Introduction

This circuit is an eight-point LED display unit that uses NMEA-0183 data from a GPS receiver. Although the GPS provides accuracy to a tenth of a degree, that kind of accuracy is not really necessary in the automotive use for which I designed this display.

Specifications

• Power: 10-16VDC at ~20mA
• Data: NMEA-0183 2.0 format (4800 baud, TTL level, RMC data sentence)

Construction

Mount eight LEDs in a radial pattern every 45°, such as illustrated below. I also put in three LED's in the center with a series resistor to make them dimmer than the eight outer LEDs.

Operation

Apply power to the GPS and to the display. After the GPS initializes, the display should show heading by illuminating one of the eight LEDs.

If the GPS sends a "satellite warning flag" or if there is an RS232 framing (baud rate) error, or if data stops for over four seconds, the display will go dark.

Technical Reference

HARDWARE

• The PIC is configured for power-on delay and brownout reset to guarantee proper resetting of the PIC. MCLR is not used.
• I wired the circuit in midair so that I could pot it in polyester resin. I first epoxied the LEDs to a piece of paper in the desired configuration, wired the circuit, then later constructed a cardboard "dam" around the periphery to contain the resin while it hardened.
  
  
  
  
• After pouring in the blue resin and letting it harden overnight, I ground the block face and edges down to the desired dimensions and buffed the surfaces. The unit is approximately 1.5" wide.
• Because I potted this unit, I brought out the programming port connections at the back of the unit, as illustrated below, in case I wanted to change the program at some point (and I already have.)
  
  

SOFTWARE

• Basic sequence: The main loop code selects the NMEA-0183 $GPRMC data sentence that will be looked for among the multiple sentences in the data stream from the GPS. The serial stream is received by the hardware UART and serviced by the RDA serial interrupt. After the beginning of the sentence is identified as $GPRMC, the remainder of the sentence is saved in RAM. When the end of the sentence is received, the main loop calls the parsing routine to extract certain data fields for display, specifically the heading data (based on true north) and the local magnetic variation that is used to convert true heading to magnetic heading. The display is updated at the rate at which the GPS sends it's data stream out (every two seconds in the case of the GARMIN-38.)
• The code doesn't capture and check the checksum characters at the end of the data sentence.
• Below is the data sentence stream that is transmitted from my Garmin 38 every two seconds at 4800 baud. The RMC sentence is the only one the repeater uses. The heading and magnetic variation fields are red. This is similar to what you should see from your GPS if you look at the data stream with HyperTerm or Procomm or similar terminal emulators.

  $GPGLL,4400.000,N,07143.125,W,020822,A*35
  $PGRMZ,584,f,3*12
  $PGRMM,WGS 84*06
  $GPBOD,347.7,T,004.3,M,MOB,R2NHB*23
  $GPRTE,1,1,c,0,TYC,R2NHB,MOB*01
  $GPWPL,4119.832,N,07205.454,W,TYC*16
  $GPRMC,020824,A,4400.000,N,07143.125,W,000.0,038.6,161201,016.6,W*7C
  $GPRMB,A,9.99,L,R2NHB,MOB,4437.379,N,07300.351,W,066.9,304.5,,V*66
  $GPGGA,020824,4400.000,N,07143.125,W,1,00,2.0,177.9,M,-32.2,M,,*7F
  $GPGSA,A,3,,,,,,,,,,,,,10.1,2.0,3.0*03
  $GPGSV,1,1,00,,,,,,,,,,,,,,,,*79
  $PGRME,15.0,M,22.5,M,15.0,M*1B
  

• If you find that you are not seeing any LED's illuminate and you know that you are getting a data stream from the GPS, it could be that you have the GPS receiver sitting on your bench inside a structure and therefore not able to receive satellites. In that case your GPS deliberately puts out the warning ("V" character instead of the "A" character you see in those sentences above.) My Garmin 38 has a simulation mode that enables me to avoid these errors and make it work on the bench.

CORRECTIONS OR UPDATES

• v1.0 - first release