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The Driveway Sentry Mk.2 Here’s an improved version of the Driveway Sentry we described in the November 2004 issue of SILICON CHIP. It detects vehicles like cars, trucks, tractors or other farm machinery moving along a driveway or through a gateway. When movement is detected, it switches on a mains-powered or battery-powered lighting system and activates an optional piezo buzzer alarm for a preset period ranging from 2-25 seconds. The Driveway Sentry Mk.2 circuit is housed in a plastic zippy case and switches on lights when a vehicle drives over a driveway detector loop. By Jim Rowe 36  Silicon Chip siliconchip.com.au +V +V HIGH GAIN AMPLIFIER LP FILTER SENSOR LOOP IC1, IC2a/d LOOP TERMINATION BOX +V  +V – START EXIT DELAY IC2b Q2 RELAY TEST BUZZER ON/OFF TRIGGER EXIT DELAY HOLDOFF EXIT DELAY TIMER PIEZO BUZZER COMPARATOR + +V ALARM ONE SHOT Q3 IC5 IC3, IC4, Q1 Fig.1: block diagram of the Driveway Sentry. The sensor loop detects a vehicle passing over it and the resulting signal is filtered, amplified and fed to comparator stage IC2b. This then triggers a monostable which turns on transistors Q2 & Q3 to drive a buzzer and activate a relay to switch on the lights. U NLIKE OTHER motion-sensing systems that use light, heat or ultrasonic sound waves to detect motion, the Driveway Sentry Mk.2 operates by sensing small changes in the Earth’s magnetic field – the same magnetic field that’s sensed by a compass. Since cars, trucks and similar vehicles contain a significant amount of ferrous metal (iron, steel etc), they inevitably produce small temporary changes in the Earth’s magnetic field when they move into or through an area. That’s how the original Driveway Sentry detected them and that’s how this new Mk.2 version works too. The original unit used a special highsensitivity shielded remote sensor unit (no longer available) but this new version simply uses a loop of sensing cable buried under the driveway. No interference Because it doesn’t generate any sensing fields of its own, the Driveway Sensor Mk.2 produces no electromagnetic interference; it’s quite “clean”. Also, because it only senses moving iron and steel objects like vehicles, it’s much more selective than other kinds of sensor. This makes it virtually immune to false alarms from birds, dogs, cats, sheep, kangaroos and other animals, falling tree branches, rain and snow, people walking past (unless they’re Iron Man!) and so on. At the same time, it can be used to detect the movement of vehicles which contain very little steel – like aluminium trailers, boats and caravans – simply by attaching a strong magnet siliconchip.com.au Main Features The Driveway Sentry Mk.2 detects moving vehicles by sensing the small temporary changes in the Earth’s magnetic field caused by this motion. It detects the changes using a rectangular sensor loop which is buried under the driveway, or concealed with two opposite ends of the loop in the expansion gaps in the driveway itself. Exit Delay: allows the system to be switched to non-sensing “sleep mode” for a period of about five minutes, to allow the owner’s vehicle to exit from the property without activating the Driveway Sentry. At the end of the Exit Delay, the system returns to its movement sensing mode. Test Button: allows the system to be manually triggered into “movement detected” alarm mode without having to drive a vehicle over the remote sensing loop. This makes system adjustment easier and more convenient. Piezo Buzzer: produces a high-pitched sound to attract your attention when movement is detected. This sound can be disabled if you prefer the system to respond silently. Relay Contacts: includes an SPST relay with mains-rated contacts. The relay is activated when the system detects movement, allowing the unit to be connected to control mains lighting or other equipment such as a high-powered siren. Alarm Duration Control: allows the duration of the system’s “movement detected” alarm mode to be adjusted between a minimum of two seconds and a maximum of about 25 seconds. Sensitivity Adjustment: allows the sensitivity of the Driveway Sentry to be adjusted over a wide range, so it can be set for reliable vehicle detection without being too sensitive and susceptible to false alarms. Low Power Consumption: unit operates from 12V DC power (normally a plugpack), with a low current drain: <25mA in Exit Delay mode (<300mW), <15mA in armed mode (<180mW) and <100mA in alarm (movement detected) mode (<1.2W). This means that the system can also be operated from a 12V SLA battery and/or solar power in rural and other remote situations. to the underside of their chassis. The magnet ensures that if they’re moved past the Driveway Sentry’s remote sensor loop, the Earth’s magnetic field will be disturbed locally and the system will activate. In short, like the original Drive- way Sentry, this new version has a multitude of motion-sensing uses around the home or farm. The system operates from 12V DC and draws very little current – less than 15mA when armed and waiting, and no more than 100mA when it senses movement and August 2012  37 FARADAY SHIELD 470 +5.7V 100nF 10k SENSOR COIL SENSOR INPUT 5 3 3 2 4 1 4 3 100 1k 2 1 +6V 22nF 4.7k 5 SENSITIVITY 1 F 1k 100nF 8 1 4.7k VR1 500 2 CON1 7 IC1 AD623AN 22k 6 4 3 5 4 1 IC2a 2 470nF 22nF 30m SCREENED 2-CORE CABLE 10 F 10k 100nF +6V +6V D3 1N4148 K 100nF 100k 6 A 100k 100nF 14 IC3b 13 4 S2 8 100nF IC3c 7 RS RT 11 10 470k 27k CT 9 Vss 8 A O14 3 DRIVEWAY SENTRY MK2 EXIT DELAY  LED2 K 150nF 10nF