Sensors Technology. Resistive based sensors and interfacing. Aalborg University Copenhagen Medialogy MED Smilen Dimitrov

Transcription

1 Aalborg University Copenhagen Medialogy MED Sensors Technology Smilen Dimitrov 1

2 Contents 1 Introduction Basic sensing principles Sensing light Sensing temperature Sensing pressure / force Resistive based sensors Interfacing: Voltage divider Interfacing: Wheatstone bridge How do we view the data acquisition system Basic sensor interfacing resistive based sensors Switch (push button [SPST] & toggle [SPDT]) Resistive switch ladder Potentiometer (slider/fader & rotary knob) Photosensitive (light dependent) resistor [LDR] Force sensitive resistor [FSR] Sensor interfacing resources PE Questions

3 1 Introduction We have previously introduced the model that conceptualizes the focus we have in ST: In these parts of the lectures, we focus on the hardware electronics part of our sensorbased interaction input system: The understanding of this part of the process requires in essence two things: understanding of the conversion from a given physical parameter into an electric parameter which is the sensing process: process of electric measurement that the sensor performs; and understanding concepts in electrical circuits which are used to perform signal conditioning. Both of these require understanding of electrical properties of matter. 3

4 Previously, we looked into the microscopic aspects of flow of electric current, and then made the transition to basic circuit theory, and analysis of elementary circuits. One of those elementary circuits - the voltage divider - is of great importance to us, as it will allow us to easily interface a variety of resistive sensors to our chosen data acquisition hardware - the Teleo Intro board. Thus, we will revisit the voltage divider, in the context of use with resistive sensors. However, not all resistive sensors can be interfaced through a voltage divider - so the Wheatstone Bridge circuit will be discussed as another possibility for interfacing resistive based sensors. Finally, we include a brief discussion of several resistive sensors, and methods for their interfacing with a data acquisition hardware. However, first a review is made, of the basic sensing principles from a microscopic perspective - in order to establish a simple model of influence of different physical parameters on resistivity. 4

5 2 Basic sensing principles Before we start looking at actual sensors and circuits, lets just summarize some sensing principles, which we may have mentioned in our previous discussions. Those principles are in essence deeply bound to the structure of matter and its electronic properties, and we might call them electronic sensing principles. Take note that this is not an extensive list of all possible electronic sensing principles we just note down the most basic ones. As we will see later, not all sensors that we will use are based on electronic principle some of them are mechanical systems, or a mix of mechanical and electronic system (consider also that the mouse is for example considered an optomechanical system), to provide a conversion of some physical value to an electric value. However, we might find modified usage of these principles in a variety of sensor devices, and therefore it may be a good idea to repeat them. 2.1 Sensing light The possibility for sensing light comes from the interaction between light, as an EM wave, and electrons, as matter, which we have previously discussed as the photoelectric effect. We are aware that light, just as any other EM wave can be seen to give energy to an electron which can also be visualized as a change of the shape of its wavefunction. Provided enough energy is given, the electron can gain enough energy to leave the atom, and become a free electron. Figure 1. Illustration of the photoelectric effect (Ref. [13]) Historically, this was used in vacuum bulb devices, where light ejected electrons from metal into vacuum and in that way an electric circuit was closed. In that sense, we remember that conductivity as a parameter is related to the number of movable free electrons and so, light can be seen to be functionally related to conductivity of a system, even in the case of the bulb. Another definition of this effect would be: "The photoelectric effect is a quantum electronic phenomenon in which photoelectrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays. Study of the photoelectric effect led to important steps in understanding the quantum nature of light, due to several attempts to explain it using both wave and particle theories, and influenced the formation of the concept of wave particle duality. [16]" 5

