AN434/0992 1/4 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 at a 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 the system, but also by the weakness of the products. this weakness can be improved by the use of 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 lenses mounted on the detector. this induces a chopper effect which generates an alternating voltage 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 and normal movement, a frequency between 0.1 and 3hz, which corresponds to the maximum sensitivity of the sensor. the output level of the sensors is in the order of mv which requires an amplification of more than 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 programmable between 10 seconds and several minutes. this then triggers the interface which drives the triac in place of a relay. 1
2/4 concepts for noisy environments figure 1. discrete components system overview 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 filter which 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 moving source 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 which prevents a movement detector from working in a satisfactory way. badly positioned sensors or lamps 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 in figure 1. vr 00 1813 human body or temperature source fresnel lens p.i.r. sensor 60 to 70db amplifier window comparator turn on delay input dead time after load switch off scr ignition pulse generator 220v ac retriggerable timer sensor d s turn-on time ldr daylight sensivity threshold band-pass filter time filter for fast daylight changes 2
3/4 concepts for noisy environments why not a microcontroller? figure 2 shows that, apart from the window comparator, all the signals are largely read as dig- ital by means of inputs to analog inputs of a microcontroller. the modern cmos microcontroller, with low power consumption, can be powered without dif- ficulty directly by the mains power supply. for protection against extended voltage input, a low cost and simple zener diode is used as show in figure 2. the power capacity of 2 i/o pins mounted in parallel is sufficient to drive osensitive gateo triacs by themselves. an operational amplifier is necessary for its high signal amplification capability. cost is not an essential factor, but carries a high prejudice against this concept. the decision to use a microcontroller with analog inputs carries a series of advantages, together with its log- ical functionality. figure 2. microcontroller system overview pb 2 pb 3 12 13 0v 8mhz 22p 22p 0v 34 p i ez o 0v 0v 0v neutra l line bta 08-600 sw version st6210 pb 5 pb 4 pb0 vdd +5v 100 reset pb1 pb 6 nmi vs s tes t oscin oscout pa 1 pa2 pa3 pa 0 0v 15 9 20 11 10 18 17 16 19 1 5 6 fus e 12v 100k 200k 200k 22k 100k 0v +5v vr0f1705 on-ti me da y light compe nsation ov ov ov ir 14 0v +5v 5v6 100 m f 6v 220nf/400v 1n4148 820-1/2w
4/4 concepts for noisy environments othe present note which is for guidance only aims at providing customers with information regarding their products in order for them to save time. as a result, stmicroelectronics shall not be held liable for any direct, indirect or consequential damages with respect to any claims arising from the content of such a note and/or the use made by customers of the information contained herein in connexion with their products.o information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. n o license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without the express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 1998 stmicroelectronics - all rights reserved. purchase of i 2 c components by stmicroelectronics conveys a license under the philips i 2 c patent. rights to use these components in an i 2 c system is granted provided that the system conforms to the i 2 c standard specification as defined by philips. stmicroelectronics group of companies australia - brazil - canada - china - france - germany - italy - japan - korea - malaysia - malta - mexico - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. http:/ /www.st.com
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