Servo - December 2015

voltage of the two batteries ranges between 8.8V and

5.5V. To measure this with the PIC, I use a voltage divider consisting of two 75K 1% resistors (shown in Figure 3).

If the combined battery voltage is for example, 7.75V, this gets presented to the analog-to-digital converter (ADC) as approximately 3.84V. The ADC, in turn, returns a value of approximately 787 which is — again approximately — 100 times the battery voltage. I reformat this number by inserting a decimal point and sending the result to the display.

I keep saying “approximately” because of accumulated inaccuracies from the actual reference voltage, the voltage divider, and the rounding of 1024 to 1000. Since I’m just interested in a low battery warning rather than a precise voltage measurement, this is good enough. If the measured voltage is below 6V, I display it in red instead of green, which provides a warning that it’s time to charge the batteries.

I recently built my own 3D printer which I used to produce a custom enclosure for this project. The bottom cover slides on, so no screws are needed for fastening it. I finished the printer just a few months before I started this project, so this was my first custom enclosure.

Creating custom enclosures was one of the main reasons for getting the 3D printer, and I was pleased to see that it worked out. The partially assembled results are shown in Figure 4. The STL files for printing the case are included in the project files at the article link. This was printed using a 0.4 mm nozzle, with 0.2 mm layers and 40% infill. Total print time for top and bottom was six and a half hours.

Software

The heavy lifting for this project is done in software. The user interface is handled by the intelligent display, and the application logic is handled by the PIC processor. 4D Systems provides a free integrated development environment (IDE) called Workshop 4, which I’ll just refer to as the 4D IDE.

You have the option of using one of four development tools which vary in flexibility and ease of use. I chose to use the easiest one — Visi-Genie — since it met my needs, and would get me up and running the fastest. With this tool, you design the user interface by dragging various prebuilt objects such as switches, gauges, and text displays onto a template that matches the size and orientation of your physical display. Once the design is complete, you can test it with the 4D IDE.

The 4D IDE test tools display the hexadecimal data signals that come from the screen when you operate the controls. You can emulate sending instructions from the processor by entering the corresponding hexadecimal sequences. As you go through this testing process, you can make note of the data sent each direction and include that in your application logic.

4D Systems has a number of detailed tutorials and demo projects to help new users get started. The current user interface design for the Deluxe Servo Tester is shown in Figure 5. The buttons are all large enough to be activated individually by my fingers. However, I’ve found that using an old Palm Pilot stylus works better, and keeps the display smudge-free. The display hardware includes a small speaker which I enlist to provide user feedback by playing a “click” sound whenever a button is pressed. The application software that runs on the PIC consists of a loop that: • Checks for a message from the display.