6 Note that the original context of the photoelectric effect, implies that electrons would be ejected from the surface of the material due to interaction with light. Figure 2. Illutration of the photoelectric effect (left, Ref. [16]) (right, applet, Ref [17]) However, in context of sensors, we are merely interested in light being able to provide enough energy to valence electrons in a material so they become free, and thereby influence conductivity of the mateerial. Nowadays, the photoelectric effect is implemented through semiconductor devices, where light is supposed to give bound, valence electrons enough energy to cross over to the conductive band, and therefore increase the number of free electrons in a semiconductor. In that way, light is functionally related to conductivity. Remember from our previous discussions, that resistivity as a parameter of a given metal conductor, is directly related to the average number of free electrons present in a metal, as well as their mobility (which is then influenced by factors like the geometry of the crystal, temperature, etc). In that sense, if light can increase the number of free electrons present in a metal - then it directly influences resistivity. This would be a simple description of the phenomenon of photoresistivity. As conductivity is a parameter inverse-proportional to resistivity, the above mechanism applies all the same, so we could alternatively call this a photoconductive effect: "Photoconductivity is an optical and electrical phenomenon in which a material becomes more conductive due to the absorption of electro-magnetic radiation such as visible light, ultraviolet light, infrared light,or gamma radiation. To be photoconductive a semiconductor must be in thermal equilibrium, which contains free electrons and holes. When light is absorbed by the semiconductor, the configuration of electrons and holes changes and raises the electrical conductivity of the semiconductor. To cause excitation the light that strikes the semiconductor must have enough energy to raise electrons across the forbidden bandgap or by exciting the impurities within the bandgap. [15]" Note that the photoelectric effect is dependent on the frequency of incoming light: "No electrons are emitted for radiation with a frequency below that of the threshold, as the 6

7 electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface (the material's work function). [16]". Thus, we can expect that different photosensitive devices will also be sensitive to the frequency spectrum of the incoming light. When we sense light, we can obtain resistance [in photoresistive materials], as the electric parameter functionally dependent on temperature. 2.2 Sensing temperature There are two aspects to sensing temperature. In metals, temperature is directly related to the movement of the ions within the crystal lattice (these vibrations of the ions are known as phonons). Figure 3. Eccentric rotation of an individual Cu-phthalocyanine molecule on a C60 surface left a molecule at 50 K temperature; right, the same molecule at higher temperature of 300K, imaged with STM (Ref. [13]) As greater temperature increases this movement, so does the probability for a free electron colliding with an ion and that is a direct influence on the resistivity of the material. In fact, all resistors change their resistance with temperature, so they can be seen in one or another way as temperature sensors although some are specially made with that purpose and are known as a thermistors: "A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. Thermistors can be classified into two types depending on the sign of k (first-order temperature coefficient of resistance). If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. [18]" 7

8 The other aspect is related to the fact that increased temperature, also means that more valence electrons will have energy to become free within a metal - so increased temperature would also lead to an increase of the number of free electrons as well. This can be applied in a thermocouple a metallic contact of two different metals, where one metal is heated. We have already mentioned that we assume a neutral metal conductor to be at potential 0, expressing the fact that its internal number of free electrons and ions is equal and it is neutral however this is just an approximation to help us with thinking in circuits. In reality, every metal has a different normal number of free electrons, which means that two metal s internal potential may and does differ. When two such come in contact, there will be obviously potential difference at the contact surface, known as the Volta effect, which may eventually disappear. When one of these metals is additionally heated, the potential difference is increased, as bound electrons are given energy and more free electrons come in the heated area, changing the potential. This ultimately changes the voltage developed at the contact surface, and can be used for temperature sensing. "In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is precision; system errors of less than 1 C can be difficult to achieve When any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the 'hot' end. Using a dissimilar metal to complete the circuit will have a different voltage generated, leaving a small difference voltage available for measurement, which increases with temperature. [19]" When we sense temperature, we either obtain resistance [in thermistors], or voltage [in thermocouples], as the electric parameter functionally dependent on temperature. 2.3 Sensing pressure / force In general, we may consider that forces acting on a material, either try to change its position (translation) or try to change its volume (scaling) here we consider that pressure is simply force averaged over an area. When we look at scaling, we should consider that in materials, this can mean change of interatomic distance, and this can influence conductive properties of the material. 8

