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BODY DETECTOR Got some loot you love which some light-fingered larrikin could lift? Some precious possession you’d prefer wasn’t purloined? Build the Body Detector: if someone comes within cooee it will catch ’em! by Thomas Scarborough ilicon Chip hip 38  Silicon by Thomas Scarborough* www.siliconchip.com.au E very human body is surrounded by an electric field – a stronger field than many people would expect. With some simple test equipment, I was able to measure this field up to a metre away. The phenomenon of capacitance is entirely dependent on the existence of electric fields. If a human body should approach one plate of a capacitor, the body’s electric field can inter-react with that of the capacitor and can cause the capacitance to increase. Again, this may easily be detected. Further, any number of metal objects may be attached to one plate of a capacitor, for example a sheet of aluminium foil or even a set of window security bars. These then become an extension of that plate. In our Body Detector circuit, we call these the sensor. In this design it is attached to the positive plate of a capacitor. While in theory the Body Detector is dependent on the electric field which surrounds the human body, in effect it’s as if an invisible field surrounds the sensor – somewhat like the “invisible” defence shields seen in the latest Star Wars movie. The principle employed here is different to alarms which detect EMF-induced eddy currents in the body. Because the Body Detector is based on the principle of body capacitance it has a high degree of immunity to AC fields, as well as being able to function well out of range of such fields. Interesting effects The electric field which surrounds Our Body Detector is housed in a small plastic case with sensitivity control, mini piezo buzzer and activation LED on the front panel. the human body has a number of interesting and useful effects. Firstly, when the body comes into direct contact with a metal object, its electric field is transferred to the object concerned. This object is instantly surrounded by an electric field, as though it were the human body itself. Therefore, as far as the Body Detector is concerned, such a metal object becomes indistinguishable from a human body, and the Body Detector may be “tricked” into thinking that a human has come near. As an example, if a sheet of aluminium foil is used for the sensor and this is placed underneath a table with a drink can on top, the Body Detector will reliably pick up a hand approaching the can. Even more useful than this, the effect could be used to protect, say, a silver tea service or a jewellery display, without any visible sign of an alarm system being present. Secondly, the average tabletop (with some exceptions) is an insulator – and of course electric fields work through insulators. A capacitor is the prime example of a device that is entirely dependent for its operation on an insulator – in this case the dielectric. The fact that the Body Detector is able to act through an insulator illustrates its effectiveness through insulators in general – it will work through materials such as glass, wood, plaster, cloth, carpet or even cement. This could prove very useful in certain applications – for instance, for detecting fingers approaching a valuable SOME POSSIBLE APPLICATIONS . . . • Intruder alarm, triggered when a doorknob is touched. • “Pressureless” pressure mat, to detect a person passing over it, or past it. As the Body Detector may be cascaded, this could extend across an entire office floor. • An invisible “panic plate”, set inside a concrete wall. Such a plate would be extremely difficult to detect. • A safety switch, to render an area a safety zone, with the possibility of shutting down dangerous machinery or child-proofing certain areas. • An anti-thief alarm, to protect a variety of metal items of value; eg, a computer, or silver tea service. • A bicycle alarm, triggered as soon as a bicycle is touched – anywhere. • An anti-tamper alarm, triggered even before a door lock or padlock can be touched. • An “off limits” alarm, to protect valuables from theft or abuse. • Anti-kidnap alarm; a child fitted with the Body Detector could not be touched without triggering an alarm. • A switch for a low-voltage bedside or night light. A large sensor would trigger the light merely with the wave of a hand in the right direction. (Note: not suitable for switching mains-powered devices). www.siliconchip.com.au October 2001  39 .01F LED1 K  A 1k D3 1N4148 GND THR 150k 100F 25V 100F 25V 2.2k 7 6 DIS 1 CV IC3 7555 4 RES VCC 2 TRIG 8 OUT 5 3 E D C VR3 500k E B C Fig. 1: the Body Detector can be split into two parts – the detection circuit (left) and the alarm circuit (right). 