Part # Application Note General Purpose datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: CD driving with ST6240B


Part Details:

APPLICATION NOTE MOVEMENT DETECTOR CONCEPTS FOR NOISY ENVIRONMENTS by Herbert SAX INTRODUCTION The sales of movement detectors, which react to human-body temperature, are increasing ata fantastic rate. No Do-it-Yourself shop proposes less than 4 models for sale if it is serious about its image,however the majority of clients are novices who wish to install the system themselves. This in-stallation often causes frustration, partly caused by a lack of knowledge of the operation of thesystem, but also by the weakness of the products. This weakness can be improved by the useof microcontrollers. MOVEMENT DETECTORS Most movement detectors available, whether using discrete components or integrated circuits,have a circuit concept as shown in Figure 1. The movement of a source of heat is projected onto the sensor by a array of Fresnel lensesmounted on the detector. This induces a Chopper effect which generates an alternatingvoltage in the sensor. The frequency is dependent on the number of Lens segments, the dis-tance and the speed of the heat source. The array of lenses is positioned so that it provides, at a detection distance of around 10m andnormal movement, a frequency between 0.1 and 3Hz, which corresponds to the maximumsensitivity of the sensor. The output level of the sensors is in the order of mV which requires an amplification of morethan 60dB. The amplifier also acts as a band-pass filter to eliminate parasitic signals. A window comparator follows which digitalises the alternating voltage. This monostable re-moves parasitic pulses providing also a high immunity to noise. A pulse longer than the mon- ostable delay time starts a second monostable which, in general, is externally programmablebetween 10 seconds and several minutes. This then triggers the interface which drives theTriac in place of a relay. AN434/0992 1/4 1 CONCEPTS FOR NOISY ENVIRONMENTS Figure 1. Discrete Components System Overview Input Dead TimeAfter Load Switch Off Fresnel Lens SCR Ignition Window Comparator Pulse Generator P.I.R. Sensor 60 to 70dB 220VAC amplifier Turn On Delay Human Body Retriggerable or Temperature Timer Source D Turn-On Time Band-Pass Time Filter For Filter LDR Fast Daylight Changes Sensor S Daylight SensivityThreshold VR001813 Two other functions are equally as important: 1. A photosensitive resistor prevents the lamp from being triggered by daylight, the level of ac-tivation can be adjusted to function in the diverse number of mounting conditions. The filterwhich follows the resistor to ignore transient changes in light level. 2. After the turn-off of the lamp by the timer, the function of the timer can be inhibited for sev-eral 100mS. This is needed for environments where the lamp is situated in the movement de-tector. There is a danger that the movement of the filaments of the lamp on cooling, them-selves a source of heat, can be interpreted in an erroneous fashion by the sensor as a movingsource of heat. This is followed by a further operation of the lamp which appears as a fault. This is the type of malfunction that is found experimentally as the most frequent reason whichprevents a movement detector from working in a satisfactory way. Badly positioned sensors orlamps can increase these problems despite the delay in retriggering. These malfunctions are found, in particular, by halogen lamps, the preferred lighting source, and are not resolved. One additional problem is the switching on of the halogen lamps, this often requires more cur- rent than either the triac in the detector or the fuses can support. The solution lies only in a mo-tion detector which presents more flexibility and more intelligence than the concept shown inFigure 1. 2/4 2


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