GPS board thoughts

I just realized that I should have breadboard-friendly pins on the GPS board, so looks like I will modify it to have those, making it a full featured board (RJ45, USB, and breadboard friendly, 2 LEDs).

I just ordered 10 6DOF accelerometers with magnetometers for a wonderful price of $5.25 each, much much better than the $30 fully assembled board that is available from Pololu and Sparkfun.  This is the next mini board that I will make as soon as finals are over. Purchased from Newark since both Mouser and Digikey have the module for more than $10.  Module is LSM303DLH, which is an ST module I believe.  These modules return 16 bit values for acceleration and magnetometer readings, which I have to determine at what level is the noise comparable to the actual signal.  It will be a little harder since I can’t exactly stick a scope on it.

It will have the following features:

  • bidirectional I2C communication (works with 5V systems!)
  • RJ45 connector
  • breadboard pins for quick prototyping
  • small form factor
  • 3V and 1.8V power supplies
  • Power LED

I try to include LED’s into my boards so that I can quickly see if its getting power.  Another idea: Enable pin to enable board, bring up voltage regulators perhaps?

Goals

My summer goals are to have a working prototype of an autonomous boat, by June 30th.  It will be equipped with the necessary navigational sensors to be able to move autonomously in a small body of water, taking depth measurements.

I am making a pcb for it in KiCad since it is open source, free, and really decent software.  My only other experience was with Eagle Cad, which was good, although I didn’t agree with the small board size limitations, and the cost.

Since these parts will be for a boat and a submarine, size and weight doesn’t matter so much.  I did some brainstorming of a simple method to have data/power lines that connect multiple modules and I thought of a good solution: RJ45 connectors/cables aka Ethernet cables!  Using these connectors, I can have 8 lines of twisted pair and shielded cables that can connect a remote sensor module to the main board.  I believe each conductor can handle about half an amp for standard 24 AWG cable.

For example, the GPS module that I created a board for, has the following lines:

  • VCC
  • Enable
  • GND
  • 3D-Fix
  • D+ and D-
  • RX and TX

With the GPS board that I made, I am able to get all of those onto the 8 conductor RJ45 cable.

One of the major problems with making embedded devices is the connectors.  With unreliable connectors, you have an unreliable robot.  So by choosing Cat5e connectors, I will have a proven connector with conductors that are twisted together and shielded possibly (shielded ethernet cable is more pricy, STP is the acronym I believe). I looked into HDMI connectors, since that has more connections, but the current rating for each conductor was less, and there currently is no ‘wiring your own hdmi cables’ solution yet, like there is for Cat5 cable.  I will get the unshielded cable since I could always shield it if it is necessary.  A golden rule about engineering is to not over build/over engineer something as that cost is unnecessary and hurts the products bottom line.

The plan is for me to design an board to interface an Accelerometer with a magnetic sensor, the main board, and a dummy board to interface the RJ45.  Since I’ll be using the ATMEGA1280, I will have many extra pins, that will be handled with a ribbon cable connection, or perhaps a few ethernet cable connections.  When I build boards for my hobbies, I like to build some future-proofness into them, in the form of extra connectors and prototyping space.