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Ideal for finding steel frames and studs, steel bracing and nails in plaster walls, this Metal Locator can also show the length of the tang in knife handles, screwdrivers and other tools. As well, it can discriminate between ferrous and non-ferrous metals. By JOHN CLARKE Metal Locator I F YOU WANT TO hang pictures, shelving or mirrors on a plaster wall in a steel-framed building it is useful to find where the metal studs are. You may wish to secure your screws to the stud or alternatively, you may wish to avoid the stud and attach directly to the plaster wall using suitable fasteners. There are also other hidden items within the wall that should be located before drilling, such as metal bracing straps, screws and nails. With the Metal Locator you can 38  Silicon Chip find the metal stud positions as well as any steel bracing, screw heads and nails. That is, provided the metal is no deeper than 25mm inside the wall. For small nails in wooden studs, the Metal Locator can detect them provided that the heads are within 10mm of the wall Specifications Detection range: up to 25mm from the underside of the case Current drain: <20mA with LED fully lit surface. Naturally, this device does not locate the timber studs themselves, nor can it find steel frames in walls that have a cladding thicker than 25mm (a very unusual wall, especially inside!). Nor can it detect power cables inside walls. The Metal Locator can also distinguish between ferrous and non-ferrous metals. Ferrous metals include mild steel, stainless steel (both magnetic and non-magnetic), wrought iron, high tensile steel, galvanised iron, tin plated siliconchip.com.au as voltage changes that can easily be amplified before driving the LED. The LED brightness varies with a change in frequency from the oscillator. An offset adjustment using VR1 allows the LED brightness to be set at a very low level to brighten with non-ferrous metals. The LED can be set at a higher level to detect ferrous metals where the LED begins to dim. The buffer stage (IC2b) between the offset control and the amplifier is there to ensure there is no gain change with adjustment of VR1. BUFFER (IC2b) OFFSET ADJUST VR1 AMPLIFIER (IC2a) FREQUENCY TO VOLTAGE CONVERTER OSCILLATOR (IC1) LED & DRIVER (LED1, Q1) (C1,C2,D1,D2,VR2) DETECTOR COIL (L1) Fig.1: the block diagram has the same functionality as the circuit below. steel (steel cans or tinplate), passivated steels and cast iron. Non-ferrous metals include copper, brass, zinc, aluminium, gold, silver, lead and tin. In the presence of ferrous metals, the LED on the Metal Locator dims. Conversely, the LED brightens in the presence of non-ferrous metal. For ferrous metals, the sensitivity knob is adjusted so that the LED is reasonably bright in the absence of the metal. The LED then dims in the presence of ferrous metal. To detect non-ferrous metals, the sensitivity is adjusted so that the LED is dim in the absence of the metal. The LED will then brighten in the presence of the non-ferrous metal. Greater sensitivity can be had with the LED just glowing in the absence of metal for detection of either metal type. The Metal Locator is housed in a compact plastic case that includes a 9V battery compartment. On the lid are the on/off switch, sensitivity control and the indicating LED. Circuit details The circuit in Fig.2 is based on just two ICs. One is a CMOS version of the 555 timer (IC1) and the other is a general-purpose LM358 dual op amp (IC2a and IC2b). IC1 operates as an unconventional astable oscillator. To explain how it works, we will compare it to a conventional 555 astable oscillator, as shown in Fig.3. This has resistor R1 between its output at pin 3 and both the trigger and threshold inputs at pins 2 and 6. Capacitor Cx is connected between pins 2 & 6 and ground. Initially, when power is first applied, the capacitor is discharged and the trigger input at pin 2 is at 0V. At this stage How it works Fig.1 shows the block diagram of the Metal Locator. It is based on an astable oscillator controlled by the detector coil, L1. The oscillation frequency changes with the presence of metal. For ferrous metals, the frequency decreases while for non-ferrous metals the frequency increases. The oscillator’s output is fed to a frequency to voltage converter. Small frequency changes are then detected REG1 78L05 +5V OUT 180k 10 F VR1 ADJUST 1k OFFSET LIN 10 F 10 K 7 IC2b D3 1N4004 A  LED1 180k 100nF 100 F 16V 8 5 6 IN GND S1 POWER ON IC2: LM358 BUFFER A K 1k 8 4 IC1 7555 C1 3 6 C1,C2 CHARGE D2 10nF L1 A C1 DISCHARGE 2 1 R1 470 OSCILLATOR 2 C2 DISCHARGE 3 K K C2 100nF D1 VR2 10k A C Q1 BC337 9V BATTERY E TP1 470 TPG FREQUENCY-TO-VOLTAGE CONVERTER METAL LOCATOR B 470k AMPLIFIER IC1, IC2 LED DRIVER 4 8 1 D1,D2: 1N4148 A SC 1 4 INDUCTOR L1: 400 TURNS OF 0.25mm ENAMELLED COPPER WIRE ON 20.5mm OD BOBBIN 2009 IC2a A K BC337 LED K D3: 1N4004 K A 78L05 GND B E C IN OUT Fig.2: the Metal