|Jun 17, 2010, 03:42 PM|
Red-Dot Sight for Keychain Hat-Cam
As great as the keychain video cameras are, there are times when they could benefit from some kind of viewfinder. When recording helicopter flights I find myself wishing I could keep things in the frame better.
When I was into high-power rocketry in the 90's, I had the same problem with my Sony 8mm video camera. When a rocket is traveling a hundred feet per second or more, if you lose it from your viewfinder, you may never find it again. I solved this problem by attaching a Tasco red-dot pistol sight to the side of the camera. Once the sight was aligned with the camera, following a rocket in flight was easy with the 1x red-dot scope. Keep the dot on or near the target object, and you can be confident the object will be near the center of the video frame.
Doing the same with a keychain hat-cam is a little different. A pistol scope is a bit too large and awkward to use with the keychain camera. Even the tubeless type (see last photo below) obstructs your view considerably. But the concept is fairly simple. Focus a bright LED down to a small dot and aim it at a clear reflector that will reflect a portion of the light into your eye. And the apparatus has to be adjustable so you can align it with the camera. Ideally, some optics would allow for collimating the beam of light so it would make the smallest possible dot that appears focused at infinity. A laser would be great for this, but it would not be healthy for your eyes.
Here is my first attempt at a red-dot sight for a keychain hat-cam. The baseplate is .032 aluminum. Hobby stores that carry the K+S Engineering line of materials have this in 4x10" sheets for under $2.
The LED is an ultrabright white LED (20,000 mcd rated), capped with a piece of red plastic, covered with heat-shrink tubing to make as narrow beam as much as possible. An even smaller piece of pre-shrunk tubing is inside the larger tube, and I used a toothpick to paint the inside of it with silver paint to keep the black from absorbing the light. I used a white LED because I have a bunch of them already; it might work better to use an ultra-bright red LED.
The reflector plate is a piece of acrylic cut into a square and fitted with an aluminum mounting bracket. The acrylic is from the protective covering that comes on poster frames and picture frames to protect the picture in the frame. I happened to have one from Walmart. Cutting the acrylic with scissors leaves tiny cracks in the edge, so you have to cut it a couple millimeters large and then sand the edges down to eliminate the cracked areas. I even beveled the left and right edges at about a 60-degree angle, which is roughly the angle the reflector is to your eye, so that when you are looking through the finished reflector the edges are almost invisible.
A 10k ohm trimmer pot is used to adjust the brightness. I added a two-pin female connector to the camera, soldered to the battery connections, for powering the sight and also for external charging of the battery. Double-sided foam tape holds the unit to the camera. I scratched some alignment marks on the camera and the baseplate for when I need to remove the camera and re-attach it.
The LED is angled downward at about 10 degrees, so the reflector bracket is bent to angle the reflector upward at about 10 degrees. This allows the LED unit to be up high out of your field of view of your right eye (mostly). Aligning the dot with the center of the frame is done by moving the reflector along the slot in the frame, and by making slight changes to the bends in the bracket legs and the baseplate where the LED is attached.
At max brightness, the dot is very visible outside on a sunny day, although there are some backgrounds where it is hard to see.
My camera normally can record continuously for 1 hour on a full charge from an external charger. If used continuously at max brightness, the LED draws enough power to reduce the battery's useful recording time to about 34 minutes.
The photos that show the dot were taken against a white sheet of paper with a fluorescent desk lamp about 10" above the paper.
In the next few weeks I will test the unit out and update this blog on results. One thing I know I need to work on is collimating the beam to make the dot smaller. In the photos it looks like a nice small dot, but when you are wearing the hat-cam and it is close to your eye, it looks more like a small red blob than a dot. I think rifle/pistol scopes have lenses that collimate the beam to deal with this. Hopefully I can find a small lens that will do this for me.
For anyone who wants to build one of these, I have included a drawing of the baseplate and bracket that could be used as templates. It is not to scale, so use your printer software to adjust the scale and try printing until the size matches the dimensions indicated. It should still work fine if it is off by a quarter-inch or so.
Here is a short clip where the sight helped me track my mSR in spite of wind and sun glare.
|Jun 24, 2010, 12:38 AM|
I suppose the next step would be some OSD with telemetry, like battery status, etc. As a minimum, i guess a timer could be integrated that would make the dot blink after a preset time.
|Jun 26, 2010, 12:38 AM|
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