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This project will sense a door opening in a large or small room and will sound a two-tone chime. By LEO SIMPSON & JOHN CLARKE While the most obvious application of this project would be as a door monitor for shop keepers, it could have applications in offices, workshops, doctors' and dentists' waiting rooms, child-minding centres and in the home. It could also be used as a sensor in a burglar alarm. In the past, the classic ways to detect the opening of a door have been either to use a microswitch mounted on the doorway or to use a light beam relay circuit. The latter method has the advantage that it does not have to be attached to the door and it can be made to work with any type of door, hinged or sliding. The disadvantage is that it must be near the doorway or an adjacent passageway and it must be carefully set up in the first place, to work correctly. The Door Minder presented here can be placed anywhere in the room; it does not have to be anywhere near the doorway. It can even be placed in an adjoining room. The prototype Door Minder was built into a small plastic case. It can be placed anywhere in the room. How does it work? When a door is closed it can be regarded as a very large piston in a close-fitting rectangular cylinder. When you push a door open you cause quite a large momentary increase in pressure in the adjoining room. The Door Minder senses this increase in pressure and sounds a two tone chime. The Minder can be used on either side of a door because it also senses a momentary drop in pressure. So it works equally well with inward- opening or outward opening doors. Nor does the room need to be tightly sealed. Windows can be open, provided they are not really large. Because it senses pressure, the Minder can be placed anywhere in the room. It work will work in very large rooms too, up to several hundred square metres (say 2000 sq ft or more). In our offices at SILICON CHIP we have three adjoining rooms. Open- I---- +8V PLUG-PACK + 47 470P. FROM 9V-12V, 300mA 25VWI 0.1 T 330k 2200 330 16VW IC2 SAB0602 ELECTRET MICROPHONE 4 + 0.33+ 8 0.1 '7r1 8(! ~SPEAKER 33k DOOR MINDER SC032-0188 Fig.1: the circuit uses a microphone, a bandpass filter stage (ICla), a comparator (IClb) and a two-tone chime (IC2). FEBRUA RY1988 53 ing the door to the first room will trigger the Door Minder in the third room, even with the windows open. It is highly effective and does not respond to wind or to loud noises. What is the pressure sensor? Possibly some of our readers are thinking that we have used a fancy expensive pressure sensor for this project but they are wrong. The pressure sensor is nothing more than a cheap electret microphone insert which can be bought for a couple of dollars. Isn't that ingenious? We think it is. The electret microphone is used with an amplifier circuit which only responds to extremely low frequencies. It does not respond to audible sounds at all. The amplifier is used to trigger a special integrated circuit which produces the chime sounds. And that is virtually all there is to it. Unlike light beam relays, the circuit uses very little power and could, if you wished, be run from batteries. The circuit The circuitry for the Door Minder comprises the electret microphone insert, a small loudspeaker, two integrated circuits, a 3-terminal regulator and a few resistors , capacitors and diodes. It is powered from a 12V DC plugpack. To describe how the circuit works, let us start right at the beginning, at the electret insert. This contains an internal field ·effect transistor {FET) which is connected as a source follower. Bias for the internal FET is provided by the 4.7kn trimpot which does double-duty as a sensitivity control. With the wiper adjusted up to the + 8V supply rail, no signal is fed to the following circuitry; with the wiper adjusted at the extreme opposite setting, maximum signal is fed to the following circuitry. ICl is a dual op amp. ICla is connected as a narrow bandpass filter stage with a gain of about 80. It responds to frequencies within the range of about 0.5Hz to 3Hz. What this means is that the amplifier will respond only to brief positive or 54 SrLICON CHIP ~ + 9V-12V, 300mA PLUG-PACK 811 SPEAKER Fig.2: install the parts on the PCB as shown here. Power for the circuit is supplied via an external plugpack transformer. negative decreases in pressure, as sensed by the electret. Note that the non-inverting input {indicated with a + sign) of ICla is set at + 3.3V by the 47kn and 33kn resistors. A lµF capacitor decouples this input from the supply. This sets the output of ICla to + 3.3V too, which is important as far as the following circuitry is concerned. ICl b is connected as a comparator. Its inverting input {indicated