The previous page described a serial baud rate detector contained within a beautiful translucent red acrylic enclosure. This project contains four probe connections (power, ground, another ground for convenience, and serial input), a pushbutton to clear the previous measurement, and a potentiometer dial to change modes (duty cycle, frequency, and bps speed). There is also a slot for the power switch elsewhere on the enclosure.
Hook clips attached to test points
Similar to other projects that require precise machining, I start by laying out the holes in Microsoft Visio. On the diagram below, the largest rectangle is the size of one side of the enclosure, with some space marked out for the internal columns where the lid screws on.
Serial BPS detection project drilling layout
The square holes are made by measuring the corners and milling out a pattern with a 1 mm end mill on a milling machine with a digital readout. I simply lower the end mill into the side of the box at any of the starting points, and then move the x-y table to the next point, cutting along the way.
The round holes are easily made with a drill. However, I put in a little extra effort on the probe connections so that they wouldn’t slide all the way into the enclosure. Instead, the center of the hole is drilled all the way through with a 2.5 mm drill for the wire. Then, a slightly larger hole is milled out to only partial depth to allow slight insertion of the test point collar.
Small drill hole and shallow larger mill hole
As you can see above, this technique produces a slight backstop. After cleaning up the test points, they fit neatly into their holes.
Filing off sides of test points to fit in round holes
The terminal loop test points came from Electronic Goldmine (G13798A 20/$1.00), but they are now out of stock. Higher quality versions (“PC test point - miniature, compact, and multipurpose”) can be purchased from Digi-Key from Keystone Electronics, with part numbers starting at 5000K for $0.36 each. Or, Mouser sells some from Kobiconn, such as part #151-301-RC for $0.30 each. Simple loops of wire are adequate, but I like the finished appearance of the manufactured loop and the variety of colors on the plastic insulated bases.
As received, the cheap test points have irregular raised edges on each side from the molding process. To fit into the round hole, the sides needed to be sanded or filed away. It is difficult to file such a small part. This was accomplished by gripping them in a drill chuck that normally attaches to a portable drill.
Drill chuck holding small green test point collar for hand filing
Another part that needed sanding was the edges of the trimpot. It is big enough and square enough to hold by hand or in a vise.
Trimming notches from trimmer potentiometer
The project enclosure (shown below) is nicely rounded, but that makes it difficult to hold in a vise during machining. I made a plate out of flat scrap PVC plastic that attaches to the enclosure using the existing lid screw holes. The plate is then held by the vise.
Case machined while attached to a plate
Countersunk holes in the plate avoid the heads of the fasteners from making the plate sit unevenly in the vise. The screw heads don’t need to sit flush with the plate. Some depth is perfectly fine.
Left: Countersunk flathead screw for flat PVC baseplate. Right: Machining lid while mounted to plate.
In the photograph on the right side above, notice that the lid can be attached to the same plate. Screw holes are being drilled into the lid to attach the circuit board, display, and nine volt battery holder. The lid becomes the back of the project box, as the LED display is bright enough to be seen through the other side of the translucent plastic.
The display doesn’t have any built-in mounting holes. So, an unrelated circuit board holds standoffs in place while they are being adhered to the display.
PCB acting as a fixture for attaching hex standoffs with epoxy
A common source of frustration with controls on enclosures is the number of ground wires that need to be attached. Previously, I’d either run one ground wire from the PCB to each item (button, trimpot, etc) or I’d run one ground wire from one item to the next item (daisy chain).
Five hole bus cut from PCB
This time, I cut a small 5-pin bus strip off a standard ready-made breadboard PCB. A single ground wire connects the bus to the motherboard. The remainder of the items on that side of the enclosure run their ground wire to the local bus. The PCB trace connects all of the ground wires together.
Finally, let’s take a look at an unusual issue I ran into when programming a certain project.