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Build a reliable Door Minder This project will sense a door opening in a large or small room and will sound a 2-tone chime. It does not have to be anywhere near the doorway as it uses an ingenious method to detect the pressure change caused when the door opens or closes. By RICK WALTERS 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 a microswitch mounted on the doorway, a pressure switch in a mat on the floor or 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 of a light beam relay 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 correct­ ly. Light beam relays can also be swamped by the Sun or by bright lighting. The Door Minder presented here can be placed anywhere in the room; it does not have to be anywhere near 54  Silicon Chip the doorway. It can even be placed in an adjoining 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 air 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 will work in very large rooms too – up to several hundred square metres (say 200 square metres or more). In our offices at SILICON CHIP we have a number of adjoin­ing rooms. Opening the door to one room will trigger the Door Minder in any of the other rooms, even with the windows open. It is highly effective and does not respond at all to wind or to loud noises. What is the pressure sensor? The pressure sensor is nothing more than a cheap electret microphone insert which can be bought for a couple of dollars. 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 two-tone oscillator circuit which produces the chime sounds. Another integrated circuit audio amplifier is used to drive a small loudspeaker. And that is virtually all there is to it. Unlike light beam relays, the circuit uses very little power and could be 6 3 .047 8 VR2 4.7k 5 IC3 2 LM386 4 10  10 25VW GND REG1 7808 OUT 33k 0.22 4 .047 220k +8V 5 IC2c 6 .047 13 D5 1N914 4.7M 12 IC2f 56k .047 IC2b 2 3 7 14 0.22 22M 0.22 8.2M 1 IC2a 74C14 D3 1N914 4 IC1b D S VIEWED FROM BELOW G 100 LL 1 MIC 47k VR1 4.7k 3.9M DOOR MINDER I GO 10k 1k D2 +3.3V +3V 150k 5 150k IC1a 3 TL072 2 +3.3V 1 2x1N914 1k D1 15k +3.6V 6 8 7 0.1 0.1 Fig.1 (right): the circuit uses a microphone, a bandpass filter stage (IC1a), a comparator (IC1b), a 2-tone chime generator (IC2) & an audio amplifier (IC3). S 1k G .01 10k 0.22 4.7M 8 9 IC2d 220k Q2 BS170 D 1k S G D4 1N914 .047 11 47k IC2e 10 10k .01 Q1 BS170 D 33k 100 16VW 0.1 +8V The circuitry for the Door Minder comprises the electret microphone insert, a small loudspeaker, three integrated cir­ cuits, two field effect transistors, a 3-terminal regulator and a few resistors, capacitors and diodes. It is powered from a 12V DC plugpack or, as already noted, from batteries. The circuit is shown in Fig.1 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. The DC supply for the internal FET is provided by the 4.7kΩ trimpot VR1 which does double-duty as a sensitivity con­trol. With the wiper of VR1 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. IC1 is a TL072 dual op amp. IC1a 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 effectively means is that IC1 will respond only to brief positive or negative changes in air pressure, as sensed by the electret. Note that the non-inverting input, pin 3 of IC1a (indicated with a + sign), is set at +3.3V by the 15kΩ, 1kΩ and 10kΩ resis­ tors. A 100µF capacitor decouples this input from the supply. This input bias sets pin 1, the output of IC1a, to +3.3V too, which is important as far as the following circuitry is con­cerned. IC1b is connected as a comparator. Pin 6, the inverting input (indicated with a minus sign), is held at +3.6V due to the resistors forming the previously mentioned voltage divider across the 8V supply. Pin 5, the non-inverting input, is held at +3.0V. The output of IC1a is connected to the two inputs of IC1b via two 1N914 IN The circuit 100 16 12V PLUG-PACK run from batteries, if you wanted to. July 1995  55 RESISTOR COLOUR CODES ❏ No ❏  1 ❏  1 ❏  2 ❏  1 ❏  2 ❏  2 ❏  1 ❏  2 ❏  2 ❏  1 ❏  3 ❏  4 ❏  1 Value 22MΩ 8.2MΩ 4.7MΩ 3.9MΩ 220kΩ 150kΩ 56kΩ 47kΩ 33kΩ 15kΩ 10kΩ 1kΩ 10Ω 4-Band Code (1%) red red blue gold (5%) grey red green brown yellow violet green brown orange white green brown red red yellow brown brown green yellow brown green blue orange brown yellow violet orange brown orange orange orange brown brown green orange brown brown black orange brown brown black red brown brown black black brown diodes, D1 & D2. Under quiescent (no-signal) condi­tions neither of the diodes conduct since the voltage