Evan Grove discusses making a shutter speed tester for photography equipment:

Mechanical shutters are the worst offenders when it comes to mechanical camera maintenance. These precise, fast-moving devices rely on springs and ratchets to work properly. As the years wear on, those springs get stretched out, and the lubricants that keep everything running smoothly get gummy and gross.

In order to give a mechanical shutter the necessary tune-up, you need to be able to check the shutter speed somehow. These shutters can move really, really quickly, though. A stopwatch isn’t going to help you here, and neither is your phone’s camera – neither is fast enough to check a shutter that’s only open for a couple milliseconds. Off-the-shelf shutter test devices are available, but they’re usually targeted at professionals and they’re wildly expensive. So, let’s make our own!

Stiff solid-core wire holds the two LEDs in a consistent spot behind the camera’s shutter. I can bend the wires so that the LEDs line up properly for different camera models. I can also adjust the wires to test different areas of the shutter, but this proved to be much more finicky. The IR photodiode didn’t pick up the IR light when the LEDs were positioned in the extreme corners of the shutter. To remedy this, I fitted the lens cap with some aluminum foil to reflect some light towards the photodiode. I didn’t expect this to work, but it actually did!

Check out the details in the post here.