The Child Leash Unleashed ECE 445 Senior Design December 2, 2004 Conway Mai . Sophia Kang Introduction The FBI estimates that 85-90% of missing persons are juveniles In approximately 725,000 cases (or 2,000 per day) the disappearance of a child was reported "Juvenile“ Category - 651,209 cases Source: Federal Bureau of Investigation, National Crime Information Center (NCIC), Missing Person File. Pet or Child? Functional Block Diagram Transponder-A Transponder-B Reader Deliverables Reliable and accurate data transmission Predefine range and signal coverage Robust alert system Tamper-proof system Uniquely identify multiple children Portable / Easily concealable package Low-cost solution and implementation Original Design Block Diagram (Reader) Transmitter PIC16F872 Receiver Display PARENT R Tell Child A to Transmit Tell Child B to Transmit E No Valid Child No Valid Child A's WAKE B 's WAKE UP Packe t? UP Packe t? C Yes Yes No Valid Child No Valid Child E A's DATA B 's DATA Packe t? Packe t? Yes Yes I No Valid Child A's END Packe t? No Valid Child B 's END Packe t? V Yes Yes Encode DATA Encode DATA Packet Packet E R Display DATA Original Design Block Diagram (Transponder) Transmitter Temperature CHILD PIC16F872 Receiver Circuit Transponder Flow Diagram IDLE STATE: Wait for signal READ Temperature ENCODE Data Transmit WAKE UP, DATA, END Packets Component Breakdown Transponder Reader Microcontroller Microcontroller PIC16F872 PIC16F872 Transmitter Receiver TXM-900-HP-II RXM-900-HP-II Contact Sensor Channel Switch MTS102 (Temp Sense) COTO-8041 (Relay) LM358N (Op-amp) LM358N (Op-amp) 8-bit ADC (in PIC) Block Diagram (Transponder) PIC16F872 CHILD Temperature Sensor To Parent Device Transmitter Block Diagram (Receiver) Child Data Receiver Display Data to PIC16F872 Parent Implementation Transmitter / Receiver Selection Key Capabilities: Transmitter Channel Selection (8 channels) [902-928Mhz] Clear To Send (CTS) Indicator Power Down Pin No internal power output attenuation! Receiver Channel Selection (8 channels) [902-928Mhz] Contact (temperature) Sensing Performance Requirements: Skin at room temperature (~24˚C-25˚C) Arms & Legs: 31˚C* Vbe vs. Temp Feet & Hands: 29˚C* 700 Vbe Voltage (mV) 650 Head, Chest & Abdomen: 34˚C* 600 550 Typical Variation: 5˚C 500 Possible Variation: 30˚C 450 400 0 50 100 Challenge: Temperature (C) Temperature Sensor Range: 10mV Amplifier insensitivity to small change for ADC *Source: http://www.science.uwc.ac.za/physiology/temperature/temperature.htm Contact Sensing Solution R14 V V 1.6M (Vambient Vbody ) 258 V R13 6.2k *assumes R14=R5 and R13=R6 Theoretical Gain = 258 V/V Tested Gain = 228 V/V Vsat = 3.759 V Example Calculation: 12mV*258 = 3.096 V Saturation condition: Difference > 15 mV Protocol Design To eliminate data corruption and noise on crowded frequency channels. Packetization WAKE UP ENCODED DATA END 8 Bit Packet 8 Bit Packet 8 Bit Packet Encoded Data Bits 0-6: ID of Transponder Bit 7: 0=Transponder not on, 1=Transponder on 7 6 5 4 3 2 1 0 Transponder Flow Diagram START: Initialize, Power Encode DATA Down Packet Transmit All Power Transmitter Packets Up Continuously (1 Sec) Gather Power Down Temperature Data Transmitter R One Child One/T wo Child Mode? Two Child E Select "A" Frequency ALERT C Signal or Idle Increment "No Increment "No Signal or Idle for too long? No Signal Signal" Counter for too long? No Signal Signal" Counter E Signal Signal Valid WAKE Increment "Bad I Valid END Increment "Bad UP Packet? Not Valid Signal" Counter Packet? Not Valid Signal" Counter ALERT Valid V Signal or Idle for too long? No Signal Increment "No Signal" Counter DISPLAY/Indicate if Transponder ON or OFF the child. E Signal Valid DAT A Increment "Bad Packet? Not Valid Signal" Counter R Alert System Alert system checks three functionalities: kbhit() returns false Data from rs232 does not match the packets sent by the transmitter In the encoded data, temperature is “bad.” Timing Issues Three timing issues to consider: Receiver has to be on before the transponder Time between powering up the transmitter and when it sends valid data (1.2 ms) Frequency channel switching (8 – 10 ms) Channel Switch Functional Test Receiver A B Range Testing 40 1 30 8 2 20 Distance(ft) 10 Parent 0 7 3 (Receiver) -40 -20 -10 0 20 40 -20 6 4 -30 5 -40 Distance (ft) Contact Circuit Functional Test Voltage Difference vs. Output Voltage y = 0.2287x + 0.4621 R2 = 0.9665 4.000 > 2.840V 3.500 Output Voltage (V) 3.000 2.500 2.000 < 2.840V 1.500 1.000 0.500 0.000 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 Voltage Difference (mV) Successes & Challenges Data integrity: Challenges Timing issues (inconsistencies): Transmitter power down Noise (same frequency): Wireless protocol Two-Child Implementation: Challenges Range: Output Power Control Communication: Transmitter power down Contact Sensing: Challenges Sensitivity: Differential amplifier of ambient temperature vs. contact temp. Tools and Concepts: Challenges Understanding wireless communication and protocol Learning new development tools Future Recommendations o Higher noise tolerant protocol o Detect multiple children to one parent o Location/Directional detection of child o Passive powered transmitter o Low power consumption o User-friendly interface Conclusion o Accomplished: One Child Communication Two Child Communication Alert System Tamper Proof System o Learned: o Teamwork and Project Management o RF Technology, PIC, C Coding o Design Procedures Acknowledgements o Mo Zhou, our TA o Marty and Chirantan o Professor Swenson o Professor Bernhardt o ECE Parts Shop Questions?
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