Seat Monitoring Device

1
Seat Monitoring Device

• Michael Ding
• Edwin Lee
• Tim Yung

2
Objectives

• Design a wireless seat sensor that will be able
  to detect if a person is sitting in a seat
• Use temperature and pressure sensors
• Connect to a server that will have the ability to
  determine which seats are full and display this
  information onto a website to be viewed

3
Benefits and Features

• Benefits
• Low overall cost
• Easy and efficient to use
• Time-saver
• Easy maintenance
• Convenience
• Features
• Non-intrusive
• Wireless transmission of seat availability data
• End users can check seat availability data
  wherever there is internet access
• Online website features a layout that is easy to
  follow
• Can be modified for a variety of different
  applications (lecture hall, library, movie
  theater, airplane)

4
Block Diagram
5
Transmitter Schematic
6
Receiver Schematic
7
Sensors

• Two Types
• Temperature Sensor
• Pressure Sensor
• When both are triggered, a person is occupying
  the seat (Boolean AND)

8
Thermistors

• Thermally-sensitive resistor
• Advantages
• Low Power Consumption
• Speed
• Simple two-wire connection
• Ruggedness
• High Sensitivity to Low Temp Changes
• Low Cost

9
Thermistors

• Two Types of Thermistors
• Negative Temperature Coefficient (NTC)
• Decrease in Resistance with Temp Increases
• Suited for temperature measurements because it
  exhibits steep resistance change as temperature
  increases
• Positive Temperature Coefficient (PTC)
• Increase in Resistance with Temp Increases

NTC Curve
PTC Curve
10
Quality Thermistor (QTLCA-14A3)

• 10kO Thermistor (room temp)
• Excellent NTC Value
• Very Accurate
• /- 1C (OC to 70 C)

11
Wheatstone Bridge Configuration

• Measuring instrument used to determine an unknown
  resistance value when three other resistance
  values are known.

12
Steinhart-Hart Equation

• Relationship between R and T.
• RT Resistance at Temperature T (O)
• A, B, C, D are constants given by manufacturer
• T Temperature in Kelvin

13
Testing Temperature Sensor

• Connected three 10kO resistors in a wheatstone
  bridge configuration with the 10kO thermistor.
• R1 R2 R3 10kO
• R4 RT
• Vin 5V

14
Testing Temperature Sensor

• Placed finger on sensor and Measured Vout with
  Oscilloscope

Vout 2.812 V
15
Testing Temperature Sensor

• R3 10kO
• Vin 5V
• Vout 2.812V
• Solve for RT

RT 7780.94O
16
Testing Temperature Sensor

• Solved for T using Steinhart-hart equation

RT 7780.94O
T 303.96 K 273.15 30.81C
17
Success!

• Very good results because the temperature was
  32.6C when I placed my finger on a digital
  thermometer for 30 seconds.

18
Temperature Threshold Calculation

• 2 Equations
• Stein-hart Hart Equation
• Wheatstone Voltage Divider

19
Thermistor Noise Issue

• V 0.0542 (V/K)T -13.66V
• Actual /- ?V .220V/2.11V
• ?T Actual ?V / slope of graph
• ?T 0.11V/0.0542V 2.03K
• Therefore Error /- 2 C

20
Pressure Sensor

• Mechanical Switch
• Four Springs (50 pounds)
• Seat Cushion

21
Testing Pressure Sensor

• Input a 5V voltage
• Do not trigger switch
• Open-Circuit when someone is not sitting in the
  seat
• Outputs constant 5V voltage

22
Testing Pressure Sensor

• Input 5V Voltage
• Trigger the switch
• Closed-circuit when someone is sitting in the
  seat
• Output 0V Voltage

23
Sensor Integration to PIC

• Take output of sensors and send into Pin 2 in
  the PIC

24
Transmitter Module
25
(No Transcript)
26
Oscillator
Without Capacitor

• Our oscillator is outputting an approximate
  square wave at 4MHz
• We used a capacitor (10 µF) from the output to
  ground in order to make the square wave more
  ideal.