9 In the example shown on the image below, a material is being compressed - and we can see that under extreme pressures, the outer orbitals of the atoms begin to merge, and metallic-like behavior starts. Figure 4. Electron density of iodine crystals at various pressures (coloured bands). As the atoms become closer, the electron density spreads between atoms. (Ref. [20]) When a material is compressed, the outer electronic orbitals of the atoms come into contact, thereby creating a continuous area where the density of electrons is not zero, which is the same concept as the free electron cloud in metals. This now becomes an area where electrons are free to move, and as electrons get more energy from the pressure to become free, the conductivity of the material is also changed. The example shown on the image on Figure 4 is extreme, and it takes great pressure to get such behavior from materials. However, for metals even small forces can influence their resistance, which is used in strain gauges: It was Lord Kelvin who first reported in 1856 that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance. This phenomenon was first put to practical use in the 1930s. Fundamentally, all strain gages are designed to convert mechanical motion into an electronic signal. A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance [1]. This is the piezoresistive effect: When the resistive (or conductive) material itself is elongated or compressed due to a mechanical input, there can be changes in the electrical conductive characteristics and this is referred to as a piezoresistive effect. [3] 9

10 A slightly different mechanism is also common, known as piezoelectric effect. Figure 5. Internal Structure of an electret (left, Ref. [2]) A sensor based on the piezoelectric effect (middle, Ref. [2]), animated illustration of piezo element (right, Ref. [21]) This effect is based on the behavior of polarized crystals under pressure: The Piezoelectric effect is an effect in which energy is converted between mechanical and electrical forms. It was discovered in the 1880's by the Curie brothers. Specifically, when a pressure (piezo means pressure in Greek) is applied to a polarized crystal, the resulting mechanical deformation results in an electrical charge. Electrets are solids which have a permanent electrical polarization. In general, the alignment of the internal electric dipoles would result in a charge which would be observable on the surface of the solid. In practice, this small charge is quickly dissipated by free charges from the surrounding atmosphere which are attracted by the surface charges. Permanent polarization as in the case of the electrets is also observed in crystals. In these structures, each cell of the crystal has an electric dipole, and the cells are oriented such that the electric dipoles are aligned. Again, this results in excess surface charge which attracts free charges from the surrounding atmosphere making the crystal electrically neutral. If a sufficient force is applied to the piezoelectric crystal, a deformation will take place. This deformation disrupts the orientation of the electrical dipoles and creates a situation in which the charge is not completely canceled. This results in a temporary excess of surface charge, which subsequently is manifested as a voltage which is developed across the crystal [2] When we sense force (pressure), we either obtain resistance [in piezoresistive devices], or voltage [in piezoelectric devices], as the electric parameter functionally dependent on force (pressure). 10

11 3 Resistive based sensors As we have seen from the review of basic sensing principles, in general different physical parameters will inflience different electric properties of different materials. We have also seen that depending on the material, we can obtain the information about the measured physical quantity as either voltage (emf) or as a resistance of the material. So, for the purposes of this course, we can define that resistive based sensors are those sensors, that exhibit a functional relationship between the resistance of the material, and the corresponding physical quantity being measured (like light intensity, force or temperature). So, if we write the measured physical parameter (light or force or temprature) generically as P, then for a resistive based sensor we can always establish a functional relationship: ( P) R sens = f Eq 3-1 The primary problem with resistive based sensors, in the context of usage with a DAQ hardware, is that what we need as excitation signal for analog inputs of a digital-toanalog converter is voltage, not resistance - that is, we would need: ( P) U sens = f Eq 3-2 or in other words - we need a functional dependence between voltage of the sensing circuit, and the measured physical parameter. Hence, we would need to build additional circuitry around a resistive based sensor, in order to extract the measured signal as a voltage, and eventually interface with a DAQ hardware like a Teleo. With this additional circuit, we would generally achieve a functional relationship between the output voltage of the circuit, and the resistance of the sensor: ( ) U = f Eq 3-3 sens R sens which then implicitly establishes a relationship between the voltage, being sampled by a DAQ input, and the physical parameter measured by the sensor. 11