across each is only 0.3V between the anode and cathode. Note that the voltage at the inverting input is higher than the non-inverting input by 0.6V and so pin 7 of IC1b is low. When the output of IC1a swings high, due to a pressure decrease sensed by the electret, diode D2 conducts and pulls pin 5 of IC1b higher than pin 6 10k 1k 1k 15k .047 220k 10  10k .01 10k .01 47k Q2 D5 D4 4.7M 0.22 56  Silicon Chip IC3 LM386 56k 8.2M 0.22 1 Q1 33k 0.22 1k MIC 150k 150k .047 1k 100uF 100uF 0.22 2x.047 4.7M 47k VR1 D3 IC1 TL072 1uF 1 220k 22M D2 1 Schmitt triggers 0.22 D1 3.9M VR2 IC2 74C14 10uF 0.1 0.1 100uF and so the output of IC1b goes high. Similarly, when IC1a’s output swings low, diode D1 conducts and pulls pin 6 lower than pin 5 and so pin 7 again goes high. IC2 contains six Schmitt triggers, two of which (e & f) are used as the chime oscillators, while the rest are for time delays. A normal CMOS gate switches at approximately 50% of the supply voltage, whether the input is rising or fall­ing. A Schmitt trigger, on the other hand, has a higher switching level for a rising input than it does for a falling one. Thus, there is a dead band (hysteresis) where the input signal can vary up and down by a fair amount, without changing the output. SPEAKER REG1 7808 5-Band Code (1%) not applicable grey red black yellow brown yellow violet black yellow brown orange white black yellow brown red red black orange brown brown green black orange brown green blue black red brown yellow violet black red brown orange orange black red brown brown green black red brown brown black black red brown brown black black brown brown brown black black gold brown 33k 0.1 12VDC PLUG-PACK Fig.2: install the parts on the PC board as shown here. Make sure that all polarised components are correctly oriented & take care with the supply polarity. PARTS LIST 1 PC board, code 03107951, 105 x 60 mm 1 plastic utility case, 130 x 68 x 44mm 1 57mm 8Ω loudspeaker 1 electret microphone insert 1 12VDC plugpack with DC plug 1 chassis mount socket to match DC plug 2 4.7kΩ miniature vertical trimpots (VR1,VR2) Semiconductors 1 TL072, TL082 dual op amp (IC1) 1 74C14, 40106 hex Schmitt trigger (IC2) 1 LM386 audio amplifier (IC3) 2 BS170 IGFETs (Q1,Q2) 1 7808 3-terminal regulator (REG1) 5 1N914, 1N4148 diodes (D1-D5) The PC board clips into slots in the side of the case, while the loudspeaker is secured using small clamps. Power can come from a 12V DC plugpack. Each time pin 7 of IC1 goes high, it charges the 0.22µF capacitor to about +7.4V and brings IC2, a 74C14 hex Schmitt trigger, into play. This will cause pin 2 of IC2a to go low, pulling pin 9 low via the series .047µF capacitor. Thus pin 8 will go high, charging the 0.22µF capacitor via D4, so that FET Q1 is turned on. The 47kΩ resistor between pins 10 and 11 of IC2e, together with the .047µF capacitor, form an oscillator which runs continu­ously. The signal at pin 10 is a 660Hz square wave. The 10kΩ resistor and .01µF capacitor at the output of IC2e provide a modest degree of filtering to make the waveform more sinusoidal. IC2f is another square wave oscillator and the signal at its pin 12 is 550Hz. When Q1 turns on, the filtered 660Hz signal is fed to its 1kΩ source resistor and then via the 33kΩ resistor to 4.7kΩ trimpot VR2. As the 4.7MΩ resistor on the gate of Q1 discharges the 0.22µF capacitor, the gate voltage of Q1 slowly falls and its resistance increases, thereby reducing the signal. This produces the audible “ding” which gradually fades. When pin 2 of IC2a goes low, as mentioned above, it produc­es a similar sequence to that previously described, pulling pin 3 of IC2b low. After the 8.2MΩ resistor charges the 0.22µF capaci­tor, pin 4 reverts to its low state, momentarily pulling pin 5 of IC2c low, which causes pin 6 to go high. This charges the capaci­ tor at the gate of Q2 to produce the “dong”. The ding-dong out­puts are mixed via the 33kΩ resistors and fed to audio volume control, trimpot VR2. The signal from VR2 feeds IC3, an LM386 audio amplifier which is used to drive the speaker. Time delays One point not mentioned so far is the avoidance of nuisance tripping. Clearly, if people are going in and out of doors fre­quently, the Door Minder circuit would be triggered into ding-donging all the time and that could drive you mad. So to avoid this, once the circuit Capacitors 3 100µF 16VW PC electrolytic 1 10µF 25VW PC electrolytic 1 1µF 16VW low leakage (RBLL) or tantalum electrolytic 4 0.22µF MKT polyester 3 0.1µF MKT polyester or monolithic 5 .047µF MKT polyester 2 .01µF MKT