Locator circuit is based on two low-cost ICs and a handful of other cheap components. siliconchip.com.au July 2009  39 +5V 8 4 OUT 3 7555 TRIG 2 THRESH R1 6 1 SQUARE WAVE OUTPUT Cx Fig.3: here’s a “traditional” 555 oscillator circuit with the frequency determined by R1 and CX. But as you can see in Fig.2, it’s possible to substitute an inductor and resistor to make it oscillate. the timer is triggered and the output at pin 3 goes high to equal the positive supply rail voltage. The capacitor now charges via R1. When the capacitor charges to the pin 6 threshold voltage (2/3 supply), the pin 3 output goes low (to 0V) and the capacitor now discharges via R1. When the capacitor voltage discharges to the trigger level voltage at pin 2 at 1/3 the supply, the pin 3 output goes high again to recharge the Cx capacitor. The process continues and so pin 3 produces a square wave output with the frequency determined by R1 and Cx. In the circuit of Fig.2, we substitute inductor L1 for R1 and R1 (470Ω) for capacitor Cx. It now operates as follows. At the instant of power being applied, inductor L1 is effectively a And here’s the proof! The top trace is the waveform at pin 3 while the green trace shows the waveform at pin 6. The waveform at pin 6 is the voltage across R1 and this shows that the current through R1 does not reverse; it merely varies between about 3.5mA and 7mA. Note the spikes generated each time the 555 changes state. high impedance and resistor R1 pulls the pin 2 input below the 1/3 supply threshold to trigger the pin 3 output to go high. Current then begins to flow through L1 and R1. As the current rises, the voltage across R1 increases until it reaches the 2/3 supply voltage threshold. This changes the state of the oscillator so that pin 3 goes low. The current through L1 does not change direction but ramps down until the voltage across R1 drops below the 1/3 supply threshold to retrigger the timer and pin 3 goes high again. The frequency is dependent upon the inductance of L1 and the resistance of R1 (which is fixed at 470Ω). L1 is an air-cored coil of wire. If metal comes close to this coil its inductance will Why Not Use A Stud Finder? Most readers know that stud finders are cheaply available from hardware outlets such as Bunnings and even from bargain stores. They often have three functions: stud, nail and power. While they are cheap and readily available, they can give misleading results when looking for screws or metal studs in walls. Nor can they discriminate between ferrous and non-ferrous metals and their sensitivity cannot be adjusted. 40  Silicon Chip change and this will alter the frequency of oscillation. For ferrous metals the inductance will increase and the frequency of oscillation will fall. For non-ferrous metal, the inductance will decrease and the oscillation frequency will increase. The frequency is around 94kHz and changes by up to 2kHz with metal near the coil. The output from IC1 is fed to a diode pump comprising capacitors C1 & C2, resistor VR2 and diodes D1 & D2. It functions as a frequency-to-voltage converter by dint of the size of C1 which is fairly small at only 10nF. This means that the DC voltage developed across C2 will vary as the frequency varies; it will be higher as the frequency increases and this allows the circuit to discriminate between ferrous and non-ferrous metals as the apparent inductance of L1 is changed. The DC voltage across C2 is amplified by op amp IC2a. This has a gain of about 470 (471 to be precise), set by the 1kΩ and 470kΩ feedback resistors. IC2a is buffered by transistor Q1 to provide a higher current drive for LED1. Offset control Op amp IC2a has an offset adjustment to enable adjustment of the LED brightness. In effect, the operating point of IC2a can be shifted up or down by varying the voltage applied to its inverting input. The varying voltage comes from IC2b, a unity-gain buffer which is fed by the wiper of the 1kΩ potentiometer VR1. Combined with the 180kΩ divider resistors, the range amounts to about 14mV. The buffer stage of IC2b ensures the gain of IC2a is kept at 471 and is not siliconchip.com.au D2 D1 C2 C1 10nF 100nF TPG S1 4148 LATE M R OTA C OL 9V BATTERY CABLE TIE 100F 4148 REG1 F 470 VR2 10k CON1 10  IC1 7555 PICAXE 4004 – TP1 100nF D3 ++ R1 with 180k LED1 A JOIN THE TECHNOLOGY AGE NOW - L1 180k VR1 1k LIN Q1 K 10F 10 470k 1k 470 04207091 IC2 LM358 10 F 100 F 10 F Fig.4 (top) shows the component layout for the Metal Locator, with the same-size photo prior to mounting in the case at left. Note the electrolytic capacitors need to be mounted folded over so they are flat on the PC board. affected by the resistance at the wiper of VR1. Any voltage change in VR1 is amplified in IC2a by 471, so the 14mV variation allows the IC2a output to be shifted over its full output range, from very close to 0V up to about 3.5V. This adjustment allows the LED to be set at the required brightness for metal detection. In effect, VR1 operates