Infrared detection and control circuits which use pyrosensor devices as thermal detectors are well-known. They are used for a variety of different purposes such as controlling lights, appliances, security devices, and electrical equipment. A pyrosensor generally consists of a layer of pyroelectric material sandwiched between two electrodes. When the temperature of the pyroelectric material changes, for example, as a result of the incidence on the element of infrared radiation from a scene being viewed, electrical charges are generated at the electrodes. The pyroelectric charge is generated only when the temperature of the element is changed. The required change in temperature may be caused by any motion radiating infrared or blocking infrared radiating within the scene viewed, for example, a person moving across a surveillance region.
It is necessary to convert the temperature change into a usable signal. Conventional methods of passive infrared detection and conversion use one or more amplifier circuits to amplify the small electric voltage generated by the pyrosensor. The signal is then filtered using high and low band-pass techniques and then fed to a signal processing circuit for analysis. The signal processing circuit in turn controls a power switch or similar device or devices.
Passive infrared detection and conversion circuits of the prior art are typically developed by setting fixed threshold voltages and providing amplifiers to produce signals corresponding to this threshold voltage level.
The conventional methods referenced above utilize band-pass filtering components to filter out small spurious or unwanted electrical signals operated by the pyrosensors which are typically caused by environmental elements such as rain and wind. Increased infrared activity also occurs during colder seasons when small infrared heat images, normally undetectable, appear larger in the cold air and become detectable. Characteristics of these spurious signals include short high-frequency cycles or constant or slow increasing/decreasing low-frequency cycles over time. Because conventional detection methods rely on fixed signal thresholds, unwanted signals can only be filtered out at the expense of sensitivity.
Measuring time variations proportional to the current output of a pyroelectric infrared detector is shown in U.S. Pat. No. 4,929,833 to Smith. The Smith patent discloses a digital infrared detector circuit which converts changes in detector current to time varying pulses and uses feedback to adjust the coincidence of a reference signal with the trigger signal to accommodate circuit variations caused by changes in infrared sensor sensitivities or ambient temperature. To supply a highly accurate feedback reference signal an internal high frequency oscillator is used for feedback control and counted down to furnish the reference signal. The conversion from detector current to time is accomplished by using the detector to discharge an accurately charged capacitor and measuring the time to recharge the capacitor.
The invention disclosed in the Smith patent has several drawbacks. First, the detector circuit depends on both the discharge and recharge time of the capacitor to determine the detector current, requiring accurate measurement of two different time periods. Second, the detector circuit as disclosed samples only about eight times per second, necessitating the use of a relatively low frequency clock. Thirdly, the detector uses a digital clock generator, a reference pulse generator, a sample pulse generator, and a trigger circuit, all of which make the circuit relatively expensive to manufacture. The detector of the Smith patent compensates for long-term drift in the analog front-end electronics by adjusting the bias of the detector to reduce the difference between the capacitor recharge time and the reference time intervals, thereby compensating the circuit itself.
Thus, there exists a need for a low cost infrared detection and conversion circuit for detecting motion and activating a switching device to control lights, appliances, and electrical equipment and other devices. There exists a need for such an infrared detection and conversion circuit which uses common, low cost electronic components, and which reduces the number of components required to effectively detect and convert passive infrared signals to electric signals for processing. Further, there exists a need for a reliable circuit design that supports wide component tolerances inherent in common components and even component substitutions without significantly impacting the signal processing portion of the circuit.