With Capacitor
27
ADC conversion

• The Analog signal inputted to the PIC will be
  referenced by the power of the PIC. The PIC has
  a resolution of 10 bits. Which means if the
  reference voltage is 5V. The measured accuracy
  is 5V/1024 4.8mV
• EXAMPLE
• If the temperature sensor is outputting 2.7V with
  a reference of 5V. The value the PIC reads will
  be (2.7V1024)/5V 553

28
Byte sending

• With the help of RS232 and the putc function we
  can send byte at a 9600 baud rate.
• Will send 4 Bytes
• 11111111 (start byte)
• 01010101(Chair 1)
• 00110111(Chair 2)
• 01100110(ON)
• 10000000(OFF)
• 00000001(Stop byte)

29
Waking up the PIC after sleep mode

• The PIC only wakes up either on an interrupt or a
  watchdog timer.
• Watchdog timer resets at a maximum of 24 seconds
• Turn off ADC to tell the PIC that ADC is
  completed and enable the interrupt of ADC
  completion at the beginning of the code

30
How Bytes are Being Sent

• Send it 5 times before PIC powers transmitter
  down and goes to sleep for 20 seconds to ensure
  signal is sent.
• Randomize intervals with prime numbers from
  400-800 We did this so that we do not create
  interference with the other transmitter.

31
(No Transcript)
32
Receiver Module

• The PIC powers the receiver by supplying the PD
  pin with voltage. The PIC will select the
  channel for the receiver which will be the same
  as the transmitters channel.(000, 903.37Hz)
• The transmitted asynchronous data will be
  continuously read by the receiver in which it
  will send to the PIC in order for it to send to
  the MAX 232 Chip to the Laptop (interface).

33
(No Transcript)
34
(No Transcript)
35
MAX232CPE

• To transfer the data from the PIC to the
  computer, the voltage levels need to be converted
  from 0V 5V to /-12V used MAX232CPE from
  MAXIM
• Connected the converted signal to the Serial Port
  of the computer using a female DE-9 connector
• Since we were only sending data one way, we used
  Pins 2 and 5 (Rx and Ground)

36
Serial Port

• Created a program (using Visual Basic) to read
  the data from the serial port
• The streamed data was stored on a buffer where
  the program popped the bytes off one by one
• Using predetermined protocol the code checked for
  Start byte (0xFF) and Stop byte (0x01)
• Middle 2 bytes of frame corresponded to the
  Device ID and whether the device was on or off
• Data was stored to a database based on Device ID

37
Serial Port interface program
38
Testing the Serial Port with RS-232 data

• Programmed a PIC to send data to the computer
  using RS-232 protocol (9600 baud, 10bit frame
  8bits per data byte, start bit, stop bit)
• PIC sent 10101010 to the PC and the PC read the
  data into decimal as 170

39
Testing a valid data frame

• We expect 4 bytes to come into the serial port
• First (0xFF) byte and fourth (0x01) byte are
  checked to determine a valid frame

40
Testing the Website

• Sent data through the PIC to the Serial Port and
  had it store into the database
• We put a time stamp to each database entry and
  displayed that on the website
• Refreshing the page would query the database for
  the most recent data

41
Transmitter PCB
42
Receiver PCB
43
Challenges
44
Challenge 1 Change temperature sensor design

• The LM235Z temp sensor operates as a 2-terminal
  Zener diode
• Voltage change was not significant enough

45
Challenge 2 - Change Pressure Sensor Design

• MSI FX1901-0001-50L (50 lbf)
• Could not trigger the sensor

46
Challenge 3 How to connect the sensors and PIC

• We first decided to connect all the components in
  parallel but when someone is sitting on the
  chair, no current will be outputted on the PIC
  and Transmitter (This is in our original review)
• The circuit below takes care of the and case by
  just putting in the voltage of the temp sensor
  into one ADC pin on the PIC.

47
Challenge 4 Connecting the Database

• Understanding multiple programming languages used
  (HTML, Visual Basic, CSS, JavaScript)
• Issue getting the data from the database to
  display correctly
• Solution Connected the database to an object in
  html, converted that data to text using visual
  basic, and displayed the data using CSS and
  JavaScript based on mouse events

48
Ethical Consideration

• Since the signals from the transmitter will be
  sent wirelessly to the receiver through the
  public RF network, the transmitter/receiver will
  be programmed to transmit/receive data as
  efficiently as possible so it does not consume
  the public resources. This way other devices that
  require the RF spectrum will have a chance to use
  the network.
• Solution Use sleep mode on the PIC