as a sensitivity control for the circuit. Trimpot VR2 provides a further range of adjustment. For optimum operation of VR1, VR2 is adjusted so the Developed as a teaching tool, the PICAXE is a low-cost “brain” for almost any project voltage at TP1 is at about half supply or +2.5V. This matches the nominal 2.5V available from the wiper of VR1 at its centre position. The circuit is powered from 5V, derived from a 9V battery and a 5V regulator (REG1). Diode D3 prevents damage to the 100µF capacitor and the 5V regulator if the battery is connected the wrong way around. The 5V supply is decoupled with a 10µF capacitor at REG1’s output and another 10µF capacitor at the supply rails for IC2. IC1 Easy to use and understand, professionals & hobbyists can be productive within minutes. Free software development system and low-cost in-circuit programming. Variety of hardware, project boards and kits to suit your application. Digital, analog, RS232, 1-Wire™, SPI, and I2C. PC connectivity. Applications include: Datalogging Robotics Measurement & instruments Motor & lighting control Farming & agriculture Internet server Wireless links Colour sensing Fun games How Safe Is That Kitchen Knife? The handles on some kitchen knives are unsafe because they have a very short tang. The Metal Locator can show just how long the metal tang goes into the handle of a kitchen knife or screwdriver. Many professional knives have the tang extending the whole length of the handles and this tang can be seen running in between the two handle sections that are riveted to the outside of the tang. But some low cost knives only have a tang that enters part way into a plastic moulded handle. They can even have imitation rivets along the handle length to give the impression that the tang runs along the whole handle length. A short tang means that a large amount of stress is applied to the handle when using the knife and it is liable to break. This can be dangerous, especially when doing heavy work such as cutting up pumpkins. Make sure you use a knife that is safe for the job. siliconchip.com.au Distributed in Australia by Microzed Computers Pty Limited Phone 1300 735 420 Fax 1300 735 421 www.microzed.com.au July 2009  41 Parts List – Metal Locator 1 PC board, code 04207091, 78 x 64mm 1 remote control case 135 x 70 x 24mm (Jaycar HB 5610 or equivalent) 1 front panel label 50 x 115mm 1 9V battery 1 9V battery snap 1 DPDT PC mount slider switch (Jaycar SS-0823 or equivalent; S1) 1 coil bobbin 20.5mm OD x 13mm ID x 10.5mm high 1 13m length of 0.25mm enamelled copper wire 1 knob to suit potentiometer 1 2-way screw terminals with 5.08mm pin spacing 4 T0-220 insulating bushes (used as spacers) 4 M3 x 4mm screws 1 20mm diameter x 12mm heatshrink tubing 1 100mm cable tie 9 PC stakes Semiconductors 1 7555, LMC555CN CMOS timer (IC1) 1 LM358 dual op amp (IC2) 1 78L05 three terminal 5V low-power regulator (REG1) 1 BC337 NPN transistor (Q1) 1 1N4004 1A diode (D1) 2 1N4148 signal diodes (D2,D3) 1 3mm high brightness red LED (LED1) Capacitors 1 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 2 100nF MKT polyester 1 10nF MKT polyester Resistors (1% 0.25W) 1 470kΩ 2 180kΩ 1 1kΩ 2 470Ω 1 10Ω 1 1kΩ linear 16mm potentiometer (VR1) 1 10kΩ 25-turn top-adjust trimpot (3296W type) (Code 103) (VR2) Here’s the completed PC board screwed into the plastic “remote control” case. The coil is not attached to the PC board – it is glued in place to the case in the cutout provided in the PC board. has a 100nF supply bypass capacitor. Construction Construction involves mounting all parts, except coil L1, on a single PC board. This is coded 04207091 and measures 78 x 64mm and is housed in a remote control case measuring 135 x 70 x 24mm. Fig.4 shows the overlay diagram. Begin by checking the PC board for shorted tracks or breaks in the copper. Check the hole sizes as well. The corner mounting holes should be 3.5mm (9/64”) in diameter, as can the two holes to anchor the battery snap leads with the cable tie. Power switch S1 also RESISTOR COLOUR CODES    No. Value 1 470kΩ 2 180kΩ 1 1kΩ 2 470Ω 1 10Ω 1 1 1 1 1 4-Band Code (1%) yellow violet yellow brown brown grey yellow brown brown black red brown yellow violet brown brown brown black black brown 42  Silicon Chip 5-Band Code (1%) yellow violet black orange brown brown grey black orange brown brown black black brown brown yellow violet black black brown brown black black gold brown mounts on the board – before assembly check that its holes are large enough and if not, enlarge slightly. Now you can begin the assembly. Install the seven resistors first. We show their colour codes in a table but it is a good idea to also check the values using a digital multimeter before installing each onto the PC board. Make sure you don’t mix up the side-by-side 470Ω and 470kΩ resistors. Doing so may not let any smoke out but it certainly won’t work when completed, either! Next, install the five PC stakes for VR1, the two stakes