (pressure, contact)
- 2. Acoustic (microphones, sonar)
- 3. Electromagnetic (microwaves, electrostatic)
- 4. Optical (visible, infrared)
Several attributes of optical sensing techniques make them prime candidates for my application. First, these schemes come in a variety of flavors, ranging from simple throughbeam interrupters to machine vision systems. Second, because they need not make contact with the "sensee," some systems can work at a considerable distance. Third, an optical sensor's field of view can often be constrained, making it a straightforward matter to discriminate between legitimate events (trick-or-treaters) and false alarms (passing cars).
One optical sensing technology particularly well suited to people detection is passive infrared (PIR). A PIR sensor works by detecting longwave (5–10 μm) IR radiation. People and other objects with a surface temperature of about 35°C emit IR in this band, so by looking for changes-in-time of incident radiation a PIR sensor can determine when someone is moving into or out of its field of view. These sensors are commonly used in burglar alarms and for automatically turning lights on and off.
A PIR sensor incorporates a pyroelectric transducer that outputs an electrical signal in response to very small changes in temperature. This transducer (see Figure 1A) is a small chip of pyroelectric ceramic that develops an electric field when a temperature gradient is applied, similar to the way a piezoelectric element develops a voltage in response to mechanical stress. IR radiation striking one side of the device will generate such a gradient. By placing electrodes on either side of the transducer, a charge output is generated in response to changes in absorbed radiation (see Figure 1B). Electrically, the device looks like a voltage source in series with a capacitor. In a practical sensor, two transducers are often wired back-to-back to minimize the effects of changes in housing temperature, and are buffered with a high-impedance voltage-follower. The output signals are on the order of 1 mV and require significant amplification.
Because PIR sensors respond to changes in temperature, they detect radiation at any wavelength they absorb. For use as human proximity/motion sensors, they need an optical filter with a peak transmission near 10 μm to screen out visible light and other undesirable radiation. That filter is the black square on the complete PIR sensor installed in a circuit board as shown in Figure 2.
A Few More Observations
One major advantage of using a PIR sensor for my Halloween application is availability—I can buy inexpensive PIR alarm sensors as assembled units, with all necessary signal processing, at nearly any hardware store.
Window glass is transparent to visible light but it strongly attenuates 10 μm IR. You need to put your PIR sensor outside because it won't work through a closed window.
You can make pretty good fake pumpkin candles by using a pair of 555 timer-based oscillators driving red and orange high-intensity LEDs. Set the timers to different rates (a few hertz) and the two LEDs will beat in a way that looks like a flickering candle. It's safe—no flames, assuming you build it right.
My goodie bowl is filIed with high-quality Halloween candy, not because it has fewer calories or is more healthful, but because it tastes better. And I get to eat the leftovers.
Ed Ramsden, B.S.E.E., a member of the Sensors Editorial Advisory Board, designs sensors for the heavy-truck industry in Portland, OR; firstname.lastname@example.org.