Department of Electrical Engineering and Electronics ELEC171 Project Practice activity 1: Investigating light sensors ● Overview Sensors are used in many electrical devices to control the device performance. The type of sensor and the application can be extremely varied. Examples include: the infrared sensors used in television remote controls to change channels ultrasonic sensors used in smart vehicle to detect nearby objects the touch sensor used on smartphones or tablets to control the operating system the electromagnetic sensors embedded in roads used to detect cars and trucks, which are used to trigger the traffic lights. Your robot car will need sensing circuits to detect the presence of walls. The basic principle of a wall-sensor is simple: your car should emit radiation of some type, the radiation reflects from the wall and a sensing element on your car detects that reflected radiation. While this basic principle is simple, there are different types of sensors and different types of circuits that can be used to perform this function. There are two main aims of this activity: to give you experience of using several different types of sensors to give you background knowledge and experience that will help you choose sensor components and circuits for your project car Activity Specifications Activity name Suggested study time Assessment Submission deadline Submission format 2020-21 Late submission Resit opportunity Marking policy Anonymous marking Feedback Practice activity 1: Investigating light sensors 5 hours Lab notes for the three practice activities will be assessed. In total, this will contribute 30% of your project mark. Online via Canvas Standard university penalty applies August resit period Marked and moderated independently Yes via Canvas / Turnitin Feedback Studio 1 Assessment 2020-21 The assessment of this activity consists only of your lab notes for the three Project Practice Activities, which are scheduled for the first 3 weeks of the semester. ● Your lab notes must be uploaded to Canvas after your lab session in the 3rd week of the semester. The date is given on the front page of this document, and on Canvas. You should upload your notes as a single pdf file, combining your notes for all three practice activities. You should scan your notes after you finish the 3rd practice activity and submit this pdf file. Your marks for the lab notes will comprise 30% of your mark for the ELEC171 project. The marking guidance for these laboratory notes is the same as for the laboratory notes marked in Semester 1. You can find the marking guidance and marking criteria on the module website. 2 Part 1: Background: Sensors and components 1.1: Background: Different types of sensor arrangements There are many devices that use light sensors, and the type of application determines the electrical circuitry required for the device. Three generic examples are described below and shown schematically in Figure 1.1. Passive light sensors: 2020-21 These contain a light sensing element that detects background light that falls onto the sensor. A schematic diagram is shown in Figure 1.1(a) An example of this type of sensor might be in an outdoor lamp used to illuminate dark spaces at night. The sensor element would detect the light level and turn the main lamp on if the background light is too low. For this type of application, only a light sensor is required. Active light sensors: These sensors contain a component that generates light as well as a component that detects light. In one type of application, shown in Figure 1.1(b), the sensor is located separately to the light source and monitors a continuous input of light from the source. The sensor element is triggered if the input light stops. An example of this type of device might be a burglar sensor in which an infrared beam is monitored. A person stepping between the light source and the light detector would trigger the alarm. A second type of light sensor contains the light source and the light detector in the same device, and monitors reflections form nearby objects. This arrangement is shown in Figure 1.1(c). An example of this type of sensor might be a robotic vacuum cleaner that uses infrared light to detect walls and objects. If there are no nearby objects, the light sensor will receive low levels of light. If an object is nearby, the source light will reflect back into the sensor, and notify the device that something is nearby. (a) (b) (c) Figure 1.1: (a) a passive sensor that detects background light, (b) an active sensor that continually detects light from a source and will be triggered if the light is interrupted, (c) an active sensor that sends out light and detects reflections from nearby objects Figure 1.2: An autonomous robot car must have sensing elements that detect the presence of walls or obstacles. These usually contain an emitting element and a sensing element that detects radiation reflected or scattered by the wall or obstacle. L 1.2: Different types of sensor components You have been given a kit of parts that contains a variety of LEDs and two photodiodes. Many of the components look very similar. Go through the part list below and make sure you can identify all the components. White LED: ● LED that generates white light, clear plastic top (small tabs on legs) LED that generates infra-red (IR) light, clear plastic top (red mark on one leg) resistor whose resistance R depends on the amount of light falling on the sensor component that detects IR light (looks similar to an LED, has a black plastic top) Normal photodiode: component that is sensitive to IR + visible light (clear plastic top with straight wire legs) Red/green LEDs and general purpose diodes (already in your kit from Semester 1 experiments) IR LED: LDR: IR Photodiode: 2020-21 Take particular care to correctly identify the three components that have a clear plastic cap. These obviously look very similar, but you can tell the difference between them if you look carefully. One component has small tabs on the wire legs. This is an LED that emits white light. One component has straight wire legs, with no tabs. This is a photodiode that is sensitive to infrared (IR) and visible light ● ● One component has straight legs, which have been marked with a red indicator. This is an LED that emits infrared light. One other point to notice is that the two photodiode components are electrically the same. (This is the reason that they have the same component number SFH 213). The only difference is that the two components have different plastic caps one is clear and one is black. The reason for this will become clear later, when you do the infrared sensor tests in Part 3. - Part 2: Sensors based on photosensitive diodes 2.1: Background: photodiodes You are already familiar with light-emitting diodes (LEDs), which emit light when current flows them. The components are composed of two different semiconductor materials. When current flows through them, electrons and holes re-combine and light is generated. Photosensitive diodes, more usually called simply photodiodes, are very closely related components that have the opposite behaviour - they generate current when light falls on them. In photodiodes, light falling on the semiconductor junction causes the charge carriers to separate and the current flows across the junction. To understand circuit performance, it is not really necessary to understand everything about the solid-state physics aspects of LEDs and photodiodes. You do, however, need to know what they do: LEDs: components that generate light when current flow through the semiconductor junction. Photodiodes: components that generate current when light falls on the semiconductor junction. 4 Figure 2.1: Examples of photodiodes. These examples look very similar to the LEDs that you have used in previous experiments. Current is generated in the diode when light passes through the plastic cap and strikes the interior surface. 2.2 Background: a sensor circuit containing a photodiode Figure 2.2 shows a circuit combination that includes a light generating circuit and a light detecting circuit. The circuit on the left should be familiar: DC circuit containing an LED. The circuit on the right include a photodiode. The light emitted by the LED in the first circuit is detected by the photodiode in the second circuit. You should note that the photodiode symbol is similar to that of the LED, but the arrows are reversed in direction because light enters the component for a photodiode. Also, for this type of circuit, the photodiode is inserted in the reverse direction, called reverse bias. This is important, for reasons you will see later. E V1 O O A R1 LED light B R2 Hilt V₁ = approximately 10 V p-p, square-wave, f = 10 kHz LED = white LED E Figure 2.2: Light generating and detecting circuits, using an LED and a photodiode. 2020-21 V2 2.3 Experiment: Testing a sensing circuit based on a visible-light LED The aim of this experiment is to successfully build the sensor-detector circuits shown in Figure 2.2, using a visible-light LED as the emitting light source. You should follow the steps below and keep a record of your experiment in your lab notebook. Using a test board, construct the LED circuit shown in Figure 2.2, using the following values for the circuit components. 5