Intro: Focusing Stand Makes Cheap USB Microscope More Useful
There are digital USB microscopes available for very low cost from merchants on eBay. I bought one with high hopes, but I was disappointed when I tried to use it. Due to the high degree of magnification, the field of view is extremely small, and the depth of focus almost non-existent. This meant that the scope needs to be held rigidly for it to be useful. Using the stand that came with the microscope, it was almost impossible to adjust the focus without moving the scope so that the specimen wasn't even in the field of view any more. And if you tried to move the scope to aim it, you would inevitably change the focus. In addition, the stand included with the scope held it at an angle with respect to the table top. Given the very shallow depth of focus, the microscope needs to be held at right angles to the plane on which the specimen rests.
What I needed was a stand which would do three things:
1. Hold the microscope so that it is perpendicular to the table.
2. Hold the microscope rigidly.
3. Provide a focusing knob, so that the focus can be adjusted accurately without disturbing the specimen.
I wanted a small rack-and-pinion mechanism for the focusing. I considered several options, including re-purposing parts from an old photo enlarger. In the end, it was the same vendors on eBay from whom I bought the microscope that provided the missing parts. I got a macro focusing rail that was very inexpensive and fit the bill perfectly.
With the microscope attached to the focusing rail, all I needed was a rigid stand to hold it in position. I built a stand out of laser-cut acrylic. (I made it at TechShop. www.techshop.ws) The result is a handy microscope that's easy to use and cost well under fifty bucks.
Step 1: Tools and Materials
The microscope is built from the following major components:
1 - USB microscope. A good place to start is to search eBay for "USB microscope". Often they are advertised as having 20x to 200x magnification, with some as high as 800x. Most have either four or eight LEDs on the front to illuminate the subject. Beware: the one I got claimed to have 1.3 megapixel resolution, but it was really only 640x480 with post processing software to enlarge the file size. When I pointed this out to the vendor, he promptly refunded half my money, so the microscope cost me only about twelve bucks! The price on eBay for these scopes is about $20 to $30 including shipping from Asia. Yes, you'll have to wait 4 to 6 weeks to build this project!
2 - Macro Focusing Stand. Search for "macro focus". They are often described as "two-way" for a single axis. The one which fits the acrylic stand described here usually has the brand "Fotomate" printed on the unit opposite the adjustment knobs. Expect to pay about $12 including shipping.
You'll also need the following tools and materials:
- laser cutter
- acrylic sheet, 9" x 12"
- cement for acrylic plastic
- dispenser for acrylic plastic cement
- clamps to hold assembly during gluing
- long nose pliers
- hot melt glue gun and glue
- small cable clamp, the right size to fit your microscope cord
- 4/40 machine screw. 1/4" long
- #4 flat washer
- 4/40 tap, tap wrench and #43 drill bit (0.089")
- self-adhesive silicone feet
Step 2: Assemble the Stand
Cut out the acrylic pieces with the laser cutter. The design for these parts is attached at the end of this article, in Corel Draw format. The plastic sheet should be nominally 1/4" thick (actually 0.227") to use the attached .cdr file. If your material if not the same thickness, you may need to adjust the drawing before you cut the material.
You can assemble the acrylic stand with low-viscosity cement like Weld-On #4 using the capillary method, or you can use thick cement, such as Weld-On #16. Since the laser-cut edges of the plastic aren't perfectly smooth, the thick cement would yield a stronger result. However, I chose to use the thin cement for a better appearance.
Peel the masking paper from both sides of all the pieces. Assemble the two upright sides and the two spacer pieces as shown. Notice that the hole on the top spacer is not centered. Make sure it is oriented to line up with the tripod screw on your focusing rail. The hole goes toward the right when you look at the microscope from the front.
Clamp the assembly firmly from the sides and apply the cement with the applicator syringe. Allow sufficient time for the joints to harden before proceeding.
Once the assembly is strong, remove the clamps. Fit the two tabs into the base plate. Clamp firmly to press the frame you just made against the base, and apply the cement. Allow to cure overnight.
Step 3: Mount the Microscope to the Focusing Rail
The microscope is attached to the focusing rail with hot melt glue.
The focusing rail comes with two captive 1/4" screws. Remove the retaining clips from both screws, and remove the screws. Save the screws for later; you'll use one of them to mount the microscope to the stand.
Position the microscope so that it extends past the end of the focusing rail by about 3/8 of an inch. That will allow the microscope to be moved as close as possible to the specimen without hitting it. Make sure the focusing knob on the scope is turned away from the rail.
Even though the body of the microscope is essentially cigar shared, the segment of it just behind the focusing knob is cylindrical. Align this portion in the slot that runs along the focusing rail. Holding the two pieces together in this position, turn them over and squirt hot melt glue into the slot from the back. Use the glue sparingly, filling the slot but not adding so much glue that it extends above the slot. Hold it securely until the glue hardens.
Step 4: Final Assembly
Although it is not strictly necessary, I thought the microscope would be more durable if I added some strain relief to the microscope's USB cable. I used a cable clamp that was a nice tight fit on the cable. Place the cable clamp in position and mark the center of it's hole on the focusing rail. Then drill with a #43 drill and tap the hole with a 4/40 tap. Fasten the cable clamp to the focusing rail with a 4/40 screw and washer.
Add four self-adhesive silicone feet to the bottom of the stand.
You're now ready to mount the microscope and focusing rail assembly onto the stand, securing it from the back with the 1/4" screw that came with the rail.