Making a movement detector using a high-gain op-amp circuit has never been easy. It is interesting to see how magazine articles present the subject; they always imply that the project is easy. The problem is that as the gain of the circuit is increased the slightest line voltage fluctuation will false trigger the unit. This arises when you try to run the op-amp circuit from the same power supply which operates the siren, or even an LED which comes on when the unit is triggered. It is still a black art to juggle the position of voltage regulators & special op-amps, capacitors & components to minimise this problem. So both for the hobbyist and commercial user there is a real attraction to have the movement detector circuit all contained in one IC. Heimann were first to put the complete electronics of a movement detector into one IC. But it was too expensive for commercial use and has been discontinued. The 20 pin DIP IC supplied here is the result of a recent American - Hong Kong Chinese venture to do the same thing more cheaply. Our tests on this IC show it to be very good and easy to use. And the choice of an efficient lens system is also important. The purpose of the fresnel lens is to break up the area under observation into optical zones so that a heat source moving from one optical zone to another will generate a heat wave on the surface of the PIR detector. It is no use having an excellent electronic circuit if the IR optical system is not very good. A lower gain circuit with an excellent lens system is far better than an excellent circuit and poor lens system. (Many cheap commercial detectors follow this philosophy.) And herein lies the problem for the hobbyist: it has always been difficult for the hobbyist to construct a curved mounting to hold the large plastic-sheet Fresnel lenses which give the most sensitive IR optics onto the PIR sensor. The physics of a creeping burglar at 25 yards (or meters) simply demands a focal length of 4 to 5cm for the lens/sensor system in order for the motion to be reliably detected. The PCB-mounted lens supplied in this kit is a good compromise. It will reliably detect motion in the 3 to 4 meters range.
The technology of PIR sensors is now well known and has been published in most hobby magazines sometime within the last 5 years. Dual sensor PIR detectors are now the most widely used motion detectors. The other technologies (mainly ultrasonic) suffer too much from false triggering or are too expensive.
The kit is constructed on a single-sided printed circuit board (PCB). It has a silk screen overlay on top to aid construction. On the bottom there is a solder mask to help in soldering.
Runs on 9 - 12vDC.
US Dollars