7 BODY DETECTOR SC 1.8pFpF 1.8 100pF VR1 500k 8.2k 6 14 SENSOR INPUT 5 A 2001 3 2 1 IC1b 4093 10k VR2 10k IC1a 4093 100F 25V 4 100F 25V _ + 9-20V DC INPUT 40  Silicon Chip A K LED 8 13 VSS RST INH DETECTOR 14 15 IC2 4017 VDD CLK +5V 16 O8 470F 16V 11 .033F (SEE TEXT) D2 1N4148 0.1F B 10M BC337 +5V OUT GND IN REG1 7805 GND D1 1N4001 Note: due to the way this circuit is triggered, it is possible NOTE: possible that that IC1 IC1 could could be be damaged damaged by by aahigh highstatic staticcharge chargeon onthe thebody body(especially (especiallyon onaavery verydry dryday). day).Minimising Minimising this this risk risk also also minimises minimisessensitivity, sensitivity,sosowe wedecided decidedtotoleave leavethe theinput inputcircuit circuitasasis. is.However, However,you youmay may wish to to make changeover easy in in case ofof damage. wishtotomount mountIC1 IC1inina a14-pin 14-pinDIL DILsocket socket, make changeover easy case damage. _ 13 RLY1 12 IC1d 4093 100k 11 9 8 10k IC1c 40933 409 PIEZO SOUNDER 10 E C B Q1 BC337 RLY1 D4 1N4001 + painting, or for detecting feet passing over a carpet – or even for detecting a hand placed over an invisible “panic plate” hidden in the wall or floor. Finally, and paradoxically, the human body itself may serve as a sensor, with its own electric field being swamped by that of another body. So the Body Detector could, for instance, be strapped to the ankle of an infant, and would serve as an anti-kidnap alarm. I first tested this concept on my 15-year-old son, to very good effect. I was not able to touch him even with the tip of a finger without triggering the Body Detector. He immediately requested such a unit for school, so that whenever anyone would touch him or prod him in class, an alarm would sound! Circuit application The simplicity of the circuit (see Fig. 1) is deceptive. I developed two previous versions of the Body Detector, one of which was published worldwide. This design is fundamentally different to the previous two, and represents a significant improvement over both. This circuit is in the “super-sensitive” category. I was able, with careful tuning, to cause the Body Detector to trigger on the approach of a person well over half a metre away. For practical purposes, however, the Body Detector will reliably pick up a hand (or a foot) approaching a 300mm x 300mm sheet of aluminium foil at a distance of 200mm – or a hand approaching a computer system unit at a few centimetres. One does not need more than this to be able to put the Body Detector to very good use. A special feature of this design is that it may also be cascaded. For instance, it may be used to sense a number of security bars around a home, or more than one area of carpet at once. All that is required is a length of three-way cable to connect separate sensor units, which are then connected in parallel. More on this shortly. Note that there is a limit to the mass of metal objects which may serve as sensors. A bicycle would probably represent the practical upper limit, although I managed to adjust the Body Detector for short periods of time to sensors up to 250kg, with hair-trigger tuning. The biggest such “sensor” was a three-wheel pick-up www.siliconchip.com.au This opened-out photograph shows the complete project. There are some minor differences between this early photo and the component layout overleaf. that I used to drive (which was sadly written off shortly before I completed this article!). The Body Detector has been specially designed with a wide variety of possible applications in mind. For this reason, it incorporates a relay which may switch low voltage devices in its own right, or switch a further (external) mains-rated relay. On the other hand, if REG1 is replaced with a micropower regulator (eg, the LP2950CZ), it could also be used for long-term battery use – for instance, when used as a bicycle alarm. If the specified regulator is used, any DC power supply (regulated or unregulated) between 7V to 20V may be used. In this case, the Body Detector will draw less than 10mA on standby. With a micropower regulator, it would draw less than 3mA on standby, which would enable it to operate continuously for more than a week from a small alkaline 9V battery. When triggered, the circuit draws around 70mA. Circuit description The circuit diagram of Fig.1 is virwww.siliconchip.com.au tually self-explanatory, so no block diagram is shown. Clock generator IC1a clocks decade counter IC2 at approximately 2MHz. Clock generator IC1b resets decade counter IC2 at about 200kHz. This means that IC2 is sequenced very rapidly from 0 to 9, then reset at around the