1. Lab 3 Speed Sensor Circuit: Electronic prototype construction
ENGR1182

2. 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

3. 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

4. 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

5. 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

6. 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,
Circuits Lab 1

7. 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

8. 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

9. 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.

10. 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,
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

11. Task 1: Review of KVL Schematic# 2 Schematic# 1 P. 11
Modified 01/20/11 by R.C.Busick, HI 221,
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

12. 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,
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

13. 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 5V (positive end)
And the lower Horizontal row connected through the black wire would ground. This would be helpful for tasks 3 and 4.
Record measurements/questions associated with task#1

14. 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,
Switch is off/open
Circuits Lab 1

15. 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,
Circuits Lab 1

16. 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,
Current follows the path of least resistance
Circuits Lab 1

17. 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

18. 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,
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

19. 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

20. 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,
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

21. 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,
Signal clip
Ground clip
P. 21

22. 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

23. 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

24. 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

25. 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,