12 For a wide class of resistive based sensors, which provide relatively wide changes of resistance in response to change of the measured physical parameter, interfacing is relatively easy through the voltage divider. However, for few types of resistive based sensors, interfacing through a voltage divider is impossible - in this case, a circuit known as the Wheatstone bridge can be used for interfacing. We outline briefly the two methods in this section. 3.1 Interfacing: Voltage divider In review, the voltage divider is actually a series connection of resistors. When fed by a DC (constant) voltage source E, each of the resistors will develop voltage on its ends, proportional to its resistance. So, in the voltage divider, the voltage input is actually the power supply E, and a voltage output is one of the resistor's voltages (commonly, we take the voltage of the resistor that is attached to ground - so on the schematic, the output would be the voltage U 2 ). E I + U 1 + U 2 Then, we can set up a loop, and tun Kirchhoff's voltage law for the only loop in the circuit, and set up Ohm's law as branch equation of the two resistors. Finally, the output voltage U 2 will be U R 2 2 = E Eq 3-4 R1 + R2 This expression confirms that in this circuit, there is a functional relationship between output voltage U 2, and the corresponding resistor R 2, that is U 2 = f(r 2 ). 12

13 So, in general, we can replace R 2 with a resistive based sensor, and keep R 1 as a fixed resistor - then the voltage developed on the sensor, which will be the output voltage of the divider circuit, will be dependent on the resistance of the sensor. In this case, as R 2 represents a sensor, it shows a functional relationship between the resistance and the measured physical parameter P, or R 2 = f(p). If we replace this dependency in the voltage divider expression: ( P) f ( P) R2 f Vo = Vi = Vi = R + R R f u ( P) Eq 3-5 it is easy to see that now we have a functional dependency between the output voltage and the measured physical parameter - although this is not the same function as the dependency between the resistance of the sensor and the measured parameter! However, now the output voltage is something that we could sample with an analog input of DAQ hardware - and that means that the corresponding numeric variables in the software environment, will also correspond to the measured parameter. Note that it is this same technique described in the excerpt: " When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity varies the current flowing through the circuit. [15]" In using the voltage divider, a common problem is the choice of the resistance for the fixed resistor. Obviously, if the resistance of the sensor would swing from 0 to infinity, the output voltage would swing from 0 to V i, regardless of the value of R 1. However, the resistance of the sensor would typically change from some minimal R 2min to some maximum value R 2max of the sensor resistance. In that case, a good first approximation is to use the mean value as the value for the fixed resistor, or: R 1 R + R 2 min 2 max = Eq However, for certain sensors, the relative change of resistance in response to the measured physical parameter is very small, and in that case the voltage divider is not practical - as it always has a non zero DC level, small signal are problematic to amplify. 13

14 3.2 Interfacing: Wheatstone bridge Some resistive based sensors, like for instance the strain gauge, provide changes of resistance which are but a few percent, or less, of some central resistance value, in response to change of a physical paramater. As such, interfacing them through a voltage divider is not practical. Usually, one can then apply a circuit known as a Wheatstone bridge. Figure 6. Different representations of a Wheatstone bridge The Wheatstone bridge consists of four resistors, arranged in two parallel branches, with two resistors in series each. The initial state analysed is the one where R 1 =R 2 =R 3 =R 4. In addition, the generator voltage E is known. The output voltage is U o - and note that it is NOT expressed in relation to the ground! Actually, the output voltage can be expressed through the node potentials V A and V B : U o = V V Eq 3-7 A B I 1 I 2 I 3 The analysis starts by identifying how many currents there are in the circuit once it is closed. There are three currents in the ciruit - I 1, I 2 and I 3. We want to find the output voltage Uo. Here, we can write two equtions from KVL, one from KCL, and 4 branch equations for the resistors (Ohm's law). 14