count of 9. If, however, a human body comes close enough to the 1.8pF sensing capacitor (connected to PC stake “A”), the capacitance rises and the frequency of clock generator IC1a drops to around 1MHz. Clock generator IC1b, however, continues at the same frequency, so that IC2 now resets around the count of 4. This means that IC2’s outputs 5 to 9 no longer go high (logic 1) and this can easily be detected and used to trigger a relay. Note that the bigger the sensor that is attached to the Body Detector, the lower the “quiescent” operating frequency of clock generator IC1a. If a 330mm x 330mm sheet of aluminium foil is used as the sensor, the “quiescent” operating frequency will drop to around 1MHz – dropping a further 500kHz when a body comes into direct contact with the foil. This “quiescent” operating frequency will drop even further with larger metal sensors – therefore VR1 and VR2 are provided to adjust IC1b to various frequencies, so that IC2 will continue to reset around the count of 9, whatever the size of the chosen sensor. VR2 serves as a “fine tune”. IC2’s output, pin 11, has a 10% duty cycle (that is, it goes high about one tenth of the time). The 0.033µF capacitor therefore “bridges” these pulses at pin 11, causing IC3 pin 2 to go high continually. But if decade counter IC2 resets before the count of 9 (when, for instance, a hand approaches the sensor), pin 2 of IC3 goes low, and the monostable timer is triggered. The output terminal of IC3 switches the relay via Q1, activates oscillator IC1c-IC1d, and illuminates LED1. The piezo alarm is optional – this would be useful particularly when testing the Body Detector when it is out of the line of sight, for instance when testing security bars from outside of a house when the Body Detector is mounted inside. October 2001  41 Parts List – Body Detector 1 PC board, 70 x 50mm coded 03110011 1 Small plastic case, (RS 284-6482 or equivalent) 1 DPDT relay, mini DIL PCB mount, 5V coil (RLY1) (Altronics S4128) 1 Low-profile piezo sounder (RS 249-889) 1 2.1mm PC-mount DC power socket 5 M2.5 nylon nuts and 10mm bolts 11 PC stakes Insulated hookup wire, various colours. Dual-in-line IC sockets if desired Aluminium foil (optional) 9V-12V battery or power supply (optional) 2.1mm power plug (optional) Semiconductors 1 MC14093BCP Schmitt trigger (IC1) Motorola brand (see text).­­­­­ 1 MC14017BCP decimal counter (IC2) 1 7555 CMOS timer (IC3) 1 LM7805 5V positive regulator (REG1) (or LP2950CZ 5V positive regulator – see text) 1 BC337 NPN transistor (Q1) 2 1N4148 diodes (D1, D4) 2 1N4001 diode (D2, D3) 1 3mm red LED (LED1) Capacitors 1 470µF 16VW PC electrolytic 4 100µF 25VW PC electrolytic 1 0.1µF ceramic 1 .033µF ceramic (see text) 1 .01µF ceramic 1 100pF ceramic 1 1.8pF ceramic Resistors (0.25W 10%) 1 10MΩ 1 150kΩ 1 100kΩ 2 10kΩ 1 8.2kΩ 1 2.2kΩ 1 1kΩ 2 500kΩ top-adjust 25-turn trimpots (VR1, VR3) (Altronics R2392A) 1 10kΩ cermet (miniature) potentiometer (VR2) A short delay is provided at switchon through the 150kΩ resistor and 100µF capacitor connecting to IC3’s reset (pin 4). This arrangement produces a negative pulse for a few seconds, so that the user has sufficient time to step out of range when the Body Detector is powered up. With the component values shown, monostable timer IC3 (and therefore the relay’s “on” time) may be adjusted over a useful 150ms to 30 seconds. If different timing periods are required, the value of the 100µF capacitor may be increased for longer time periods (and vice versa). The output of monostable IC3 provides current for switching transistor Q1, which in turn controls relay RLY1. Regulator REG1 is employed especially to ensure stability for clock gen42  Silicon Chip erators IC1a and IC1b. The specified device consumes around 7mA. Any similar 5V positive regulator may be used, provided that it is rated 150mA upwards. My experience is that it makes quite a difference which brand of 4093 IC is used. The one specified here is manufactured by Motorola. Other makes may not function properly. Circuit stability Stability is a challenge with any circuit of this order of sensitivity. This is because the quantity being measured – in this case body capacitance – is so small that minute variations within the circuit itself may swamp the quantity being measured. This circuit largely overcomes the twin problems of temperature varia- tions and supply voltage fluctuations in such a way that it attains an unusually high degree of stability. Each of my previous designs convincingly solved only one or the other of these two problems – this one overcomes both. Firstly, the frequency of clock generator IC1a is converted to a decimal number through decade counter IC2. Then it is effectively