Handheld measurement equipment for measuring extremely low frequency electromagnetic fields Intermediate Report S u b m i t t e d b y A s h u t o s h B a h e t i, N e h a B a h e t i, S h i l p a S a r a w a g i, V a i b h a v M e h t a This report shows the further work done by us both on detecting magnetic field and electric field. Magnetic field is detected using Loop Antenna with amplifier and filter for 50 Hz frequency and Electric field is detected using Simple helical antenna followed by two stage amplifier and filter.also both the above circuits consists of peak detector along with rectifier which is then feeded into Linear Dot /bar display driver for driving 10 different LED s
Magnetic Field Detection 1.1 Introduction With technology advancing at the pace it is, Overhead Power lines, FM and TV towers are rising up all over the country. In a city like Mumbai where any amount of available space is exploited to build homes, offices, buildings etc, it is the people who are the ones who suffer. Little do they know that the Electric and Magnetic fields to which they are exposed to be exceeding the standard set norms. The harmful effects of the same are innumerable and vary from various types of cancer, including leukemia, brain and breast tumors. The characteristic biological effects of magnetic field appear to be functional changes in the central nervous system, endocrine and immune systems [1]. The alarming rate at which these diseases and harmful effects are affecting people motivated me to work on detection of Extremely Low Frequency (ELF) field detection, in particular, the magnetic field. The appliances that we use are no doubt a boon to us, but we don t realise that sitting close to a power supply all day, or glaring at the television and computer screen is affecting and more so killing us silently. Below are some values of electric field and magnetic field near everyday use appliances. [2] Visit http://www.bbemg.ulg.ac.be/uk/2basis/gtintro.html for live demo. Table1
1.2 Principle of Magnetic Field Induction When a current carrying conductor is formed into a loop or several loops to form a coil, a magnetic field develops that flows through the center of the loop or coil along its longitudinal axis and circles back around the outside of the loop or coil. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. [3] Number of turns is inversely proportional to frequency based on the derivation below. S = l 1 *l 2 1.2.1 Simple Test To simply detect the strong magnetic field near the SMPS of the oscilloscope (50 KHz) it is enough to make a magnetic loop (which acts as a dipole antenna) from the DSO s probe. The 1MΩ resistor inserted between the ends helped to increase voltage levels due to the induced current in the loop. The Figures1 & 2 show the different voltage levels Figure1
Figure 2 From the above figures 1 & 2, we can gauge the difference in magnetic field intensity levels near the Switched-Mode-Power Supply (SMPS) of the oscilloscope. 1.3 Detection of ELF magnetic field The electromagnetic fields at 50 Hertz are a combination of electric field and magnetic field at this frequency. The magnetic field being weaker in magnitude ( E/H= 120П) compared to the electric field requires a more precise way to only detect it and thus prevent it from being overpowered by the stronger electric field. The magnetic loop antenna used for the purpose causes induced current to flow, which we can convert to a corresponding voltage and then amplify this obtained voltage. The principle on which this is based is the Faradays Law of Induction: where a variable magnetic field induces a voltage at the ends of a coil of an electric wire. 1.4 Circuits and their corresponding results 1.4.1 LM308: Using this opamp, testing was done to detect magnetic field of 50 Khz frequency. As frequency is high, numbers of turns are very less. Exactly 5 turns of copper-enamelled wire was wrapped around a pencil to act as a coil.
Circuit Diagram Figure3 Place Max. ac voltage Min ac voltage Input +200mV -200mV Near SMPS(DSO) 2.4V -2.6V Away 1.6V -400mV Table2 Comments: The 100k resistor from pin3 to ground is necessary for providing a dc path for the induced current. Also it was observed that on touching ones hand to the loop, the voltage increased to +13.2 Volts. This means that the loop is detecting electric field and that our body through our hand is providing a path to ground.
1.4.2 LM358 Circuit Diagram Figure 4 Place Highest Voltage(AC) Lowest Voltage(AC) Normal position 20mV -20mV Near CRO screen 100mV -24mV Top of CRO(near SMPS) 254mV -100mV Near Power supply 104mV 0 Table3 Comments: The results are consistent with the fact that due to a stronger magnetic field, a higher voltage is induced. 1.4.3 AD708 A simple instrumentation amplifier is built using Analog Design AD708. The low offset voltage drift and low noise allows the designer to amplify very small voltages without sacrificing the overall system performance. [4]
Circuit Diagram [5] Figure 5 Figure 6 The number of turns and the area enclosed by the coil are of utmost importance. The dual supply needed for AD708 is achieved by using TC7660- a voltage inverter IC. Note that the voltage induced
across the coil s end is in µv and the output ripple of TC7660 is in mv. Thus the latter circuit has been configured to lower the ripple. The values are tabulated in Table 4 below Place V(ac) mg(readings from available gaussmeter) Switchboard(lab entrance) 0.287 11 1 st switchboard 1.735 21 2 nd switchboard 2.114 21 Near AC 1.892 30 Table 4 Comments: 1. The first switchboard near the lab entrance is well shielded, resulting in lower voltage and mg readings. 2. This circuit can measure variable magnetic field, not the DC fields from a magnetic, but if the magnetic is suddenly moved, the ac multimeter will show an instantaneous reading and go back to zero again. 1.4.4 Two possible configurations 1.4.4.1 Current inducing configuration Circuit Diagram