15 However, instead of going through the full solving process, we can try to solve the circuit short-hand. As we want to find U o, we can first try to express the potentials V A and V B - which are basically the voltages developed on the resistors R 2 and R 1, respecitvely. V A = U R2 = R2 I 3 VB U R1 = R1 I 2 = Eq 3-8 Then we replace into Eq 3-7: U o = V V = R Eq 3-9 A B 2 I 3 R1 I 2 Obviously, we need to find the currents I 2 and I 3. For this, we can apply a trick - we can use the fact that both series branches are attached directly to the ends of the voltage source - so we can apply Ohm's law directly to the branches - using the fact that series resistances add up: I 2 = E R + R 1 3 I 3 R 2 E + R = Eq Now we simply replace these currents in Eq 3-9: E U o = R2 I 3 R1 I 2 = R2 R1 Eq 3-11 R2 + R4 R1 + R3 E R2 = E R2 + R 4 R1 R + R 1 3 U o Eq 3-12 We have finally obtained the output voltage only expressed through the known variables. Note that out starting analysis case was when R 1 =R 2 =R 3 =R 4 (=R) - in that case note that the output voltage is 0. This state is known as a balanced bridge. Now, let us assume that we use a resistive based sensor instead of R 2 in an otherwise balanced bridge (that is, R 1 =R 3 =R 4 =R ). In that case the output voltage expression simplifies as: R2 1 = E R2 + R 2 U o Eq 3-13 So, we again obtain a functional relationship between the output voltage and the resistance of the sensor. 15

16 Note that a strain gauge in a Wheatstone bridge like this, will produce a voltage signal in the range of milivolts - which is practicably unusable with a DAQ hardware like the Teleo. That is why a signal from this bridge is usually amplified - however, again take note that the output voltage U 0 is not expressed in relation to ground! Thus, a differential amplifier must be applied (or even better, an instrumentation amplifier) - which are circuits we will briefly discuss at the end of the course. For other resources about Wheatstone bridge, consult [22], [23], [24]. It is also advisable to use Falstads circuit simulator applet, to visualise the flow of current from either drawing perspective. Figure 7. Visualising the flow of current in Wheatstone bridge using Falstads applet (Ref [25]) Try also changing one of the resistor values in an otherwise balanced bridge, to see the effect of change on the output voltage. 16

17 4 How do we view the data acquisition system As said before, one of the important aspects in the macroscopic schematic representation is that we account for both microscopic effects and for energy relations. And this is also extremely important in the case of the focus we have in ST. Actually, in our sensor based interaction process we have the following consecutive stages: - The user performs a movement an interactive action and changes a physical parameter. - The sensor measures the change of physical parameter, and provides a corresponding change of an electric parameter. - The entire sensing circuit provides a voltage signal, based on the changes of the electric parameter of the sensor, conditioned for use with the Teleo - The Teleo accepts the voltage signal from the sensing circuit on its analog input(s), performs A/D conversion, and sends the sampled value to a PC. In this sense, seen from an elementary circuit perspective, we realize that whatever kind of sensing circuit we may use, it still must in the end provide a voltage value conditioned for the input voltage range of the Teleo, which is 0V 5V. Hence, whatever sensing circuit we use, it must act as a voltage generator on its output. In the same sense, the analog input of the Teleo must act a resistor in one or another way and in this way we can see the elementary circuit structure; which helps us realize that we are sending information from the sensing circuit to the Teleo since the sensing circuit is the generator that invests energy, it also generates and sends the information; on this level the Teleo simply consumes it as a load. The important thing to realize that the full circuit connection is established through the ground connector, as displayed on the following image: AIN0 I + V sens GND Sensing circuit 17

18 This applies to all the inputs of the Teleo notice however that the ground is the common wire for all the rendered elementary circuits: AIN1 I1 + V sens Sensing circuit AIN0 I0 + V sens GND Sensing circuit Finally, let us remember that the Teleo offers a +5V power supply connector, which we can use to power our sensing circuit. Lets realize that in such a situation, we have two energetic aspects: as one, the Teleo power connector acts as a generator, and our sensing circuit acts as a load; and the other, our sensing circuit acts as a generator, and the Teleo analog input connector acts as a load. Thus, we can adopt a rough electrical model of the Teleo, where each of the analog inputs is represented as an equivalent resistance, and the +5V connector is represented as a voltage generator. 18

