William's Workbench

Tonight I etched a printed circuit board for my Nav/Com radio prototype. The toner transfer method using photo paper and a laser printer gave acceptable results. The board would have turned out better had the copper clad board I used been in better shape. It was a good deal on eBay and was a little rough with several pitted areas. A scouring pad cleaned it up pretty good. Below is the board after etching (ferric chloride rubbed on with a sponge, about 3 minutes to etch) and drilling.

I inserted the 7 segment displays to check alignment. This project has been delayed again and again so I can hardly wait to light it up.

Here is a paper printout of my panel design to give an idea of what the finished product will look like. The display opening will have some kind of frosted or tinted plastic diffuser.

Unlike the wide body boys there do not seem to be any affordable options for us general aviation simmers when it comes to the radio stack. Sure there are some fantastic products out there, the SimKits line for instance, but they are priced higher than the Chief of Finance will allow. There are also some cheaper alternatives but in my opinion they sacrifice too much in realism. So, like my instruments, I decided to build my own.

I am starting with a nav/com unit based on the Bendix King KX165 like the image above. The biggest obstacle was finding a suitable display that wouldn’t break the bank. After a lot of searching I came across a 4 digit 7-segment display at Futurlec that was priced reasonably and the correct size. Using the dimensions provided at Futurlec I mocked up a display. While the spacing is tighter than the real thing I think it is close enough for my purposes.

With the display picked out and on order I worked on a face plate design. I am thinking about using 1/8″ plexi covered with a printed label. Here is my CAD design.

For the most flexibility I decided to build a microcontroller based circuit to communicate with the host PC and control the display. Like my instruments this nav/com unit will connect via USB and be controlled by my usbSimCentral program. I proved out my display driving circuit on a breadboard because I was concerned about being able to refresh 20 digits quickly enough. It works fine with no flicker.

Here is the schematic for the display board. This includes the 7-segment displays and shift registers as well as the encoders and buttons.

The PCB design is a little more complicated than needed because I am etching a single sided version with jumper wires for the prototype. If there is enough interest I plan to sell a kit version and will have boards professionally made for that.

The next step is to etch the two boards. I also need to order a few parts as I do not have the surface mount shift registers or encoders. This is a work in progress, stay tuned for updates.

I completed the build of my ASI (air speed indicator) for my flight simulator project. There were some minor issues with the PCB but nothing a few jumper wires couldn’t fix. The most frustrating “bug” was with the USB connector. For some reason the connectors I ordered have an unconnected ground pin. Ground is instead connected to the case or shield of the connector. Once I added a jumper from the connector case to the ground pin everything worked great.

You may have noticed an empty IC socket and unused connectors on the PCB. That is because I designed the board to support two stepper motors, two limit switches, two encoders and a switch but the ASI only uses one stepper and one limit switch.

Time to build instrument number 2!