Motion detection of an object can be achieved by using some stimulusand sensing its reflection as in radar, or by sensing the naturalsignals generated by an object. This article discusses motion detectionof an object by sensing the infrared signal radiated from the object.
Infrared radiation exists inthe electromagnetic spectrum at a wavelength that is longer thanvisible light. Infrared radiation cannot be seen but it can bedetected.
Objects that generate heat also generate infrared radiation andthose objects include animals and the human body, whose radiation isstrongest at a wavelength of 9.4 micrometers. To detect this signal, a transducer is required thatconverts the infrared signal to a form detectable with conventionalcircuitry.
What are pyroelectric detectors?
Pyroelectric sensors are made of a crystalline material that generatesa surface electric charge when exposed to heat in the form of infraredradiation. When the amount of radiation striking the crystal changes,the amount of charge also changes and can then be measured with asensitive FET device built into the sensor as shown in Figure 1, below.
The sensor elements are sensitive to radiation over a wide range, soa filter window is added to the TO5 package to limit incoming radiationto the 8 to 14 micrometer range that is most sensitive to human bodyradiation.
Figure 2 below shows how anexternal resistor “R” connected to a radiation source converts the FETcurrent to a voltage. A well-filtered power source from 3 to 15 voltsshould be connected to the FET drain terminal.
The output voltage is a function of the amount of Infrared Radiation(IR) sensed at the input. Unfortunately, the output is also affected byvibration, radio interference, and sunlight.
To overcome this problem the sensor has two sensing elements, asshown in Figure 3 below, connected in a voltage-bucking configuration. This arrangement cancelssignals caused by vibration, temperature changes and sunlight.
A body passing in front of the sensor will activate first one andthen the other element whereas other sources will affect both elementssimultaneously and be cancelled. The radiation source must pass acrossthe sensor in a horizontal direction when sensor pins 1 and 2 are on ahorizontal plane so that the elements are sequentially exposed to theIR source.
The distance from the front of the sensing elements to the front ofthe filter window is around 0.045 inch (1.143 mm), so a lens isrequired for detecting motion any more than a few feet away.
Figure 4 below shows a Fresnel lens placedin front of thePIR detector. A Fresnel lens isa Plano Convex lens that has been collapsed on itself to form a flatlens that retains its optical characteristics but is much smaller inthickness, therefore having less absorption losses.
The Fresnel lens is made of an infrared transmitting material thathas an IR transmission range of 8 to 14 micrometers. It is designed tohave its grooves facing the IR sensing element so that a smooth surfaceis presented to the subject side of the lens, which is usually theoutside of an enclosure that houses the sensor.
Pyroelectric sensors at work
In order to use the output voltage to detect human motion, the signalmust first be conditioned. Figure 5below shows how the FET source terminal connects through apull-down resistor to ground and feeds into a two-stage amplifierhaving signal-conditioning circuits.
The amplifier is typically bandwidth limited to below 10 Hz toreject high frequency noise and is followed by a window comparator thatresponds to both the positive and negative transitions of the sensoroutput signal.
To build the above circuit with discrete components we may requirefour op-amps: two op-amps for the amplification stage and two for thecomparator stage.
Two op-amps are required for the comparator stage to respond to boththe positive and negative transitions of the sensor output signal.Connecting the conditioned output signal to a microcontroller competesthe sensor.
Using a microcontrollerwithintegrated analog capabilities eliminates the need forexternalactive components to buffer, amplify and detect a moving infraredsignal source.
Figure 6 below shows anapplication block diagram using a microcontroller. The signal conditioncircuit is built using simple R and C components.
A 100-kohm resistor is connected to the source of the PIR detector.A 0.15 micro Farad capacitor is connected in parallel to the resistor.This combination of components makes a 10 Hz low-pass filter.
A 0.16 Hz high-pass filter is made with a 1 microfarad capacitor anda 1 M-ohm resistor. Amplifier-1 provides a gain of 16 to the signal. Itis brought out to the buffer and is high-pass coupled into the input ofAmplifier-2 to provide another gain of 16 for a total gain of 256.
The signal is then fed to a 13-bit ADC where the signal is digitizedat 240 samples per second. The digital values from the ADC are softwarecompared for threshold levels on either side, and an indicator isactivated to identify the detection of human motion.
Geethesh N.S. is PSoC Consultant,and Martin Cornish is Field Applications Engineer at CypressSemiconductor Corp.