SC 3 560 10k 2012 IC3a 2 IC4 4060B MR 10 1 IC3: 4011B 16 Vdd 12 9 11 12 5 EXIT DELAY IC3d B C E Q1 PN100 1N4148 A K Fig.2: the circuit uses five low-cost ICs. IC1 (AD623AN) provides most of the signal gain for the loop sensor signals, while 7555 timer IC5 forms the monostable. Counter stage IC4 and its associated circuitry provide an exit delay. is “alarmed” or activated. Thus it can be operated from a 12V battery and/or solar power as an alternative to a DC plugpack supply. How it works The heart of the Driveway Sentry Mk.2 is a rectangular loop of shielded multi-conductor cable. This can either be concealed in the expansion joints of a driveway or laid under the driveway or gateway to be monitored. The ends of the loop are fed into a small waterproof box, where the starts and finishes of the various conductors are terminated to form a multi-turn loop. When tiny, low-frequency AC voltages are induced in the loop turns as a result of magnetic field disturbances, they are fed back to the Driveway Sentry’s main box via a twin-shielded 38  Silicon Chip cable, amplified and used to trigger the alarm circuit. Because the sensor loop also tends to pick up a significant amount of electrical noise, it needs to have a Faraday shield. This job is done by the screening layer of the loop cable which is connected (at one end only) to the shield braid of the output cable. This provides an electrostatic shield without also forming a shorted turn. Fig.1 shows how it works. The tiny voltages induced in the loop are first passed through a fairly drastic lowpass filter to attenuate all noise, hum and spurious signals above about 13Hz. This is possible because the signals we want to detect are of a very low frequency – only a few Hertz. The filtered signals are then fed to a high-gain amplifier (IC1, IC2a & IC2d), where they are amplified by up to 500,000 times. They are also further filtered, giving an overall attenuation of about 40dB for any spurious signals at 50Hz and above that may be picked up. The amplified signal is then biased to a DC level of 3V and fed to one input of a comparator (IC2b). Here it is compared with a reference DC voltage of 4.4V at the second comparator input. When the peak value of the amplified sensing loop signal exceeds this reference level, the output of the comparator switches low. The resulting negative-going pulse is then used to trigger IC5, a monostable pulse generator (or one-shot). When this happens, the output of the one-shot switches high, turning on transistor Q3 and energising the relay. The relay contacts can be used to siliconchip.com.au OUT 470nF 4.7k 220k 1k 220 F D1 1N5819 REG1 7806 +6V +11.7V IN GND VR2 500k 47 F 16V 68k 100nF IC2d 12 POWER LED1 1k 14  6 7 IC2b 5 +4.4V TEST S1 100nF 12V – DC IN 1000 F 25V A IC2: LM324 10 220k 180k 9 IC2c WARNING: WIRING INSIDE THIS AREA OPERATES AT 230VAC. CONTACT COULD BE FATAL! 8 IEC MAINS INPUT PLUG TRIGGER FUSE 1 +6V 47k VR3 500k 100nF ALARM DURATION 8 IC5 7555 7 2 5 K D2 1N4004 – MAINS OUTLET RELAY1 LK1 1N4004 K switch power to a siren, turn on security lights or trigger a security system. At the same time, the high level at the output of the one-shot can be used to turn on transistor Q2 which activates a small piezo buzzer mounted in the Driveway Sentry’s control box. However, if you don’t want this internal buzzer to sound, it can be disabled. The TEST pushbutton switch can be used to temporarily ground the positive input of comparator IC2b. This forces the comparator’s output low, triggering the one shot in the same way as a signal peak from the high-gain amplifier. So the TEST button allows you to do things like adjust the alarm duration without having to drive a vehicle over the cable loop. As shown in Fig.1, the rest of the circuitry is used to provide the Sentry’s N A 10k B C E 1N5819 A K Q2 PN100 B E B E “Exit Delay” function. This operates by holding off the one-shot for a fixed period of about two minutes after power is first applied to the Driveway Sentry or after the “START EXIT DELAY” pushbutton is pressed at any later time. With the one-shot prevented from triggering during that time, you are able to leave in your own vehicle before the Driveway Sentry is re-armed. Circuit description Now let’s have a look at the full circuit in Fig.2. The sensor loop is at upper left. For clarity, it’s shown with only two turns, although with the recommended 9-conductor screened cable there will actually be nine turns. The loop is connected to the input of the main circuit in the Driveway Sentry via a length of screened 2-core 7806 BC337 B C E Q3 BC337 PN100 K A C 4.7k LEDS siliconchip.com.au E A BUZZER ON/OFF 100nF A N +11.7V 3 1 47 F RBLL 100 PIEZO BUZZER A 10A + 4 6 + K 11 +3.0V CON2 A 1.5k TRIGGER SENSITIVITY 13 K E GND IN C GND OUT cable. This ends in a 5-pin DIN plug which mates with input socket CON1, a 5-pin DIN socket. The very weak signals from the sensor loop then pass through the main low-pass filter, formed by two 4.7kΩ resistors, two 22nF capacitors and a 1μF capacitor. They are then fed to the inputs of IC1, an AD623AN instrumentation amplifier which provides most of the signal gain. The 100Ω resistor and 500Ω trimpot (VR1) connected between pins 1 & 8 of IC1 allow its gain to be varied between 168 and 1001 times, without significantly changing its common-mode rejection. Note that the sensor loop’s Faraday shield and the input cable’s shield are not connected directly to earth but instead go to the half-supply bias voltage that’s fed to both inputs of August 2012  39 TRIG SENS 100nF