19 5 Basic sensor interfacing resistive based sensors Here we take a look at several basic sensing devices that we can use for user interaction. All of these react to changes of a given physical parameter by changing the resistance on their terminals, so we can consider them a class of resistive based sensors and most of them can be interfaced to the Teleo through a voltage divider, which as a circuit will provide the sensed change of resistance as change of voltage. 5.1 Switch (push button [SPST] & toggle [SPDT]) A switch is one of the basic structures fr electrical interfacing. It is basically a mechanical system designed to bring two conductors into an electric contact at command of the user mostly expressed by applying force/pressure. In some way they can be considered force/pressure sensors however, they cannot provide a measure of how much force/pressure is applied, as the force sensitive resistor can. They can be found in almost all electronic appliances and form the basic structure of many interaction devices, such as the keyboard or the joystick. Let us mention that whenever we might touch two conducting wires together and separate them at will, thereby controlling the flow of electric current, we are effectively implementing a switch. However, as electronic components, we usually refer to two types of switches: push button [SPST] and toggle [SPDT]. [10] The common structure of a push button is displayed on the image on the left. It is a component with two terminals to which wires are soldered. The terminals are not internally connected by default this is a state when the switch is OFF. However, internally there is a movable contact, which can establish electric connection between the terminals for as long as force is applied to the button; in which case the switch is ON. As soon as the force stops, an internal spring lifts the movable contact, and the push button automatically is reset to OFF state. This type of a switch is also known as SPST Single Pole Single Throw switch. 19

20 We can consider the switch as a resistive element, since when the state is ON, the resistance between the terminals is R = 0, and for state OFF, =. [10] swicth R swicth [10] The structure of a toggle switch is rendered on the images above. Here we usually encounter a component that is again a mechanical system that reacts to force, though this time with three terminals. The center terminal is a common connection, which can be connected either to one end terminal or the other; the end terminals cannot be connected to each other. When force is applied the connection to the common terminal flips from one end terminal to the other and it stays like that even when force is not applied anymore (we state that the state has been toggled). Let s note that this distinction in operation between a toggle and a push button is also for instance implemented in the button and toggle objects in Max/MSP The symbols for a push button and a toggle, in the sense of difference in number of terminals, can be seen on the table below: [12] We should keep in mind that if we look at a pair of a common connector and an end terminal of a toggle switch, electrically they behave exactly as the two terminals of a push button. So we will look at some examples using an SPST switch but we should be aware that depending on our interaction specification, we could use either a push button or a pair of a common and end terminal of a toggle switch, as we find fit. 20

21 We can see the function of a switch through an elementary circuit example. In general, the switch is connected in series with the voltage generator and the resistor. The analysis is simple when the switch is OFF (infinite resistance) the circuit is open (broken) and current does not flow that means that the current is zero. In that case, by Ohms law, the output voltage V o on the resistor will be: V o = R I = R 0 0 Eq = When the switch is ON (zero resistance) the circuit is complete and current flows according to Ohms law and as this is an elementary circuit, the output voltage V o on the resistor will be equal to that of the generator V o = V i (or V o = V CC or V o = E). That is actually why we need to interface a SPST switch to the Teleo as rendered on the image on the left. What we are actually doing is establishing an elementary circuit where current can flow, where the generator is taken from the Teleo, and the resistor is the internal resistance of a Teleo analog input. The circuit closes internally through the ground of the Teleo. 21

22 In case we want to use a different voltage generator, all we need to do is to make sure that the ground of that generator and the Teleo are connected, so a circuit can be closed: Finally, let us mention that if we interface a switch in parallel with the resistor as shown on the image, when the switch is ON it is actually a SHORT CIRCUIT so we might burn the generator AVOID! 22

23 5.1.1 Resistive switch ladder As it is obvious from the previous example, we need to use an entire channel of the Teleo to interface a single switch, which limits us to use of four switches with a single Teleo. However, there is a possibility to interface more than one switch to a single Teleo channel using a simple ladder of resistors, forming a voltage divider, interfaced with switches. In general, such a circuit can support several switches in any case, if we use N switches, we need to use N+1 resistors. For the case of two switches, the schematic of such a circuit is displayed below: Depending on which switch is ON, the state in the circuit changes, and hence we generate a different output voltage V o, telling us which of the switches was pressed. In this case we have three situations both switches OFF, S1 ON - S2 OFF and S1 OFF - S2 ON. 23

