Capturing Lightning

I built an analog circuit to trigger a DSLR camera when it detected lightning. I was invited to speak about it for an informal gathering of Michigan Engineering students.

There is no video, but you can download my slides: http://chris.dzombak.name/talks/pdf/2011-10-capturing-lightning.pdf

This talk appears on my site: http://chris.dzombak.name/talks/2011-10-capturing-lightning.html

Transcript

1. Capturing Lightning
   Chris Dzombak
   @cdzombak
   for the Haus Series
   2011-10-18
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2. About Me
   •CSE senior
   • formerly CE
   •Software developer at Nutshell
   • Android, JS/JQuery, HTML, CSS3, PHP, MySQL
   •Hobbies: photography, lighting design, journalism
   •And making cool stuff
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3. Capturing Lightning
   •Device to sense lightning and trigger a camera (any DSLR)
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4. Accomplishments
   •Reliably senses barely-visible test flashes
   •Triggers camera
   •Microsecond-order delay
   •Not much real-world testing yet (scheduling issues)
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5. Lightning
   •Lightning strikes composed of several individual “strokes”
   •Spaced tens of milliseconds apart
   •Strokes are microseconds long
   •On average; stats vary widely
   •http://www.youtube.com/watch?v=dukkO7c2eUE
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6. Delay
   •Delay through my device is single microseconds
   •Trigger delay on the Nikon D700 is ~47 ms
   phototransistor output
   camera trigger signal
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7. So, in theory, it works.
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8. How?
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9. How?
   •Simple.
   •Phototransistor → Capacitor → Op Amp → Comparator →
   Trigger (555 + transistor)
   •And some LEDs indicate status
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10. How?
    •Phototransistor connected (with
    another R) from +Vcc to ground
    •Light causes voltage drop “above”
    phototransistor
    •Capacitor filters out slow changes
    Vcc (+3 V)
    Gnd
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11. How?
    •Op amp amplifies these fast light-
    level changes (voltage drops)
    •Pretty high gain, chosen
    experimentally
    •Currently about 20
    input from phototransistor/capacitor
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12. Aside: Biasing the Op Amp
    •Op amps need +Vcc and -Vcc
    •But we only have 0 and 3V!
    •Solution: connect op amp’s -Vcc to ground
    •+Vcc is still +3 V
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13. Aside: Biasing the Op Amp
    •Op amp can now drive 0 V to 3 V
    •Side note: Chose rail-to-rail op amp
    •Need to “bias” our input so “zero” value is between 0 and 3 V
    •Unlike EECS215 circuits
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14. Aside: Biasing the Op Amp
    •We know input to the op amp will be driven lower, not higher
    •don’t care if it goes high
    •Use voltage divider to get 0.93 × Vcc
    •Buffer through another op amp for high impedance
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15. Aside: Biasing the Op Amp
    Vcc (+3 V)
    Gnd
    stable, high-impedance
    bias voltage: 0.93Vcc
    capacitors ensure Vcc and
    Gnd nets stay stable
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16. input from phototransistor/capacitor
    Aside: Biasing the Op Amp
    bias voltage net
    The rest of this
    should look
    familiar from
    EECS215.
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17. How?
    •Comparator takes two signals
    •Outputs 1 if a•Potentiometer across Vcc/Gnd lets user choose a threshold
    •In practice, this is probably unnecessary
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18. How?
    •Comparator compares op amp
    output to sensitivity chosen on
    potentiometer
    •Drives low if triggered; high
    otherwise
    op amp
    output
    LED lights when
    triggered
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19. How?
    •555 circuit drives a 50ms
    signal high to trigger camera
    •Can’t just drive camera from
    comparator; signal is too fast
    comparator
    output
    output
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20. How
    •Finally, transistor connects two pins on camera’s multipurpose
    connector.
    •47ms later, *click*!
    555 output
    ground
    (connected to camera ground)
    camera trigger pin
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21. Challenges
    •Testing
    • considering building a lightning simulator
    •Single-supply op amp
    • and other factors (choosing rail-to-rail amp, choosing a proper gain)
    •Low-voltage parts
    • most 555’s are 5V
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22. Recommended Reading
    •TI - Op Amps for Everyone
    •Immensely useful: basics, analysis, single-supply, noise, active
    filter design
    •http://www.ti.com/lit/an/slod006b/slod006b.pdf
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23. Questions?
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