VR2 500k + 10k 220k 1k TEST 220 F IC2 LM324 S1 1000 F D2 4004 CON3 IC5 7555 4011B 100 LK1 10k 47k 100k 100nF POWER VR3 100nF 100nF 100nF IC3 100nF 4060B 4148 27k 470k 1.5k D1 5819 12VDC IN LED1 REG1 7806 + D3 IC4 BUZ + LED2 NO BUZ 47 F LL EXIT DELAY 500k S2 Q2 EXIT DELAY PN100 + CON2 500 100k 150nF SENSITIVITY Q3 BC337 4.7k 1 F 68k 22nF 22nF 10nF VR1 100nF 10k 220k 22k 470 4.7k 100 1k 4.7k 4 1k CON1 IC1 AD623 TO EARTH TERMINAL OF GPO SOCKET 4.7k 470nF + 5 SENSOR IN 10k YRT NES YAWEVIRD 2 1 0 2 © 2 KRA M 12170130 1k 470nF 180k 100nF 100nF 560 10 F Q1 PN100 47 F 100nF TO RELAY COIL LUGS PIEZO BUZZER Fig.3: install the parts on the PCB as shown on this parts layout diagram. Take care to ensure that all polarised parts are correctly orientated and be sure to make the leads to the relay coil lugs at least 80mm long. IC1. This bias voltage is derived from a voltage divider consisting of two 10kΩ resistors and is bypassed using 100nF and 10µF capacitors. It’s then used to bias IC1’s inputs via the 1kΩ resistors connected between pins 2 & 3 and 2 & 1 of CON1. This means that there is virtually no DC voltage between the sensor loop conductors and their shielding, which improves the noise performance. The amplified signals from IC1 emerge from pin 6 and then pass through another low-pass RC filter formed by a 22kΩ resistor and a 470nF capacitor. They then pass through IC2a, one section of an LM324 quad op amp that’s used as a buffer to ensure that this RC filter is very lightly loaded. The buffered signals are then fed to the inverting input of IC2d via a 1kΩ resistor and a 220µF coupling capacitor. IC2d provides the rest of the signal amplification, with its gain adjustable between five and 500 times via trimpot VR2. It also acts as a low-pass filter due to the 470nF feedback capacitor. Its -3dB point varies with the gain setting so that only signals below 40Hz are amplified. Note that IC2d only amplifies the AC component of the signals, with their mean value set to +3.0V by a voltage divider consisting of two 220kΩ resistors. From there, the greatly amplified signal from pin 14 of IC2d is fed via a 40  Silicon Chip 1kΩ resistor to pin 6 of IC2b, configured as a comparator. Here it is compared with a +4.4V reference voltage at pin 5, as set by a 68kΩ/180kΩ voltage divider. When the signal applied to pin 6 of IC2b exceeds this +4.4V reference level, IC2b’s output (pin 7) switches low, providing a trigger pulse for monostable IC5, a 7555 CMOS timer. The trigger pulse from IC2b is fed to pin 2 of IC5, while pins 6 & 7 are tied together and connected to a timing circuit consisting of a 47kΩ resistor, trimpot VR3 and a 47µF low-leakage capacitor. VR3 allows the one shot’s “alarm time” duration to be adjusted from about 2-25 seconds. When IC5 is triggered (ie, pin 2 pulled low), its output at pin 3 switches high. This turns on Q3 which in turn activates Relay1 to switch power through to the GPO mains outlet. At the same time, Q2 is turned on to activate the piezo buzzer, provided link LK1 is set to its upper position. Exit delay The exit delay circuit consists of a simple RS-flipflop (IC3b & IC3c) plus IC4, a 4060B 14-stage binary divider with its own clock oscillator. When power is first applied or when S2 is pressed, the flipflop is switched into its reset state (pin 4 low) by the temporary low on pin 8. This low on pin 4 is applied to the reset pin (pin 12) of IC4 and as a result, IC4 starts counting. At the same time, gates IC3d and IC3a (used here as inverters) apply a logic low to pin 4 of IC5, its reset input. This prevents IC5 from triggering in response to pulses from IC2b. The timer’s counting proceeds for a little over two minutes, after which IC4’s O14 (pin 3) output finally goes low. This negative-going pulse is coupled via a 10nF capacitor back to pin 6 of IC3b, which switches the flipflop back into its set state. When this happens, pin 4 goes high and switches IC4 back into its reset state, thus stopping its oscillator and counter. At the same time, gates IC3d and IC3a apply a logic high to the reset pin of IC5, allowing it to be triggered again by any low-going pulses from IC2b. So the Driveway Sentry is armed (or re-armed) after a 2-minute delay. If you want a longer exit delay, simply replace the 150nF capacitor with a higher value (eg, 330nF for five minutes). During the exit delay time, there is a logic high on pin 10 of IC3c, the lower flipflop gate. This is used to turn on transistor Q1, which allows current to flow through LED2. This LED is therefore only illuminated during the exit delay period. Power supply The power supply section of the siliconchip.com.au This view shows the completed PCB, ready for installation in the case. Note that the two LEDs must be stood off the board by 20mm (see text) so that they later protrude through matching holes in the front panel (case lid). Driveway Sentry is very straightforward. Power comes from an external 12V DC plugpack, with Schottky diode D1 providing reverse polarity protection. The output from D1 is decoupled using a 1000µF electrolytic capacitor and then fed to regulator REG1 which provides a stable +6V supply. This +6V rail powers all of the circuit except for the relay which is powered directly from the cathode of D1. Diode D2 across the relay coil protects Q3 from damage by quenching any back-EMF spikes that are generated when the relay turns off. LED1 provides power-on indication, with the 1.5kΩ resistor limiting the current through the LED to about 7mA. x 113 x 63mm. As stated, the remote sensor loop and