compared with itself over time – typically 50 times per millisecond. This yields far better results than if a standard beat frequency oscillator (BFO) is used. Secondly, the fast clock generator IC1a is built almost identical to the slower clock generator IC1b, so that any temperature variations in IC1a are more or less mirrored in IC1b. As far as possible, the temperature coefficients of all the capacitors and resistors surrounding these two gates should be matched – this is important. I used a relatively expensive potentiometer for VR2, so as to match its temperature coefficient to the other resistors surrounding IC1a and IC1b. Thirdly, the .033µF capacitor is used to mask the effects of voltage transients, by damping any voltage-induced jumps in clock generator IC1a. In fact this capacitor, although it is only one component, is crucial to the functioning of this circuit, since transients would otherwise render the circuit unstable, particularly at higher sensitivities. This may be appreciated by tapping the sensor very rapidly. If it is tapped rapidly enough (thus mimicking a transient), the Body Detector will fail to trigger. The value of this capacitor may be increased in some applications (for instance, when used as a bicycle alarm) to about 0.1µF. This creates a delay of two or three seconds before monostable IC3 triggers, leaving just enough time to switch off the alarm before it triggers. One final threat to the circuit’s stability came from the switching actions of IC3 and the relay. In fact, initially, this seriously interfered with the functioning of the circuit. Therefore D3 is employed in such a way as to take IC3’s trigger input pin 2 high (logic 1) when monostable IC3 triggers. Pin 2 then remains high for a fraction of a second after the timing period has ended. This effectively masks the switching actions of IC3 and the relay. The effect of D3 may be appreciated by holding your hand to the sensor www.siliconchip.com.au Fig.2: here’s how it all goes together on the PC board. Note that there are a few differences between this version and the early prototypes photographed. A hole is drilled in the side of the case to expose the power supply socket while the sensor solder pin is attached to the side of the case by means of a small bolt and solder tag. Note that the tag should not be soldered while on the case – it may melt the plastic. The relay outputs are routed to three solder pins on the PC board (pins C to E), and these may be used to wire up an external load. You could drill an appropriate hole in the side of the enclosure, or to use a suitable socket. Calibration continually. As IC3’s timing period comes to an end and LED1 extinguishes, a fraction of a second’s delay is seen before LED1 illuminates again. These measures to a large extent make the Body Detector free from temperature and supply voltage variations. A prototype of the Body Detector was tested over a 70°C temperature range (-20°C to +50°C) at a useful sensitivity, using a 300mm x 300mm sheet of aluminium foil as the sensor and there was no spurious triggering. Construction The Body Detector is built up on a single PC board measuring about 70mm x 50mm and coded 03110011. Details of the component layout are shown in Fig.2. All the components should fit into place without difficulty. First solder the link wires and solder pins, the power socket, resistors, presets and relay, then the diodes and LED, continuing with the capacitors and transistor. Attach VR2 and the piezo sounder to the relevant solder pins by means of insulated hookup wire cut to suitable lengths. LED1 was soldered to PC pins in such a way as to slot directly through an appropriate hole drilled in the top of the plastic case. Finally, solder the ICs into place, being careful not to overheat any of the pins. Dual-in-line sockets may be used if desired. Observe anti-static precautions, the most important of which is to ground your body immediately before handling these devices (a simple solution would be to touch a metal tap). If the specified case is used, regulator REG1’s pins need to be inserted deeply into the PC board to provide maximum headroom. Finally, bolt a solder tag to the case, connecting this to solder pin www.siliconchip.com.au A by means of a short length of wire. Be careful to observe the correct polarity of the electrolytic capacitors, and the correct orientation of Q1, the diodes and ICs. The cathodes of the diodes are banded, while the anode of LED1 has the longest lead. Finally, check that there are no solder bridges on the board. The Body Detector may be housed in a suitable case, with VR2 being mounted on the front panel for easy fine-tuning. The piezo sounder and LED1 may also