with a - sign) is held at + 3.6V due to the 330kn and 270kn resistors forming a voltage divider across the 8V supply. The noninverting input is held at 3.0V with the 300kn and 180kn resistors. The output of ICla is connected to the two inputs of ICl b by diodes Dl and D2. Under quiescent conditions neither of the diodes conduct since the voltage across each is only 0.3V. Note that the voltage at the inverting input is higher than the noninverting input by 0.6V and so the output of IClb is low. When the output of ICla swings high, diode D2 conducts and brings the non-inverting input of IClb higher than the inverting input and so the output of IC1 b goes high. Similarly, when ICla's output swings low, D1 conducts and pulls the inverting input lower than the noninverting input and the output of IC1 b again goes high. Each time the output of IC1 b goes high, it triggers the two-tone chime IC, the SAB0602. This produces a rich, heavily modulated chime sound which is attention-arousing without being too obtrusive. The output from IClb is divided using a 1okn and 2.7kn resistor divider before being applied to the input of IC2. This is necessary because when the output of IC1 b is PARTS LIST 1 9VDC or 1 2VDC plugpack 1 PCB, code SC03-2-188, 87 x 58mm 1 plastic utility case, 1 30 x 6 7 x 41mm 1 57mm an loudspeaker 1 electret microphone insert 1 1 OµF 16VW PC electrolytic 1 1µF 1 6VW RBLL or tantalum electrolytic 1 1µF 1 6VW PC electrolytic 1 0 .33µF metallised polyester 2 0. 1µF metallised polyester 1 .0068µF metallised polyester Semiconductors 1 TL082, TL072 dual op amp 1 SAB0602 two tone chime ringer 1 7805 3-terminal regulator 2 1 N4148, 1 N914 diodes Resistors (0.25W, 5%) 1 x 3 .9M!l, 1 x 330k~, 1 x 3ookn 1 %, 1 x 270kn , 1 x 180kn, 2 x 4 7kn, 2 x 33kn, 1 x 10kn, 1 x2 .7kn, 1 x470n, 1 x 2200, 1 x 4. 7k0 miniature vertical trimpot. Capacitors 1 330µF 16VW PC electrolytic 1 47µF 25VW PC electrolytic Miscellaneous Hookup wire, solder, etc Construction ' View inside the case. The PCB clips into slots in the side of the case while the loudspeaker is secured using small clamps. low, it sits at about + 1.5V or so, which is not low enough to prevent continual triggering of IC2. Hence, the voltage divider which fixes the problem. IC2 drives an 811 loudspeaker via a 330µ,F capacitor. The 0.1µ,F capacitor and 33k11 resistor set the operating frequency of the chime tone, while at the output, a 0.33µ,F capacitor provides a degree of high frequency filtering. As mentioned above, power for the circuit is provided by a 9V or 12V DC (nominal) plugpack. This is fed to 7805 3-terminal regulator which has its output set to about + 8V by the associated 47011 and 22011 resistors. This setting was used to satisfy the supply voltage requirement of between 7V and 11 V DC for the SAB0602 chime IC. L _J 0 --, 0 Fig.3: the PC artwork is reproduced here actual size. r All the components, with the exception of the loudspeaker, are mounted on a printed circuit board measuring 87 x 58mm and coded SC03-2-188. No special procedure needs to be followed when assembling the board apart from ensuring that tlll the polarised components such as the diodes, electrolytic capacitors and ICs are wired in the right way around. This is shown on the component layout diagram. Most electret microphone inserts do not have their leads labelled but most tend to be sold with specs showing how they are connected. Make sure you obtain this info when purchasing. The 3-terminal regulator is laid flat on the PCB and the tab held down with a wire lug soldered to the board. When installed flush on the bottom of the case, there is adequate clearance between the components and the speaker magnet. The loudspeaker can be mounted onto the front panel using an expoxy adhesive or small clamps and screws. We used the latter. Before doing that though, you need to drill holes in the lid to let the sound out. You also need to drill two holes in the case itself - one for the twinlead from the DC plugpack and the other to allow changes in pressure to be sensed by the electret. Setting up This is easy. Apply power and measure the voltage oh the output pin of the 7805 regulator. It should be close to + 8V. Check that the same voltage appears at pin 8 of ICl and pin 2 of IC2. Now check the voltage as pins 1, 3, 5 and 6 of ICl. They should be close to the values nominated on the circuit. Now set the trimpot to about half setting and open a door. The chime should sound. Place the Minder where convenient and that's all there is to it. Incidentally , the Siemens SAB0602 is very similar to the SAB0600 which is a three-tone chime IC. The latter is already available from a number of kitset suppliers and could be substituted if the SAB0602 is temporarily unavailable. ~ FEBRUARY1988 55