its associated termination box connect to the main unit via a 2-core shielded cable. Construction The assembly is straightforward with most of the parts mounted on a PCB coded 03107121 and measuring 140 x 84mm. The only parts not on the board are the remote sensor loop, the output relay and the mains input and output connectors. With the exception of the sensor loop, the parts are all housed in a standard UB2 jiffy box measuring 197 Table 2: Capacitor Codes Value 1μF 470nF 150nF 100nF 22nF 10nF µF Value 1.0µF 0.47µF 0.15µF 0.1µF .022µF .01µF IEC Code EIA Code   1μ 105 470n 474 150n 154 100n 104   22n 223   10n 103 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o siliconchip.com.au No.   1   2   1   2   1   1   1   1   4   4   1   4   1   1   2 Value 470kΩ 220kΩ 180kΩ 100kΩ 68kΩ 47kΩ 27kΩ 22kΩ 10kΩ 4.7kΩ 1.5kΩ 1kΩ 560Ω 470Ω 100Ω 4-Band Code (1%) yellow violet yellow brown red red yellow brown brown grey yellow brown brown black yellow brown blue grey orange brown yellow violet orange brown red violet orange brown red red orange brown brown black orange brown yellow violet red brown brown green red brown brown black red brown green blue brown brown yellow violet brown brown brown black brown brown 5-Band Code (1%) yellow violet black orange brown red red black orange brown brown grey black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown red violet black red brown red red black red brown brown black black red brown yellow violet black brown brown brown green black brown brown brown black black brown brown green blue black black brown yellow violet black black brown brown black black black brown August 2012  41 OUTPUT CABLE TO SENTRY CONTROL BOX CABLE GLAND NYLON CABLE TIE IP65 SEALED ABS ENCLOSURE 115 x 65 x 40mm (SHOWN WITH LID REMOVED) WIRE SOLDERED TO PCB COPPER UNDERNEATH SHIELD F9, O2 S9, F8 S8, F7 OUTPUT CABLE SHIELD, PCB COPPER & LOOP SHIELD AT FINISH END JOINED AT RH END TERMINAL NOTE: LOOP CABLE SHIELD NOT CONNECTED AT THIS END CABLE GLAND 'START' END OF SENSOR LOOP S7, F6 S6, F5 S5, F4 S4, F3 S2, F1 S3, F2 S1, O1 SENSOR LOOP TERMINATION BOARD CABLE GLAND NYLON CABLE TIES 'FINISH' END OF SENSOR LOOP Fig.4: here’s how to connect the wires from the sensor loop and the output cable inside the loop termination box. The 12-way terminal block is mounted on a 104 x 38mm piece of blank PCB material. Note that an earth wire must be soldered to the copper on the underside of the PCB and connected to the earth screw terminal at far right. Below: inside the loop termination box. Use cable glands and silicone to seal the cable-entry points, to keep moisture out. 42  Silicon Chip Fig.3 shows the parts layout on the PCB. Begin the assembly by fitting the five wire links (or 0Ω resistors) to the board (note: if you have a double-sided PCB, these links aren’t required). The resistors can then be installed, taking care to install the correct value at each location. Table 1 shows the resistor colour codes but you should also check each resistor using a DMM before installing it. Follow with the non-polarised capacitors, then fit the polarised (electrolytic) capacitors. Make sure you fit the latter with the correct orientation, as shown on Fig.3. In particular, note that there are two different 47µF electrolytics. One is a low-leakage (RBLL) type and this goes in just below IC5. The other is a standard RB type and this is installed just to the right of REG1. Now fit the five IC sockets, taking care to orientate their notched ends as shown on Fig.3. In particular, note that IC1 & IC2 face in the opposite direction to IC3, IC4 & IC5. The diodes and transistors can now be installed. Be sure to orientate these parts correctly and take care not to get the transistors mixed up (Q3 is the BC337). Follow these with REG1, which is mounted horizontally at lower left. Its installed by first bending its leads down through 90° some 6mm from the device body. That done, it must be attached to the PCB using an M3 x 6mm machine screw, star lockwasher and nut before soldering its leads to their respective pads. Next on the list are the three trimpots (VR1-VR3) and the piezo buzzer. Note that the PCB provides multiple mounting holes for the buzzer, to cope with different buzzer pin spacings. Follow these parts with the 3-way SIL header strip for LK1, then install input socket CON1 and the 2.5mm DC power socket CON2. Make sure these parts are seated flush against the PCB before soldering their pins. The two LEDs can now be installed. These must be orientated as shown (ie, with the longer anode lead to the top). They must also be stood off the board by 20mm. This can be done by pushing each LED down onto a 20mmhigh cardboard spacer that’s inserted between its leads before soldering the connections. You can now complete the PCB assembly by plugging the five ICs into siliconchip.com.au their sockets. Be sure to install the correct IC at each location and make sure they are correctly orientated (IC1 & IC2 face in the opposite direction to IC3-IC5). Note also that IC3, IC4 & IC5 are all CMOS types, so take the usual precautions to minimise the risk of electrostatic damage. If possible, earth yourself before picking them up and avoid touching their pins. The Driveway Sentry’s PCB assembly is now ready for testing. IEC MAINS INPUT CONNECTOR (FUSED) INSULATE METAL STRIP WITH SILICONE SEALANT E N A NYLON CABLE TIES A N E Test and set-up For the initial testing, there’s no need to connect the remote sensor loop to the PCB assembly. However, you will need to temporarily connect a 27Ω resistor between