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 1
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 2
3. Capturing Lightning •Device to sense lightning and trigger a camera (any DSLR) 3
4. Accomplishments •Reliably senses barely-visible test flashes •Triggers camera •Microsecond-order delay •Not much real-world testing yet (scheduling issues) 4
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 5
6. Delay •Delay through my device is single microseconds •Trigger delay on the Nikon D700 is ~47 ms phototransistor output camera trigger signal 6
7. So, in theory, it works. 7
8. How? 8
9. How? •Simple. •Phototransistor → Capacitor → Op Amp → Comparator → Trigger (555 + transistor) •And some LEDs indicate status 9
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 10
11. How? •Op amp amplifies these fast light- level changes (voltage drops) •Pretty high gain, chosen experimentally •Currently about 20 input from phototransistor/capacitor 11
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 12
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 13
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 14
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 15
16. input from phototransistor/capacitor Aside: Biasing the Op Amp bias voltage net The rest of this should look familiar from EECS215. 16
17. How? •Comparator takes two signals •Outputs 1 if a<b; 0 if b<a •Potentiometer across Vcc/Gnd lets user choose a threshold •In practice, this is probably unnecessary 17
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 18
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 19
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 20
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 21
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 22
23. Questions? 23