to terminate inductor L1 and the two stakes for test points TP1 and TP GND. Install diodes D1 to D3 and take care to orient these correctly. IC1 and IC2 can be installed, making sure that the 7555 timer is placed in the IC1 position and LM358 in IC2. Each IC must be oriented with the notch as shown on the overlay diagram. You might find some ICs don’t have a notch but will have a small dimple marking pin 1. Q1 and REG1 can then be installed but make sure each is placed correctly, as they look very similar to each other. LED1 can be installed, again taking care to get the orientation correct. The top of the LED should be 15mm above the PC board. Next, the capacitors can be installed. The three electrolytic types need to be oriented with the polarity shown but they also need to lie down to provide clearance in the box. A capacitor code table is provided to help identify the 100nF and 10nF capacitors. Trimpot VR2 can be installed either way around. Switch S1 is mounted as high as possible on the PC board but with about 1mm of pin length under the PC board to allow soldering. CON1 can now be installed. Cut the shaft of the 1kΩ potentiometer (VR1) to a length of 12mm. VR1 sits vertically with its back on the PC board surface and is secured in place by soldering the potentiometer case to the associated PC stakes. So that solder will adhere to the surface the passivated coating on the pot case must be removed by scraping with a knife or file where the PC stakes are positioned. The potentiometer terminals are soldered to the remaining three PC stakes. The 9V battery leads pass through one of the battery compartment holes in the plastic case before inserting them into the screw terminals. A cable tie secures the wires in position. The PC board is raised by about 1mm siliconchip.com.au Target Ferrous metals: LED dims Non-ferrous metals: LED brightens Helping to put you in Control Control Equipment A close-up of the coil (L1). It’s about 400 turns of wire on a plastic bobbin. Adjust for partial LED brightness away from any metal ON L A T E M R O T A LOC SILICON CHIP siliconchip.com.au This full-size front panel artwork fits into the recess on the top of the remote control case. by placing a TO-220 bush into each mounting hole from the underside of the PC board. This raises the PC board sufficiently so the switch slider is above the top of the case lid. Secure the PC board to the case with four M3 screws that screw into the integral support bushes of the case. Winding the inductor Inductor L1 is wound with 400 turns of 0.25mm enamelled copper wire on a plastic bobbin. The windings are jumble wound. This means windings do not have to be placed neatly sideby-side, layer-by-layer. The winding is held in place with a 12mm length of 20mm heatshrink tubing over the outside of the bobbin. There is no need to shrink the tubing down. The bobbin is secured to the base of CAPACITOR CODES Value µF Value IEC Code EIA Code 100nF 0.1µF 100n 104 10nF .001µF 10n 103 siliconchip.com.au the case in the cut-out area reserved for it at the front of the PC board. We used silicone sealant to glue the bobbin in place. Scrape off the enamel coating on each wire end with some fine grade abrasive paper and then solder them to the two PC stake terminals – it doesn’t matter which way around. Test & set-up Apply power and check that there is 5V between TP GND and pin 4 & 8 of IC1 and 5V between TP GND and pin 8 of IC2. Depending on the regulator, the voltage could be anywhere between 4.85 and 5.15V. Connect your multimeter between TP GND and TP1 and adjust trimpot VR2 for a reading of about 2.5V. Now set VR1 to its centre position and adjust VR2 until the LED just lights. Using it When the Metal Locator is first switched on and the LED is adjusted so that it glows dimly, there is a start up drift over about 10 seconds. During this period the adjustment will have to be altered to track the change in LED brightness. It is best to wait for the warm up period before using the Metal Locator. As mentioned the sensitive area is directly under the target printed on the top side of the case (which of course lines up with the middle of coil L1). So for detecting metal in a plaster wall, the case is slid over the wall to detect a change in the LED brightness. The adjust knob will need to be set to show some LED brightness in the absence of metal objects. The sensitivity to metal is dependent on this adjustment. If the LED brightness is set too high then there will not be a noticeable change in brightness with the unit in proximity to a metallic object. The LED will dim for ferrous and SC brighten for non-ferrous metals. 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Comes with DaqFactory software and drivers for Windows and Labview. $475+GST RS485 Converter This simple RS232 to RS485 non isolated converter features an auto baud rate up to 115200bps. $59.95+GST Contact Ocean Controls Ph: 03 9782 5882 www.oceancontrols.com.au July 2009  43