Shutter speed tester - first phase
Developing camera shutter speed tester based on AVR microcontroller and phototransistor as a light sensor
A camera shutter opens for a precise moment to allow the exposure of photosensitive film. This is usually in the range of 1 second to 1/1000 second. However, old camera shutters can be slow, causing overexposed pictures or getting stuck during longer exposure times. This can significantly affect the quality of pictures or render the camera useless. This issue can be caused simply by age, dirt, degrading lubricants, improper servicing in the past, etc.
Key features:
- 0.001 millisecond resolution
- 0–5 second measurement range
- Deviation calculation from preselected shutter speed
- Flash sync testing
In this project, I would like to create a device to check the proper function of a shutter, especially after a camera repair, restoration, renovation, or CLA (Clean, Lubricate, Adjust). A non-flickering light source is placed in front of the camera and a light sensor is placed where the film would normally be. When the shutter operates, the light creates a short pulse, which is converted to an electrical signal by the sensor. This tester measures the pulse width between the shutter opening (light is detected, HI signal) and the shutter closing (light is not detected, LOW signal).
It uses a phototransistor as a light sensor because of its quick reaction. I chose a type with the same rise and fall time (0.015 milliseconds), so it does not affect the pulse width; it only creates an insignificant delay. A microcontroller monitors the rising edge and falling edge of the signal, time is measured by a build-in 16-bit timer/counter. All measurement (timer/counter readings) is performed under a high-priority input capture interrupt, outside of the main program loop. The timer/counter clocking frequency is 1MHz, which provides a resolution of 0.001 milliseconds. For maximal accuracy, it incorporates an external precise crystal. In case of any inaccuracy at this level, it will not significantly affect the result, because it is one hundredth of the expected shortest time (1/10,000 second is 0.1 milliseconds).
The tester can also check the shutter-flash synchronization. Supply voltage is monitored, and the battery status is shown. The display backlight intensity is adjustable from the settings page. The user interface is operated via a single rotary encoder.
In the final phase of development, the PCB with parts will be enclosed by a 3D printed case, but I am fully focused on software finalizing and testing right now.
