GPS Receiver (minor revision)

In my previous post I brought up several ideas regarding how I wanted to upgrade my GPS receiver. After further investigation, most of my ideas are not practical for a variety of reasons. The PIC microcontroller I was planning on using does not support the math functions I would need to calculate directions and distances between waypoints. Even the saving of waypoints became far too complicated when I attempted to code it (due primarily to the way the GPS passes information). This was made even more difficult because the larger LCD I was planning on using would have required a much larger case than I wanted to use and thus was eliminated as well.

Long story short, the only change made to my GPS was its external appearance. This was accomplished by getting a new frontpanel and replacing the two slide switches with push-on-off switches, as you can see in the photo. The left button is for power and the right turns the LCD's backlight on and off. Since the backlight toggle is performed via a digital input on the PIC I had to modify the circuit slightly by adding a 1M ohm pull-down resistor to the digital input which the pushbutton wires to. This is required because in the previous version the single pole double throw slide switch would either connect the digital input to +5V or ground; since the circuit now uses a single pole single throw pushbutton it cannot perform this same functionality. Therefore, the pull-down resistor grounds the digital input when the switch is not on and then when the switch is turned on its resistance is so high that it is effectively an insulator to ground in comparison to the straight +5V from the power supply circuit.

As I mentioned previously I could most likely completely rebuild this project using the much more powerful Arduino and a larger LCD to accomplish my design goals. However, such an undertaking's cost would approach that of a commercially available GPS unit with considerably more functionality.

GPS Receiver Revision Ideas

In the seven months that I have been using my GPS Receiver I have been thinking about a lot of improvements I could make that could greatly improve the usability and power of the unit. The following is my wish list of improvements.

1. Rechargeable battery - while running off of a standard 9V has its advantages; having a battery with more capacity, lighter weight, and smaller size(assuming I would use a Lithium-Polymer battery) would be very handy
   
2. Ability to save GPS Waypoints - I had thought about this from the beginning of the project but was somewhat lazy and didn't look into it too much; now that I have used the unit for a few months I realize that adding this capability is a must
3. Distance & Direction calculations between GPS Waypoints - this idea came about from doing the calculations by hand on a calculator which isnt' difficult but still a pain
4. User Interface Upgrades - this aspect of the revision directly follows the ideas listed above, especially upgrading the display and adding a rudimentary menu structure to allow the saving of waypoints and performing calculations; I also want to change the backlight switch from a slide switch to a press and hold momentary pushbutton

Since I first developed this wish list I have done some analysis into what I would have to do to realize these upgrades. I considered the possibility of completely rebuilding the unit based around the Arduino, especially since I could upgrade the software via the USB port at any time. The arduino, however is somewhat overkill for this application (16K Flash & 512 bytes EEPROM) and is also a rather large board to fit into a small handheld device with an LCD and GPS receiver. The arduino does come in another form factor called the Arduino Mini. While this is much smaller, at $37 for the unit itself and another $20 for the USB adapter, it is a larger financial investment than I am willing to make for an upgrade of a device such as this (considering I already spent $100 on the current unit).

The rechargable battery has also turned out to be somewhat unfeasible. I did find a small LiPoly battery charger board for $17 and a 1100mAh LiPoly battery for $12 at Sparkfun. This isn't too much money, especially since a rechargeable battery pays for itself over time, but since the charger only works with single cell batteries (which only put out 3.7V) it is not compatible with a 5V system such as this and using multiple cells could get complicated.

After further research I realized that the PIC16F84A that I based the current unit off of contains 64 bytes of EEPROM which could be used to save waypoints. I recalled, however, that my current GPS program is running toward the limits of the PIC's 1024 word Flash memory. The solution to this problem comes from the fact that the PIC16F84A has been replaced by the PIC16F628A. The 16F628A is a pin-for-pin equivalent to the 16F84A so I can reuse my existing circuit board and simply program the new chip and drop it into the existing socket. The 16F628A has the added benefit of 2048 words of Flash and 128 bytes of EEPROM. This should allow the 16F628A to have enough program memory to implement my UI changes and the calculation options I would like to add as well as store up to 32 waypoints (4 bytes per waypoint). The best part is that I have a couple of these lying around so it costs me nothing. The only items I will have to purchase will be a larger LCD, a new case and a couple of switches.

Panel Meter Clock (Part 2)

In order to make the panel meter faceplates read time instead of current I had to make a new set of scales for the three gauges. I started with the templates available on The Chronulator website (I particularly liked the VU meter as you can see in the photo). Since these are vector graphics images you can easily resize them without losing detail like you would in a bitmap image. I used a free conversion tool called FreeSVG to convert the files from PDF to SVG (Scalable Vector Graphics) format which can be read by the free, open source vector graphics editor Inkscape. Note: I believe that the new version of Inkscape will include the ability to open PDFs.

I used Inkscape to resize the faceplates to match the dimensions of the panel meters I had purchased (which were larger than the template). I also inverted the color scheme of the template since text on a white background is more visible than with a black background. I printed the new faceplates out on 4"x6" glossy photo paper (which really shows off the colors of the scale better than regular paper). After cutting out the new gauges with a razor blade, I then used rubber cement to glue each of the completed faceplates onto the original aluminum gauges. I allowed the finished gauges to dry overnight and they went on without issue.

This is an excellent project to get your feet wet with the Arduino, and it looks great too.