24 These situations are illustrated on the diagrams below: Thus, what we need to do is find the output voltage V o for each of these states. 1) When all switches off We have a voltage divider formed between R 1 and the series connection of R 2 and R 3. Hence the output voltage will be R1 V o E R1 + R2 + R3 = Eq 5-2 2) When S1 ON We have no voltage divider (R 2 and R 3 are short-circuited by S1), so the output voltage is the voltage of the generator: 3) When S2 ON V o = E Eq 5-3 We have a voltage divider formed between R 1 and R 2. Hence the output voltage will be R1 R + R V o 1 2 E = Eq 5-4 Thus, we obtain three unique voltages for the three states of the circuits. The Teleo will be then able to sample those voltages and provide a variable value that indicates the status. Let us just mention that the switches are not exclusive, and the lower one (in the schematic) takes precedence that is, if both S1 and S2 are pressed, the circuit reports that S1 is pressed. 24

25 The problem with the circuit is also that the resistor values need to be carefully designed so to get equal steps for each state of the circuit as we can see that there is no linear relationship between the output voltage and the which switch is pressed. In the end, in the sense of the Teleo, such a circuit can be easily implemented by soldering resistors on a Veroboard, and using the Teleo itself as a power supply as displayed on the assembly diagram below: 25

26 5.2 Potentiometer (slider/fader & rotary knob) A potentiometer is a three terminal electronic element, which electrically is composed of a single chunk of resistive material, which has two fixed conductive terminals on the end, and a movable contact that can glide on the resistive surface known as a taper or wiper. The schematic symbol for a potentiometer is displayed on the right. [11] There are two general versions of potentiometers, which are identical in their electric properties, but different in their mechanical ones and in that sense, different in their context of interaction use. There are potentiometers that allow linear displacement of the wiper, known as sliders or faders, and those that allow for a rotary displacement of the wiper, known as rotary knobs construction diagrams of both are rendered below. R l [11] l1 [11] l2 l1 l2 R1 R2 In essence, the chunk of resistive material, has some cross section S a given length l and a resistivity ρ. That means that the whole piece of material can be seen as a resistor with resistance R, which can be calculated as: R l ρ S = Eq 5-5 This is the resistance that is measured between the two fixed ends of the potentiometer. However, the presence of the movable wiper in electric contact with the resistive material, effectively divides this resistor into a series connection of two resistors, R1 and R2. Their respective resistances can be calculated as: R = 1 ρ l1 S R 2 ρ l2 S = Eq

27 That situation is displayed on the schematic below and it gets easier to see that the potentiometer thus represents a voltage divider in itself; in essence, the resistance R1 is seen between the wiper and one end terminal, and the resistance R2 is seen between the wiper and the other end terminal the total resistance R being the resistance between the two end terminals: E So, in essence, the user performs an interactive action by applying force or torque (rotational force) to the potentiometer, and by doing that, the user displaces the wiper internally. Hence the individual resistances R1 and R2 change to reflect the new position of the wiper. So, according to the above diagram, if the potentiometer is powered by some voltage supply E, then the output voltage Vo will be according to the voltage divider: V o = R2 R + R 1 2 E Eq 5-7 Obviously the two lengths obtained by the separation of the wiper must add up to the total l = l 1 + l 2 Eq 5-8 Thus the two resistances, R1 and R2 must always add up to R, regardless of the position of the wiper: R = R 1 + R 2 Eq

28 If we replace that in Eq we get V o l1 ρ R2 = E = S E = f 1 R R ( l ) E Eq 5-10 So we get that the output voltage of the potentiometer used as a divider changes linearly with the displacement of the wiper. As we now have a direct relationship between voltage and a length of a resistive block, determined by the wiper position, which is directly set by user interaction this means that we have a function (mapping) between the user s actions and voltage, so we can use the potentiometer with the Teleo to get user data, based on the sampled voltage Vo. All we need to do is power the end terminals of the potentiometer (with the Teleo power supply on one end, and the Teleo ground on other) and the voltage Vo will be provided in the middle terminal, which can be then connected to an analog input. 28

