Lab 3 Speed Sensor Circuit: Electronic prototype construction ENGR1182

Lab Overview In today’s lab we will: Learn about and use basic electronic components used in lab Understand the principle of operation of a speed sensor Build a prototype speed sensor using a Light emitting diode (LED) and a phototransistor

Electronic components used Operation Schematic symbol Actual snapshot Resistor Resists the flow of electrical current Non-polarized Light Emitting Diode (LED) Emits light when current flows through it Polarized Photo-transistor (diode) Allows current to flow, from top leg (collector) to bottom leg (emitter) when light falls on it base Circuits Lab 1

Transistors Can act like a “switch” Can act as an amplifier Current limited Phototransistor is a special type of transistor Instructor: The basic element of digital electronics is the transistor. It can act like a switch. For this type of transistor, called a bipolar junction transistor, there are three parts, called the base, collector, and emitter. When current flows to the base of the transistor, the “switch” is turned on, allowing current to flow from the collector to the emitter. Circuits Lab 1

Prototype board and DMM Setting to be used throughout this experiment Examples of electrically connected holes Most people are familiar with batteries. Batteries can be used in portable equipment or in cases where an electrical source is needed on a temporary basis. And in situations where the voltage source does not have to be regulated (battery voltage decreases with power draw). AC to DC power supplies are used in many, many applications. They convert ‘house’ power (120 VAC) to a direct current and voltage. They can be voltage or current regulated, or can be unregulated. In lab we will use a regulated power supply to provide the stable 5 volt direct current source needed by the circuits. Two styles of power supplies, as pictured above, are used in the labs. Digital Multimeter - DMM (used for measuring voltage and resistance) 5 V DC Power Supply Circuits Lab 1

Principle of operation: Speed Sensor Why do we need to study a speed sensor? To measure speed of the ball at different points in the roller coaster project BASIC PRINCIPLE: Measure time between when the leading and trailing edges of the ball pass a given point. Given the diameter of the ball, an estimate of the speed that the ball is traveling can be found. Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Circuits Lab 1

The Speed Sensor Circuit When the ball passes between an LED and a phototransistor, the light is blocked. 100 This is a graphic representation of the speed circuit that will be used later in the course. Today, students will build a proto-type of this circuit using a prototyping board (or "breadboard") and test it out. Circuits Lab 1

The Speed Sensor Circuit Lets see how it works 100 This is a graphic representation of the speed circuit that will be used later in the course. Today, students will build a proto-type of this circuit using a prototyping board (or "breadboard") and test it out. Output Signal Circuits Lab 1

Calculating Speeds The clear plastic around the LED is a lens that focuses the light into a beam. The beam has a spread of about 6 degrees. The edges of the ball are not flat. Therefore, the measured time is shorter than the actual time.

Speed Calculations Leading Edge Trailing Edge Diameter of Ball (D) Time axis Measure (t2-t1) The correction factor for the LED/Phototransistor pair and the 1” Dia. coaster ball was measured to be 8.3%. So the speed can be calculated using: s=D/[1.083*(t2-t1)] Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 When ball passes through Point A - we trigger – transition from 0 to 1 (leading edge of square wave) When ball passes through Point B – we reset – transition from 1 to 0 (trailing edge of square wave) We observe this square waveform using a Virtual Oscilloscope (same as in ENG-181) The problem in building such a circuit boils to: Sensor circuits: Sensing the ball moving over Point A and/or Point B – we will use optic and mechanical switches for this purposes. These circuits will generate signals when ball passes through these points. In our lab set up, we will use a mechanical switch at Point A and a optical switch at Point B. They could be interchanged. Variations of the switch circuits could also be used (laser-based, different micro-switch, some form of pressure transducer, etc.) . Square wave generation: Generating a square waveform based on the signals we obtain from the sensor circuits. We will use what is commonly called a D-Flip flop for this purpose. We will not examine the internal design/working of a D-flip flop. All we will know is : It is an IC (chip) - We will use 5 of its pins for building one speed trap. Lets call its output Q – where the square wave is generated (Q can be 0 Volts or 5 volts - equivalently 0 or 1) It has SET and RESET input pins – where we will connect the output of our 2 sensor circuits While the flip flop is powered on, it is in a unknown state – we set it to a known state (Q=0) by another small mechanical switch circuit (similar to the ones used in our sensor circuit) Circuits Lab 1