pins 1 and 3 of CON1 as a passive “stand in” (ie, between the two outer pins). That done, connect a plugpack or another source of 12V DC to the DC input socket (CON2). If all is well, both LEDs should immediately light – LED1 to indicate that power is present and LED2 because the exit delay timing circuit has begun counting. LED2 should now remain on for about two minutes after power-up. Similarly, it should also light and remain on for about two minutes after you press button S2. Next, set trimpot VR3 to about midrange and check that link LK1 is in the “buzzer” position. Now wait until LED2 goes out, showing that the exit delay circuit has timed out, then press TEST button S1. The piezo buzzer should immediately sound for about 10 seconds. If the buzzer operating time is not to your liking (ie, it’s too short or too long), this can be easily changed by adjusting trimpot VR3. The adjustment range is from about 2s up to about 25s. The only other adjustment to be made to the Driveway Sentry is to vary the sensitivity of the sensor loop. This is done by adjusting trimpots VR1 and (if necessary) VR2 after the system has been installed and the remote sensor loop connected. For the present, set VR1 fully anti­ clockwise and VR2 to midrange. GP OUTLET, PANEL MOUNTING (REAR VIEW) RELAY NYLON SCREWS NYLON SCREWS EARTH LEAD CONNECTS TO CENTRE OF SCREW TERMINAL BLOCK ON PCB RELAY COIL LUGS CONNECT TO END TERMINALS OF SCREW TERMINAL BLOCK ON PCB (1) INSULATE METAL STRIP ON IEC SOCKET WITH NEUTRAL-CURE SILICONE SEALANT (2) SECURE IEC SOCKET & RELAY TO CASE WITH NYLON SCREWS, NUTS & WASHERS (3) COVER MAINS WIRING WITH PRESSPAHN INSULATION Fig.5: install the mains wiring as shown here. Be sure to use mains-rated cable for these connections and make sure that all connections are securely crimped. The wires must also be routed and strapped to the tabs on the mains outlet socket using cable ties as shown in the photo below, so that it’s impossible for a wire to come adrift and contact other wiring. Below: route the mains wires and secure them with cable ties as shown in this photo (note: Fig.5 doesn’t show the exact routing for the sake of clarity). Be sure to insulate the metal strip on the IEC socket with silicone sealant. INSULATE METAL STRIP Making the sensor loop As mentioned earlier, the sensor loop consists of a 25m-length of screened 9-conductor “computer” cable, with the individual conductors connected in series to form multiple turns. The free ends of this multi-turn loop are then siliconchip.com.au August 2012  43 PRESSPAHN INSULATION MATERIAL 9 9 9 9 FOLD DOWN 90 o ALONG DOTTED LINE 95 18 13 7 23 20 35 33 35 103 Fig.6: this diagram shows how cut out and fold the Presspahn insulation material that’s used to cover the mains wiring. Don’t leave it out – it isolates the mains wiring from the parts on the PCB. The Presspahn cover has a cut-out to clear the relay and is fitted in position as shown here. connected to a length of screened twocore extension cable which connects to the main unit. In addition, one end of the loop cable screen (ie, the braid) is connected to the screen of the extension cable, so that the Faraday shield can work correctly. This is all achieved by bringing both ends of the loop cable and one end of 44  Silicon Chip the extension (or output) cable into a small IP65 enclosure, dubbed the “loop termination box”. This enclosure measures 115 x 65 x 40mm and houses a small blank PCB fitted with a 12-way terminal block to facilitate the various connections. Fig.4 shows assembly details for the loop termination box. As you can see, it’s really very simple, with the PCB supporting the 12-way terminal block for the necessary interconnections. The copper under the PCB is connected to the cable screens (at terminal 12), to provide a measure of screening inside the box. All three cable ends are brought into the box via cable glands, with the two loop cable ends entering on one side and the output cable end entering on the opposite side. A Nylon cable tie is fitted tightly around each cable just after it emerges from its gland, as an added precaution against the cable being pulled out accidentally. Having stripped and secured the cables to the box, it’s just a matter of wiring their leads to the screw terminal block as shown in Fig.4. The wires at the “start” end of the loop cable are connected in turn to screw terminals 2-10 on the PCB, while the “finish” ends are connected to terminals 3-11. That way, the conductors end up connected in series, to form a 9-turn loop with its overall start at terminal 2 and its finish at terminal 11. This means that the two inner conductors of the output cable must also be connected to terminals 2 and 11, as shown. You need to take special care with the shielding wires and braids, to ensure correct operation of the Faraday shield. Make sure that the loop cable’s shield wire at the “finish” end only is connected to screw terminal 12. The shield braid of the output cable is connected to the same terminal. In addition, a separate lead (shown green) must be run from this terminal and soldered to the copper on the underside of the PCB. By contrast, the loop cable’s shield wire is cut short at the “start” end and is not connected to anything. It can be covered with a small piece of insulating tape if you wish, so that it cannot short against anything. Nothing is connected to screw terminal 1, which is just a spare connection. Twist each pair of wires together before inserting them into the terminal block. For single wires, you will