Exposure Tester

thingiverse

This is a tool to test the exposure time of a camera. I have a trail camera that I suspect is taking much longer exposures than it is claiming, so I built this to test it. The idea is to have a rotating pointer that the camera will take a picture of. If you know exactly how fast the pointer is rotating, you can calculate how long the exposure is by measuring the angle the blurred pointer covers in the picture. To make measuring the angle easier, there is an LED at the center of rotation, and another at the tip of the pointer. I built the pointer in FreeCAD, but when I went to build the mount, I wanted to use a threaded hole. I looked up how to create threads in FreeCAD, and found it was really complicated. So I made the mount in OpenSCAD where I had already created spiral_extrude(). This made creating the threaded hole much easier. To make the mount, you will need to also download spiral_extrude(): https://www.thingiverse.com/thing:1958354, and edit the mount.scad file to tell OpenSCAD where to find the spiral_extrude.scad file. You do this by editing the "use" line in the file. The pointer also has a AAA battery holder mounted on the tail end. This acts as a counterweight, and powers the LEDs. I used 100 Ohm resistors in series with each LED, and used hot melt glue to attach a power switch. I printed the pointer twice, once with 1mm holes for the LED wires, and again with 1.5 mm holes. In each case, the printer completely filled the holes with plastic. I changed the hole size to 2mm, because that is what I eventually had to drill the holes to in order to fit solder joints through the holes (the LEDs I had on hand had very short leads). The motor I used was this one: https://www.amazon.com/gp/product/B01M0FVHM7/ref=oh_aui_detailpage_o03_s01?ie=UTF8&psc=1 It is 18 RPM with 60 Hz power. I expect very few people will actually build this. Not many people have a need for this. Update: I need a faster motor. 18 RPM is 1 rotation every 3 1/3 seconds. The camera is claiming 1/12 of a second exposure, so this results in a very small angle. This along with the LEDs looking much larger late an night makes the angle hard to measure accurately. A faster motor should solve this issue. Update 2: I rebuilt this with a 120 RPM motor, that is 1/2 second per rev. 1/12 second should be 60 degrees of rotation. I measured 55 degrees, so the camera was not lying about its exposure times. The new motor was bigger than the original and had wires coming out the side, so I had to re-design the motor mount. The amazon page for the motor has drawing for a different motor that is almost, but not quite the same. The wires on this motor come out at about 85 degrees from the line through the mounting screws instead of 90 degrees as the diagram shows. Not enough to be obvious, but enough to require me to re-print a corrected mount. I re-printed the pointer as well, I added a mount for a switch. This test also proved that 2mm is the smallest hole I can print and still have an actual hole. The switch I designed for is this one: https://www.digikey.com/products/en?keywords=gf-126-3011 The mounting holes are rather small, smaller than an M2 screw. I had some very small sheet metal screws that I used to mount it. The LEDs I used are: https://www.digikey.com/products/en?keywords=c503D-WAN-ccbeb151 I used 100 ohm resistors in series (these do not need to be particularly bright) The battery holder I used is: https://www.digikey.com/product-detail/en/keystone-electronics/2469/36-2469-ND/303818 The two offset holes are at a slightly different angle than the first battery holder I used, and I did not correct this in the pointer design. This means the battery holder is skewed by a couple of degrees. The motor I used is this one: https://www.amazon.com/gp/product/B0714PGC5G/ref=od_aui_detailpages00?ie=UTF8&psc=1 Be aware that the quoted RPM is for 50 Hz, and the motor is only marked for 50 Hz, but it does work at 120 RPM at 60 Hz.