29 5.3 Photosensitive (light dependent) resistor [LDR] The photosensitive resistor is a two terminal electronic element, which reacts to changes of light intensity I falling on it by changing its resistance, or we can establish the mapping (function): R LDR = f (I) Eq 5-11 The general look of a photosensitive resistor is displayed below: In the context of interaction use, a human user can perform actions that block the amount of light falling onto the photoresistor (by creating shadows for instance), thereby changing the amount of light falling on the photoresistor, and thus changing its resistance. In order to obtain changing voltage from this resistance, we obviously need to interface with a voltage divider, so we need to place an additional resistor, as rendered on the schematic below. In this case the output voltage is given by the voltage divider: V o = R R + R LDR LDR E It is trivial to see that the output voltage, and thus the digitized data we obtain in software, will be a function of the light intensity, and thus functionally related to user s interaction: f ( I) V o = E R + f ( I) 29

30 5.4 Force sensitive resistor [FSR] The force sensitive resistor is a two terminal electronic element, which reacts to changes of force (pressure) on its surface by changing its resistance, or we can establish the mapping (function): R LDR = f (F) Eq 5-12 The general look of a photosensitive resistor is displayed on the right. In the context of interaction use, a human user can perform actions that exert pressure on the force sensitive resistor (by stepping on it for instance), thereby changing its resistance. In order to obtain changing voltage from this resistance, we obviously need to interface with a voltage divider, so we need to place an additional resistor, as rendered on the schematic below. In this case the output voltage is given by the voltage divider: V o = R FSR R + R FSR E It is trivial to see that the output voltage, and thus the digitized data we obtain in software, will be a function of the applied force/pressure, and thus functionally related to user s interaction: f ( F) V o = E R + f ( F) 30

31 5.5 Sensor interfacing resources Falstad s Analog Circuit Simulator Applet [6] Chapter about Sensors online course Input/Data Acquisition System Design for Human Computer Interfacing [7] SensorWiki The aim of this project is to provide a thorough review of the main types of sensing technologies used in musical applications. More than 30 techniques are described, along with their sensing principles and examples of actual devices that implement those principles. [8] 31

32 And finally Be open minded a lot of things can be used as a sensor Check for instance this slide [9]: 32

33 6 PE Questions Discuss some of the basic sensing principles. Discuss how are resistive based sensors interfaced to a DAQ hardware (Teleo) 33

34 Resources and references [1]. The strain gage, [2]. Piezoelectric sensors, [3]. Resistive sensors, [4]. Electrical safety, [5]. How can electric current *NOT* cause a shock?, [6]. Falstad Analog Circuit Simulator Applet, [7]. Input/Data Acquisition System Design for Human Computer Interfacing, [8]. SensorWiki, main page, [9]. Resistive sensors, slides, [10]. Switches, Basic Car Audio Electronics, [11]. Potentiometers, Basic Car Audio Electronics, [12]. Switch, Wikipedia, [13]. Omicron NanoTechnology GmbH. In-situ High Temperature STM Studies of Surface Dynamics 2. _of_surface_dynamics_2/index.html~omicron [14]. Petrucci, Harwood, Herring. Photoelectric effect (movie). Chapter 9, Electrons in Atoms. General Chemistry - Principles and Modern Applications - Media Portfolio. [15]. Photoconductivity - Wikipedia, the free encyclopedia. [16]. Photoelectric effect - Wikipedia, the free encyclopedia. [17]. Fu-Kwun Hwang. Photoelectric effect (applet). NTNUJAVA Physics Simulations. [18]. Thermistor - Wikipedia, the free encyclopedia. [19]. Thermocouple - Wikipedia, the free encyclopedia. [20]. Prof Simon Redfern. Materials under extreme conditions: Lecture 2. [21]. Retrieved from (not online anymore) [22]. Wheatstone bridge - Wikipedia, the free encyclopedia. 34

35 [23]. efunda. Introduction to Wheatstone Bridges. [24]. Allaboutcircuits.com. Bridge circuits : DC METERING CIRCUITS. Volume I - DC. [25]. Paul Falstad. Circuit Simulator Applet. Math, Physics, and Engineering Applets. [26]. 35