have to strip back a little more insulation, double the wire over and maybe add a little solder to make it thick enough to be gripped when the terminal screw is tightened. Once it’s all wired up, tighten the outer sleeve nuts of the cable glands to make the entry points watertight (add silicone sealant if necessary). The box siliconchip.com.au Use Double-Crimp Spade Connectors Note that the spade connectors used to terminate the mains wiring must be double-crimp types. This means that the metal collar inside each connector extends almost back to the wire entry hole. That way, both the bared wires and the insulation are crimped by the metal surround, to give better retention. Don’t use single-crimp types which crimp the copper only, as the wire can more easily come loose. lid can then be fitted, along with its neoprene gasket, and fastened in place using the screws supplied. The only step remaining is to fit the other end of the output cable with a 5-pin DIN plug, to mate with input socket CON1 on the main Driveway Sentry PCB. Note that the two inner conductors must be soldered to pins 1 & 3 of the plug, while the screening braid goes to pin 2 (ie, the centre pin). Preparing the case The drilling details for the box and its lid are available in PDF format from the SILICON CHIP website. These should be downloaded and printed out, after which the individual sections can be cut out and used as drilling templates (they can be temporarily attached to the box/lid using sticky tape). Most of the holes can be made by simply drilling and (if necessary) reaming them to size. Be sure to always use a small pilot drill to start the larger holes, to ensure drilling accuracy. The two holes for the mains input and output connectors at the righthand end of the box are inevitably more complex. These are best made by first drilling a series of small holes around the inside perimeter of the area to be removed. The holes can then be joined using a handheld jigsaw, after which the centre pieces can be knocked out and the edges de-burred and filed to a smooth finish using needle files. Mains wiring The next step is to mount the relay inside the case, with its switched output lugs nearest the adjacent end and the coil terminals towards the middle. It should be secured using M3 x 12mm Nylon screws, with metal flat and lock siliconchip.com.au Driveway Sentry: Parts List 1 UB2 Jiffy box, 197 x 113 x 63mm 1 PCB, code 03107121, 140 x 84mm 1 110 x 100mm piece of Presspahn insulation material 1 PCB-mount mini piezo buzzer 2 panel-mount SPST pushbutton switches 1 PCB-mount 5-pin DIN socket (CON1) 1 2.5mm concentric DC input connector (CON2) 1 3-way PCB terminal block (CON3) 1 panel-mount fused IEC male input connector 1 M205 10A fuse 1 GPO mains outlet, flush panel mounting 1 12V SPST 20A chassis-mount mains relay (Ocean Controls RLY-008) 2 8-pin DIL IC sockets 2 14-pin DIL IC sockets 1 16-pin DIL IC socket 5 6.3mm fully-insulated female spade connectors (see text) 2 fully-insulated 4.8mm female spade connectors 1 150mm length of blue insulated mains-rated wire 1 200mm length of brown insulated mains-rated wire 1 400mm length of green/yellow mains-rated wire 2 120mm lengths of insulated hook-up wire 4 M3 x 25mm tapped spacers 9 M3 x 6mm machine screws 4 M3 x 12mm Nylon screws 8 M3 Nylon nuts 1 M3 hex nut 7 M3 star lockwashers 4 M3 flat washers 1 500Ω multi-turn trimpot (VR1) 2 500kΩ horizontal trimpots (VR2,VR3) 12 small Nylon cable ties 1 150mm length tinned copper wire 1 3-way pin header 1 shorting link Semiconductors 1 AD623 instrumentation amplifier (IC1) 1 LM324 quad op amp (IC2) 1 4011B quad CMOS NAND gate (IC3) 1 4060B CMOS counter (IC4) 1 7555 CMOS timer (IC5) 2 PN100 NPN transistors (Q1,Q2) 1 BC337 NPN transistor (Q3) 1 7806 6V positive regulator (REG1) 1 5mm LED, green (LED1) 1 5mm LED, red (LED2) 1 1N5819 Schottky diode (D1) 1 1N4004 1A diode (D2) 1 1N4148 100mA diode (D3) Capacitors 1 1000µF 25V RB electrolytic 1 220µF 16V RB electrolytic 1 47µF 16V RB electrolytic 1 47µF 25V RBLL low-leakage electrolytic 1 10µF 16V RB electrolytic 1 1μF MMC 2 470nF MKT polyester 1 220nF MKT polyester 1 150nF MKT polyester 10 100nF MMC or MKT polyester 2 22nF MKT polyester or greencap 1 10nF MKT polyester or greencap Resistors (0.25W 1%) 1 470kΩ 4 10kΩ 2 220kΩ 4 4.7kΩ 1 180kΩ 1 1.5kΩ 2 100kΩ 4 1kΩ 1 68kΩ 1 560Ω 1 47kΩ 1 470Ω 1 27kΩ 2 100Ω 1 22kΩ Sensor Loop Assembly 1 IP65 sealed ABS enclosure, 115 x 65 x 40mm 1 blank PCB (ie, copper on one side), 104 x 38mm 4 M3 x 6mm machine screws 1 12-way barrier screw terminal block, 96mm long (Altronics P2130A) 2 M3 x 15mm machine screws and nuts 3 cable glands (for 3-6.5mm cable) 3 Nylon cable ties 1 25m length of screened 9-conductor “computer cable” (Jaycar WB-1578) 1 10-30m length (to suit) of screened 2-conductor heavy duty microphone cable (Jaycar WB-1530) 1 5-pin DIN plug, line type 1 50mm-length spaghetti tubing August 2012  45 STREET END GAPS BETWEEN DRIVEWAY SLABS SENSOR LOOP LOOP TERMINATION BOX HOUSE OR GARAGE END OUTPUT CABLE TO MAIN SENTRY UNIT Fig.7: the loop sensor arrangement. The loop can either be buried just under the driveway or installed in the expansion slots of a concrete driveway, with the loop sides buried in a shallow trench. The loop termination box and the output cable should also be buried. washers under Nylon nuts on the top of the relay mounting flanges inside (do NOT use metal screws). A second Nylon nut at each location is used to lock the first into position. That done, use neutral-cure silicone sealant to insulate the exposed