Strobe Light using DotStar LEDS and ESP32.
The problem with using DotStar LEDS for this is that the LEDS are addressed one-at-a-time; it's a shift register. A fast strobe flash requires clocking the DotStar strip at a high frequency. For example, the images displayed here use a 288 LED strip clocked at 15 MHz, giving an on-time of 780 microseconds (1/1280 second). Exposure time for the 'double bounce' was 2 seconds at ISO 1600.
Unfortunately, the standard Adafruit_DotStar library doesn't operate at that speed. Best I could get with v1.2.0 ("BusIO version") set to 15MHz with 288 LEDs was 1/174 second on-time (software SPI) and 1/83 second on-time (hardware SPI).
Included is dotStar.cpp, a optimized version of the Adafruit_DotStar library v1.1.5. The modifications bring the 288-LED on-time to 1/526 second for software SPI, and 1/1280 second for hardware SPI. It should be possible to run the SK9822 LEDS at 30MHz clock, but it seems to depend on the length of the strip. With 288 LEDs, 19MHz is the highest clock frequency that works reliably.
- ESP32 (https://www.adafruit.com/product/3405)
- DotStar LEDS(https://www.adafruit.com/product/2241)
The program can be compiled with either PlatformIO or Arduino IDE.
The first time it boots it will create a Soft Access Point with SSID: strobelight and PSK: carbondioxide.
Open http://strobelight.local/config to configure the WiFi
http://strobelight.local/ allows for selection of the strobe parameters:
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LEDs length of strip
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Clock MHz SPI clock frequency in MHz
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Fall Height distance object will fall in camera frame
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Strobes/Fall number of strobe flashes during fall
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Strobe begin flashing at rate determined by settings
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Flash single strobe flash
Before taking a picture:
- Dark Room Shield LEDS so that light doesn't spillover onto background
- Set Shutter Speed Set shutter speed long enough to capture the object's motion. One or two seconds
- Set White Balance Turn on the strobe and focus on a sheet of white paper to capture the white balance. The RGB LEDs aren't actually 'white'
- Set Exposure Place the object in frame, open shutter, and press Flash to get a single strobe flash. Adjust ISO (or ISO and aperture if available) to get proper exposure.
Fifteen MHz clock, one second exposure, ISO 1600