be mounted on the front panel. The PC board is fixed to the bottom of the case with four small nuts and bolts. Begin by turning VR1 and VR3 fully anti-clockwise, and VR2 to a centre position. Plug in the power supply, which is a regulated or unregulated DC supply between 9V and 20V if the specified regulator is used (a regulated supply is better – 9V or 12V is ideal). Be sure to observe the correct polarity. If at any time the circuit does not behave as described, switch off immediately, and check the wiring carefully. Now turn up multi-turn preset pot VR1 (this may require several clockwise turns) until LED 1 illuminates and the piezo buzzer sounds. Then back off VR1 until the piezo just stops The input to the Body Detector is this case-mounted solder lug, which can be connected to a range of “sensors” as discussed in the text. The lug should not be soldered “in situ” because you may well melt the plastic case. Fairly obviously, this pic was taken before we glued the front panel label in place. October 2001  43 The full-size PC board pattern can be used to check commercial boards or, if you’re keen, to make your own. Likewise, the front panel (right) can also be used “as is” or a photocopy made. Both the PC board artwork and front panel artwork can also be downloaded from www.siliconchip.com.au sounding. Touch the solder tag which is wired to solder pin A with a moist finger. The sounder should now beep and the LED illuminate. Next, connect the “sensor” tag (which is wired to solder pin A) to a sensor; eg, a sheet of aluminium foil about 300mm x 300mm is ideal. Note again that it is vitally important that there should be a good connection between the sensor and circuit board, otherwise adjustment could be a hit and miss affair. If possible, use soldered connections. The piezo sounder should now be making noise and the LED should illuminate. Now slowly turn multi-turn preset pot VR1 anti-clockwise until the piezo sounder falls silent, and the LED extinguishes. Your body may affect the tuning, so use a plastic or insulated screwdriver and stand back from the circuit from time to time to see whether the piezo sounder falls silent. Too large a sensor (eg, the kitchen stove!) could exceed the range of the circuit, so that the LED does not extinguish – the circuit’s range can be extended by increasing the value of C3. Adjust preset VR1 in such a way that potentiometer VR2 (on the front panel) continually triggers the circuit when turned fully clockwise but bare- ly triggers it when turned back. VR1 is used to roughly match the circuit to a given sensor while VR2 is used for fine-tuning thereafter. The Body Detector should now react when your hand approaches the sensor, from a distance of few centimetres. With careful adjustment, a distance of 20cm+ should easily be achieved. All in all, it is sensible to calibrate the Body Detector so that it is sensitive enough to safely trigger, yet not so sensitive that it comes too close to its trigger threshold, which may lead to instability. Finally, adjust VR3 (turning this clockwise) to set the monostable and relay to the desired time period. In use A wide variety of metal sensors may be tried. Always be sure to make a secure connection between the circuit and the sensor. Try different shapes and sizes of aluminium foil – also a grid made of aluminium foil. You may also experiment with larger objects such as a bicycle or a fridge door, which should serve quite well as sensors. In the case of very heavy metal items, a lighter sensor may be mounted on their surface, without any physical connection to the object itself, to far better effect. B O D Y SENSITIVITY SILICON CHIP www.siliconchip.com.au D E T E C T O R Remember that the unit’s sensor is also capable of picking up body presence through various materials – even through insulators such as glass. Cascading There are two parts to the circuit – the “Power Circuit” and the “Sensor Circuit” (see Fig.1). A few sensor circuits may be constructed (without the power circuit) and cascaded – that is, wired in parallel – with the main unit which contains the power circuit. A three-conductor cable is required, connecting the +5V and 0V rails and the output of the sensor at point B (the junction of the .033µF capacitor, D2 and D3) to the same point on the main circuit board. Each sensor would be individually adjustable for sensitivity. Thus it would be possible to protect larger areas, or a greater number of items, than would be possible with a SC single “sensor” board. *The author may be contacted at scarboro<at>iafrica.com MINI SUPER DRILL KIT IN HANDY CARRY CASE. SUPPLIED WITH DRILLBITS AND GRINDING ACCESSORIES $61.60 GST INC. 44  Silicon Chip www.siliconchip.com.au