Figure 7 The Figure 7 shown above depicts a current induced configuration of the sensing coil. The first stage opamp is a voltage follower circuit, followed by a non-inverting configuration. AC voltage variation from 0.448-1.111V was observed. 1.4.4.2 Voltage induced configuration Circuit Diagram Figure 8 Place R2 V(ac) V(dc) Normal Position 330kΩ 0 2.1 mv Near Supply 330kΩ 0.154V 105.6mV Normal Position 1MΩ 0V 0V Near Supply 1MΩ 0.3V 0.5V Table 5 Comments: The readings perfectly comply with the expected results. The output of the 2 nd opamp connected to the multimeter shows that considerable magnetic field is induced at the ends of the sensing coil. We note from Table 5 that when R2 is replaced by 1MΩ, the gain is increased, thus justifying the higher values of the reading compared to the case when R2=330kΩ. References
[1] Ahlbom, A., 2001. Neurodegenerative diseases, suicide and depressive symptoms in elation to emf.bioelectromagnetics supplement. Stockholm, Sweden, 5: 132-143. [2] http://www.who.int/peh-emf/about/whatisemf/en/index3.html [3] http://www.ndt-ed.org/ EducationResources/CommunityCollege/ MagParticle/Physics/CoilField.htm [4] Datasheet of AD708 Electric Field Detector INTRODUCTION: Nowadays due to increase in the electricity consumption the demands of electricity is rising at a larger rate. Due to this increase in demands the fields nearby the switch boards, Power supplies and Overhead power transmission lines is increasing at alarming rate. So there is a need of some device which can be used to do measurements of these electric and magnetic fields releasing from the respective supplies. Electric field is something which is generated due to the voltage induces in the power lines of electricity and magnetic fields get generated due to current induction in the supply. Today there is
an extra need of being careful as these radiations in one or the other way is harming us to a great extent and one of the major impacts of these radiations is Cancer. This device will help us to know at what places we have to be for what duration of time as excessive exposure to ELF fields may cause harmful effects to body so we have to be very careful for that. This device is used to know where we should keep our resources or where we should not. AIM: The aim of this device is to measure the electric field near by 50 Hz transmission lines, near our switchboards and near our surroundings. PRINCIPLE: The principle behind this gadget is to take the very small voltages which are induced in helical antenna structure and this voltage is then amplified using LM324 IC where 50 Hz frequencies are filtered and amplified through two stages of amplifier. This antenna basically receives only electric field in the near field zone as it induces voltage more than current while loop antenna when both ends used with a resistance will induce more current in it. Though the strength received is not high so we have to take care of high amplification using LM324. Then the output of Op-Amp is passed on to LM3914 which is a dot/ bar display IC which shows the strength of the signal using the set of 10 LED s. So in this way we can come to know the strength of the field nearby us in our houses, offices and many other places. The higher the strength of field nearby the more the LED s will glow and more will be the sound of a buzzer. COMPONENTS REQUIRED: LM324 LM3914 Resistances, capacitances, diode 10 LEDs ANTENNA (winding wire 22 SWG) Battery 3.7V 1800mAh Buzzer (Initialize at 0.6 Volt) SCHEMATICS & EXPLAINATION: The figure-2 shows complete schematic which can be divided into several parts which are embedded in the circuit :
1).An antenna which is helical in nature having these specifications : Antenna Specifications : Total turns: 136 Diameter: 7mm Winding wire: 22 Gage 2).This antenna circuit is then followed by two amplifier stages which is on IC LM324 quad Amp IC. The first order amplifier stage is of gain 10 and the following second order operational Amplifier has a gain of 16. These both Op- Amps have a filter also where I have kept 22nf capacitance which contributes to a frequency of 75Hz using the formula. I have used something similar to figure-1. The first Order amplifier has R3=100k, R1=10k and C1= 22nf The second order amplifier has R3=100k, R1=6.2k and C1= 22nf Op- Figure-1
These two amplifier stages are connected by a capacitor of 1uf.The figure-2 shows the first and second Order Operational amplifier Design circuit. The second Order amplifier stage has rectifier circuit in it as it has one Diode as well connected in the feedback Loop. This is then connected with the peak detector circuit to give the constant DC output by a capactior of 10uf and Resistor of 4.3k. Figure-2 3). This output is then passed into two circuits a). To drive a Buzzer b). To drive dot/bar display LM3914 IC a).to drive a buzzer using this output I have again used a amplifier stage which circuit I have shown in figure-3. This amplifier is Non inverting Amplifier using
a gain of 1+R2/R1.When the Output of operational Amplifier is high then the Buzzer sound is very good. More close to 50 Hz field more will be the sound of a Buzzer. Figure-3 b). The output of LM324 is also passed into an Indicator circuit which is a linear voltage display IC. This IC is LM3914 which is a dot/bar display driver which glows LED s according to increase in the voltage as my antenna move close to 50Hz field. This IC has some calculations in order to set reference for voltage for respective LED glowing and current for LED which is shown in figure-4. The values of R1= 3.5k and for R2= 1k. So the value for ILED= 12.5/ R1=12.5/3.5= 3.57mA and the value for Ref out V = 1.25 *(1+ R2/R1) = 1.25*(1+1/3.5) = 1.607 V
Figure-4 These all circuits I have simulated on software MultiSim and Proteus but I do not have any antenna option in them so I prepared real time hardware. 4). The most important part is choosing of system Switch which makes it a device. We have used a four button switch which has long side internally short and opposite sides are used to pass 3.7V from battery to the system. If we keep it pressed then the system is on otherwise the complete system is off and we have connected an LED with the output of this switch. When this switch is pressed LED indicates the system is on or off. If LED glows on pressing Switch then system has 3.7V and intensity of LED shows how much battery is charged. Figure-5 shows the circuit diagram of button.
Figure-5 5).Switch for Buzzer : we have used a 6 pin On-off switch name as Mini Push to On/Off Switch for switching buzzer on or off.