metal strip on the IEC input connector. That strip links the Active input pin and the fuseholder and it runs at mains potential (230VAC) when power is applied. So insulating it is a good idea to prevent accidental contact. You can now mount the IEC mains input connector and the mains output socket on the righthand end of the case. Use M3 x 12mm Nylon screws to hold the IEC connector in place, along with flat washers and two Nylon nuts on each screw. Fig.5 and its accompanying photo show how the mains wiring is installed. Be sure to use mains-rated cable for all this wiring. You will need to crimp 6.3mm fully-insulated female spade connectors to the wires that go to the relay contacts and to the IEC connector. In each case, it’s a matter of stripping back about 5mm of insulation from the wire, then pushing it into the connector and crimping it with the tool. Check each crimp connection as it is made, to make sure it is securely terminated – you must not be able to pull the wire out of the connector. Note that you must use a professional ratchet-driven crimping tool for this job (eg, Jaycar TH-1829 or Altronics T-1552). Don’t even think about using a cheap, non-ratchet crimper; they are not up to the job for a project like this as the pressure applied to the connectors will vary all over the place and this will result in unreliable and unsafe connections. Note also that some IEC input connectors have 4.8mm terminals, in which case you must use 4.8mm spade connectors to suit. These should also be fully-insulated types or, if necessary, you can insulate them yourself using heatshrink tubing. Once all the spade connectors have been fitted, plug the leads into the IEC connector, then connect the Neutral lead to the GPO mains socket. The lead from the Active terminal on the IEC connector is terminated in a second spade connector and this connects to one of the relay output terminals. The other relay output terminal connects to the Active terminal on the GPO mains socket. The two earth leads can now be run to the GPO mains socket. One of these leads is run from the Earth terminal on the IEC socket, while the second lead is routed back next to this lead and ultimately connects to the earth track of the PCB. You will need to make this latter lead about 250mm long. Be sure to route the mains wires as shown in the accompanying photo (note: Fig.5 shows the connections but doesn’t show the exact routing for the sake of clarity). Once all the connections have been made, use cable ties to strap the wires to the tabs on the GPO mains socket (see Fig.5). Five more additional cable ties are also used to strap the wires together and should be installed as shown in Fig.5 and the photo. These cable ties are vital to ensure that an individual lead can’t come loose and contact other terminals, even if the box receives a sudden jolt. In particular, be sure to strap the One end of the case carries the IEC input socket and the mains output socket (GPO) while the other end provides access to the loop input socket, the DC connector and the sensitivity trimpot (VR1). 46  Silicon Chip siliconchip.com.au Switching Other Devices If you don’t wish to switch the mains then the IEC socket, the flush-mount GPO socket and the mains wiring can all be omitted.You can then simply use the relay output contacts to switch a low voltage or to trigger some other piece of gear, eg, a burglar alarm. Note, however, that it will still be desirable to earth the Faraday shield of the loop sensor and this can be done by running a lead from the PCB earth terminal to a metal stake driven into the ground. The PCB is mounted on the lid of the case on four M3 x 25mm spacers. Make sure that the extension wires soldered to the switch terminals pass through their respective holes on the PCB and don’t forget to solder them. Earth wires to the GPO mains socket tab and strap the Earth and Neutral wires together at the IEC connector. Presspahn cover As shown in the photos, a Presspahn cover is used to physically isolate the mains circuitry from the low-voltage circuitry and the PCB. This fits vertically over the relay and is folded over the top of the IEC connector and mains socket to form a complete enclosure. Fig.6 shows the dimensions of the Presspahn cover. It can be cut to shape using a sharp pair of scissors, while the hole for the Earth lead that runs to the PCB can be cut out using a sharp hobby knife. The fold line is then lightly scored, after which the top section is folded down through 90°. Check the mains wiring carefully before installing the Presspahn cover. It’s a good idea to use a multimeter (set to Ohms) to check all the connections between the IEC connector and the mains socket (GPO). The Earth lead is critical – use the DMM to confirm continuity between the Earth pin of the IEC socket and the Earth of the flush-mounting GPO socket. siliconchip.com.au Do the same for the two Neutral connections (the two Active terminals should be open circuit since the relay contacts will be open). Check also to ensure there are no shorts between Active & Neutral on both the IEC connector and the GPO socket, or between either of these two terminals and Earth. Once that’s done, feed the Earth lead that runs to the PCB through the hole in the Presspahn cover. The cover can then be slipped into position over the relay (see photos) and secured using some hot-melt glue or neutral-cure silicon sealant. Final assembly Now for the final assembly. The first step is to download the front panel artwork (in PDF format) from the SILICON CHIP