The FindIT Flashlight: Responsive Tagging Based on Optically Triggered Microprocessor Wakeup

1
The FindIT Flashlight Responsive Tagging Based
on Optically Triggered Microprocessor Wakeup

• Hong Ma and Joe Paradiso
• Responsive Environments Group, MIT Media Lab
• October 1, 2002

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Ubiquitous Computing

• Embedding of sensing, communication, and
  computation into everyday objects.
• Perhaps nothing will be lost again

• Give everyday objects a sense of identity and
  awareness of their environment

3
The Ideal Tags

• Respond to external stimuli
• Last forever
• (or long enough)
• A simple transmitter
• Low cost

4
Possible Technologies

• Bar code reader
• No response from the tags
• Passive RFID with rectenna
• Does not harvest enough power to produce a
  response
• Coded optical receivers (remote control) with
  battery

5
Our Solution The FindIT Flashlight

• Find objects from 10 feet away with 8-bit optical
  code
• Wake up to coded optical beam
• Respond by a flashing LED
• Ambient current draw 500nA
• Runs up to 10 years on a lithium coin cell
• Prototype show, size can easily be reduced by ½.

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Demo
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System Block Diagram
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Detailed Receiver Circuit
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Low Power Design Artifacts

• Avoid linear amplifiers
• Linear amplifiers draw at least 50uA
• New low power comparators can draw as little as
  300nA
• Run processors at extremely low duty cycle.
• Our processors are on 0.001 of the time

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Coding Scheme

• On-off keying on half of the transmitted
  waveform.
• 1MHz instruction cycle to decode 2kHz waveform
• Unsynchronized bits means that rotationally
  symmetric codes cannot be uniquely identified
• Necklace theorem
• 8 bits gt 36 independent codes
• 32 bits gt 70 million codes

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Battery Life

• Panasonic BR1225 3V lithium coin cell with 48mAh
  capacity

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Application Scenario

• Digital storage media
• Content not obvious from packaging

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Conclusion

• Optical tags that runs for over 10 years on a
  single coin cell battery.
• The tag can produce a response based on a coded
  optical message from 8-10 feet away.