Arduino camera focal plane shutter speed tester
Circuit uses Arduino Nano 5V and its internal pullup resistor. 2 phototransistors are connected between D2 and GND, and second between D3 and GND. D2 and D3 because hardware interrupts are available on those pins.
Digital input pin is set to HIGH, but when light hits phototransistor, that pulls the volatage of the pin to the ground so reading of the pin returns LOW. I use interrupts to capture the exact moment the pin is pulled low or goes high. In interrupt handler routine only the microsecond time is saved.
1604 a.k.a. 16 x 4 character LCD is connected via I2C by 4 wires: SDA, SCL and power supply 5V and GND.
The output on LCD shows exposure time for each sensor as well as curtain travel time of curtain extrapolated to 36mm frame witdth. My sensors (phototransistors) are 30x20mm apart, positioned diagonally for measuring horizontal as well as vertical shutter, so the travel time is multiplied by 1.2 to give result for 36mm or 24mm travel.
uses https://github.com/JChristensen/Timer v2 library https://bitbucket.org/fmalpartida/new-liquidcrystal for LCD via I2C
To find the address of your I2C LCD, use I2C scanner from https://github.com/todbot/SoftI2CMaster (also part of New Liquid Crystal library)