website. This should be printed out, laminated and attached to the front panel using double-side tape or silicone. The holes in the panel artwork can then be cut out using a sharp hobby knife. Once the panel is finished, mount the two pushbutton switches (S1 & S2), then attach four M3 x 25mm tapped spacers to the back of the box lid at the PCB mounting points. Secure these spacers using four M3 x 6mm machine screws. That done, cut four 20mm lengths of 0.5mm tinned copper wire and solder these to the switch terminals. These form extension leads which will later pass down through matching holes in the PCB when the latter is mounted on the spacers. Next, cut two 80mm lengths of medium-duty hookup wire and crimp one end of each wire to a 4.8mm fullyinsulated spade connector. Check that these connections are secure, then connect the opposite ends of these two leads to the terminal block on the PCB – see Fig.3. The earth lead should also now be connected to the terminal block. Do the screws on the terminal block up nice and tight, then fit a cable tie to the three wires as shown in the photo. Another cable tie can then be used to bind the relay wires about 40mm from the connectors. The PCB can be mounted on the spacers on the rear of the lid. Basically, it’s just a matter of offering the board up to the lid while making sure that the extension leads from S1 and S2 pass through their corresponding PCB holes. At the same time, you have to make sure that LED1 and LED2 go through their matching holes in the lid. Once everything is correct, secure the PCB to the stand-offs using M3 x 6mm screws and star washers. Do the screws up tightly, then solder the extension leads for switches S1 and S2 to their PCB pads. The assembly can now be completed by connecting the two spade connectors to the relay coil terminals, then carefully lowering the PCB/lid assembly into the box. Note that it August 2012  47 affect the sensitivity. If you cannot get reliable triggering, reverse the loop by turning it over. This means that you must test the complete unit before burying the loop. VR2, the trigger sensitivity adjustment, is basically a back-up and is normally left in the midrange position. It need only be moved from this position if you run out of range with VR1. Sensor loop installation This is the completed loop termination box, together with the sensor loop cable (light grey) and the extension cable (black) that runs back to the main unit. Make sure that the box is properly sealed against moisture. will be necessary to bend the leads from the terminal block straight up from the PCB so that they will clear the Presspahn cover. Make sure that the Presspahn cover is correctly positioned before securing the lid using the four small self-tapping screws supplied. The Driveway Sentry Mk.2 in now complete and ready for installation and sensitivity adjustment. Both the sensitivity control (VR1) and the trigger sensitivity control (VR2) can be adjusted after the box is fully assembled, via small access holes (one in the lid and the other in the lefthand end of the case). The same goes for the alarm duration trimpot (VR3). Sensitivity adjustment To test the unit, the sensor loop must initially be laid on top of the driveway and connected to the main unit. You’re then ready to adjust the sensitivity. It’s simply a matter of setting VR2 to midrange and adjusting trimpot VR1 clockwise to make the Driveway Sentry more sensitive, or anticlockwise to make it less sensitive. This will have to be done on a trial and error basis, with a vehicle driven over the sensor loop after each adjustment. The best setting is where it reliably detects the smallest moving vehicle likely to enter or leave the driveway but don’t make it more sensitive than necessary. If you simply adjust VR1 for maximum sensitivity (ie, fully clockwise), the unit may be prone to giving false alarms due to passing radio transmitters or mobile phones, or during electrical storms. Note that the loop direction will The remote sensor loop can either be buried just under your driveway (eg, under pavers) or it can be installed in the expansion joints of a concrete driveway. As shown in Fig.7, two of its opposite sides lie in the narrow gaps between the concrete driveway slabs, while the other two sides run alongside the enclosed slab on either side. The loop termination box can be located adjacent to one side, with the output cable running away to the main control box inside your house. In practice, the loop termination box can be buried and the output cable run in a shallow trench back to the house, so that it doesn’t get damaged. Make sure that the cable glands have all been properly sealed using silicone before burying the loop termination box, to prevent water damage. If you are on a rural property, the loop sensor can simply be buried under the driveway in a shallow rectangular trench. Using it When the Driveway Sentry is armed and detects movement, it immediately produces an alarm sound from the buzzer and operates the relay. The relay contacts can be used to switch on a security floodlight, other lighting or perhaps a siren. The Alarm Duration can be set by adjusting VR3 using a screwdriver through the front-panel access hole. Finally, note that any fixed mains wiring to lights etc should be installed SC by a licensed electrician. Issues Getting Dog-Eared? Keep your copies of SILICON CHIP safe with these handy binders REAL VALUE AT $14.95 PLUS P & P Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. 48  Silicon Chip siliconchip.com.au