Task 1: Review of KVL Schematic# 2 Schematic# 1 P. 11 Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Instructor notes: Make sure to remind the students that this is a series connection of resistors In step 2, the BJT switch will be parallel to the LED Schematic# 2 Schematic# 1 P. 11

Correspondence between circuit board and schematic connections This connection is made by the vertical bars on the backplane 5 V power supply Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Instructor notes: Make sure to remind the students that this is a series connection of resistors In step 2, the BJT switch will be parallel to the LED P. 12

Recording Voltages VR1 and VR2 Mention the fact that once the AC adapter is plugged in, the upper horizontal row connected through a red wire on the bread board would be @ 5V (positive end) And the lower Horizontal row connected through the black wire would be @ ground. This would be helpful for tasks 3 and 4. Record measurements/questions associated with task#1

Task 2 : Construction and analysis of Green LED circuit Switch is on/closed Green LED does not glow Current Flow Current Flow Green LED glows Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Switch is off/open Circuits Lab 1

Task 2 : Direction of Current flow when switch is off or open Green LED Glows At node a, total current i breaks into i1 and i2; i.e. i=i1+i2 Since open switch provides infinite resistance, no current flows through it; therefore i1=0 and i=i2 a Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Circuits Lab 1

Task 2 : Direction of Current flow when switch is on or closed Green LED does not Glow a At node a, total current i breaks into i1 and i2; i.e. i=i1+i2 Since closed switch provides zero resistance, all current flows through it therefore i1=i and i2 =0 Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Current follows the path of least resistance Circuits Lab 1

Applying of what we learned to RC speed sensor Ball position In between the transistor and LED Not in between the transistor and LED Transistor (modeled by a switch in task#2) OFF ON LED Circuits Lab 1

Task 3 : Construction and analysis of phototransistor circuit Visually distinguishing between a Red LED and a phototransistor Red LED (labeled “LED” on the lens side) has a flatter lens Phototransistor’s lens has a more bullet-like shape (labeled “Photo-diode” on the front) Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Members of the staff : CHECK PROPERLY if the students have actually left the bread boards as depicted in the picture. Do not sign the checklists blindly. Circuits Lab 1

Task 3 : Construction of speed sensor circuit Complete the circuit by hooking up a Red LED in front of the photo transistor while making sure there is enough space for the given piece of track to fit in the space, in between the Red LED and the photo transistor   Members of the staff : CHECK PROPERLY if the students have actually left the bread boards as depicted in the picture. Do not sign the checklists blindly. Circuits Lab 1

Task 3: Verification Ensure that the Red Led and phototransistor are aligned such that Red LED beam is centered on the phototransistor lens Verify that the phototransistor circuit works by placing an opaque object between Red LED and the phototransistor Circuit works properly if the green LED turns “ON” when the light beam is interrupted. Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Verify that the phototransistor circuit works by placing an opaque object (finger, Buck ID card, etc.) between the Red LED and the phototransistor Circuits Lab 1

Task 4: Using speed sensor circuit for velocity measurements Connect the “BNC Connector” to the matching BNC connector labeled “Channel 0” on the rear panel of the PC. BNC connector Connect the “Ground Clip” to any ground point in the circuit. Connect the “Signal Clip” to the output of the Phototransistor (non-black side) If signal clip is connected to the black/ground end of the phototransistor you will not get any lab view plots Modified 01/20/11 by R.C.Busick, HI 221, 247-6223 Signal clip Ground clip P. 21

Task 4 The output of each speed sensor will consist of a pulse, where the output voltage goes from low to high to low as the ball passes (like a square wave) We're interested in the time (∆t) between when the leading and trailing edges of the ball pass the sensor. ∆t

Task 4 Follow instructions given in your lab procedure to finish task 4 Use Alt+PrintScrn to copy your Lab view plot with speed sensor data (i.e. coaster ball velocities) in a word document. You need to attach this screen capture in your lab memos Answer all the measurement/discussion questions in the lab procedure

Task 4 Record the results on your worksheets This lab will be performed in teams of 2 students (or 3 if only 3 team members) If you come across a faulty electronic component, do not replace it with a neighboring team, instead bring it to the instructional staff’s attention

Lab Memo Only one set of data per team should be selected for the single team memo. Modified 01/20/11 by R.C.Busick, HI 221, 247-6223