Capturing Lightning
Chris Dzombak
@cdzombak
for the Haus Series
2011-10-18
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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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Capturing Lightning
•Device to sense lightning and trigger a camera (any DSLR)
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Accomplishments
•Reliably senses barely-visible test flashes
•Triggers camera
•Microsecond-order delay
•Not much real-world testing yet (scheduling issues)
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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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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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So, in theory, it works.
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How?
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How?
•Simple.
•Phototransistor → Capacitor → Op Amp → Comparator →
Trigger (555 + transistor)
•And some LEDs indicate status
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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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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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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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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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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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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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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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How?
•Comparator takes two signals
•Outputs 1 if a
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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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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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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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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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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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