ST08 – Resistive based sensors and interfacing 1 Resistive based sensors and interfacing Lecturer: Smilen Dimitrov Sensors Technology – MED4

ST08 – Resistive based sensors and interfacing 2 Introduction The model that we introduced for ST

ST08 – Resistive based sensors and interfacing 3 Introduction We have discussed –The units of voltage, current and resistance, from both a microscopic and macroscopic (electric circuits) perspective –The definition of an elementary electric circuit –Ohm’s law and Kirschoff Laws –Solving and measurement of voltage divider circuit –Solving of more complicated circuits Now, ready to look into actual sensor circuits

ST08 – Resistive based sensors and interfacing 4 Basic sensing principles A brief overview of basic sensing principles from a microscopic perspective, before we start with circuit-theory level –Not an all-inclusive list… Sensing light Sensing temperature Sensing pressure/force –Electronic sensing principles – not all sensors are necessarily built on them; some sensors can be a mix of mechanical and electronic system

ST08 – Resistive based sensors and interfacing 5 Sensing light Basis in photoelectric effect – a valence electron can gain enough energy from a light wave/particle to leave the atom, and become a free electron. In vacuum bulbs: In materials (conductive or semconductive) - this increases number of free electrons per unit volume, which directly influences what we call resistivity ρ – which influences resistance R Photoresistivity (or alternatively, photoconductivity) – dependant on frequency and intensity of light When we sense light, we can obtain resistance [in photoresistive materials], as the electric parameter functionally dependent on light.

ST08 – Resistive based sensors and interfacing 6 Sensing temperature Basis – phonons – increased motion (vibration) of ions in a crystal lattice Increases effective area of ions, and probability of collision with a free moving electrons influences the average speed of electrons – which again influences resistivity; for all resistors Thermistors: PTC/NTC Temperature also influences possibility for increasing number of free electrons –Thermocouple - a metallic contact of two different metals, where one metal is heated – thermoelectric effect When we sense temperature, we either obtain resistance [in thermistors], or voltage [in thermocouples], as the electric parameter functionally dependent on temperature.

ST08 – Resistive based sensors and interfacing 7 Sensing pressure / force Forces acting on a material, either try to change its position (translation) or try to change its volume (scaling) – pressure is simply force averaged over an area. 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). Piezoresistivity – resistance is related to volume (length and cross-section area) of conductors; but compression can also lead to metallic behaviour. Piezoelectricity – when a pressure is applied to a polarized crystal, the resulting mechanical deformation results in an electrical charge. Deformation disrupts the orientation of the electrical dipoles and creates a situation in which the charge is not completely canceled.

ST08 – Resistive based sensors and interfacing 8 Basic sensing principles So, in sensors (electric sensor materials) it is important: –Which mechanism gives rise to the sensor effect; which determines: –Which electrical parameter changes in response to the measured physical paramater (or in other words – which is the electric parameter functionally dependent on the measured physical parameter). … which is important, as this determines what kind of a circuit do we need, in order to obtain a usable signal – voltage – that we can interface with (that is, that we can sample with DAQ hardware)

ST08 – Resistive based sensors and interfacing 9 Resistive based sensors Resistive based sensors have resistance as the electric parameter functionally dependent on the measured physical parameter (light, temperature, pressure..) As circuits, we can use either a voltage divider, or a Wheatstone bridge, in order to obtain voltage dependent on the changing resistance of such a sensor: And thus a voltage, that changes ultimately because of the change of the measured physical parameter: … which is what we need in interface with the sensor, using a DAQ system – and obtain the change of the measured parameter, as a change of data in a software environment.

ST08 – Resistive based sensors and interfacing 10 Interfacing: Voltage divider For a wide class of sensors – photoresistors, force sensing resistors (FSRs).. What resistance to choose for the fixed resistor? First guess:

ST08 – Resistive based sensors and interfacing 11 Interfacing: Wheatstone bridge Some sensors have too small of a change in resistance – for them Wheatstone bridge must be used: Can’t be used directly with a DAQ – a differential amplifier is needed first.. I1I1 I2I2 I 3

ST08 – Resistive based sensors and interfacing 12 How do we view the data acquisition system Any sensing circuit can be seen as a variable voltage source, closing a circuit with an “analog in” Any analog input can be seen as a very big equivalent resistance

ST08 – Resistive based sensors and interfacing 13 Switch (push button [SPST] & toggle [SPDT]) Simplest ( from an electrical perspective ) – resistance changes between 0 and ∞ Based on establishing electric contact between conductor, upon application of force

ST08 – Resistive based sensors and interfacing 14 Switch (push button [SPST] & toggle [SPDT]) Availability: micro-switches (SPST), and regular ones (SPST and SPDT):

ST08 – Resistive based sensors and interfacing 15 Switch (push button [SPST] & toggle [SPDT]) - interfacing If switch is OFF:If ON: Vo = Vcc Beware of possible short circuits when using (for example: switch in parallel with resistor):

ST08 – Resistive based sensors and interfacing 16 Resistive switch ladder One switch – one analog input; with this method, can interface more – however, can only detect one switch at a time Analysis: calculate voltage divider for each state of each switch: –All off: –S1 on: –S2 on:

ST08 – Resistive based sensors and interfacing 17 Potentiometer (slider/fader & rotary knob) Three-terminal element, made of single chunk of resistive material Linear displacement – slider/fader, rotary displacement – rotary knob (trimmer or potentiometer) Resistance seen between an end terminal and wiper: Thus, a voltage divider is formed between these two resistances Can be used as two- terminal “variable” resistors

ST08 – Resistive based sensors and interfacing 18 Availability: trimmers, potentiometers (rotary), faders (linear) Potentiometer (slider/fader & rotary knob)

ST08 – Resistive based sensors and interfacing 19 Photosensitive (light dependent) resistor [LDR] A two terminal electronic element, which reacts to changes of light intensity I, falling on it, by changing its resistance Interfacing – through voltage divider – to obtain voltage dependent on measured light intensity:

ST08 – Resistive based sensors and interfacing 20 Force sensitive resistor [FSR] A two terminal electronic element, which reacts to changes of force (pressure) on its surface by changing its resistance Interfacing – through voltage divider – to obtain voltage dependent on measured light intensity:

ST08 – Resistive based sensors and interfacing 21 Force sensitive resistor [FSR] Availability: FSR and bend sensor Also strain gauge (Wheatstone !)