polyester Resistors (0.25W, 5%) 1 22MΩ 2 47kΩ 1 8.2MΩ 2 33kΩ 2 4.7MΩ 1 15kΩ 1 3.9MΩ 3 10kΩ 2 220kΩ 4 1kΩ 2 150kΩ 1 10Ω 1 56kΩ Miscellaneous Hookup wire, solder. has been triggered to produce a “dingdong”, it can’t be triggered again for about seven or eight seconds. This is achieved by the time constant consisting of the 0.22µF capacitor and 22MΩ resistor at pin 1 of IC2a. Once D3 has charged up the 0.22µF capacitor, it takes a significant time to discharge and this prevents re-triggering of the circuit. As mentioned above, power for the circuit is provided by a 12VDC (nominal) plugpack. This is fed to a July 1995  57 label where the crosses are, then place the panel on the lid of the box and mark the holes with a felt pen. The lid can now be drilled. This done, carefully drill the Dynamark® holes one or two sizes smaller, mount the speaker on the lid and affix the label. You also need to drill two holes in the case itself – one for the DC socket and the other to allow changes in air pressure to be sensed by the electret micro­phone. The latter can be drilled in one side of the case, near the microphone. Setting up Fig.3: the PC artwork is reproduced here actual size. + + + + + + + + + + + + + + + + + + + + All the parts, with the exception of the loudspeaker, are mounted on a PC board measuring 105 x 60mm (coded 03107951). This is mounted in the base of a standard zippy box measuring 130 x 68 x 43mm. No special procedure needs to be followed when assembling the board, although it is better if the two links and all the resistors are fitted first. Ensure that all the polarised compon­ents such as the diodes, electrolytic capacitors and ICs are inserted the right way around. This is shown on the component overlay diagram of Fig.2. As can be seen from the photos, we used a socket for IC2. This was done to allow us to check variations in the performance of Schmitt trigger ICs but otherwise a socket is not necessary. Most electret microphone inserts + Construction do not have the their leads labelled but tend to be sold with specifications showing how they are connected. Make sure you obtain this information when purchasing. Ours had an external metal screen with an earth lug which was not connected to either pin. We earthed this lug with a piece of tinned copper wire. The 3-terminal regulator is laid flat on the PC board. When installed in the case, there is adequate clearance between the components and the speaker magnet. If you do have clearance problems, because you use different components, file the PC board where it sits on the guides, to allow it to rest on the bottom of the case. The loudspeaker can be mounted on the front panel, using a silicone or epoxy adhesive, or small clamps and screws. We used the latter. Before doing that though, you will need to drill holes in the lid to let the sound out. Drill small holes in the Dynamark® DOOR MINDER 7808 3-terminal regu­lator which has an output of +8V. Fig.4: this artwork can be used as a drilling template for the loudspeaker grille. 58  Silicon Chip This is easy. Apply power and measure the voltage on the output pin of the 7808 regulator. It should be close to +8V. Check that the same voltage appears at pin 8 of IC1, pin 14 of IC2 and pin 6 of IC3. Now check the voltage at pins 1, 3, 5 and 6 of IC1. They should be close to the values nominated on the circuit of Fig.1. Now set trimpot VR2, near the LM386, fully clockwise (look­ing from the edge of the board) and trimpot VR1, next to the electret microphone, to about half setting and open a door. The chime should sound. Place the Door Minder anywhere convenient and that’s all there is to it. Adjustments Because of the possible spread in Schmitt trigger (IC2) levels, you may have to adjust one or two components. After the “ding”, there should be a short silence, then the “dong”. If they overlap, change the 8.2MΩ resistor on IC2b pin 3 to 10MΩ. Conversely, reduce it if the silent period is too long. If the tone duration is too long, reduce the 4.7MΩ resistors at the gates of Q1 and Q2 or, conversely, increase them for longer chime durations. As mentioned above, the 0.22µF capacitor and 22MΩ resistor at pin 1 of IC2a prevent a double chime as the door is opened and then closed. If you want a longer delay in your situation, in­ crease the capacitor to 0.47µF or even 1µF. Finally, note that when the Door Minder is not chiming, it will produce a low-level buzz. This is normal and is due to radiation of the harmonics of the 550Hz and 660Hz square wave oscillators into the mixing circuit asSC sociated with VR2.