Oct 31: Corrected pin numbers for how to wire the SN74AHC125 IC.
In this guide I will hook-up the LS20031 GPS receiver to a PC (most likely a laptop) to interface it to Google Earth. This is a 3-part process:
- Wire the LS20031 GPS receiver to the FTDI serial-USB interface.
- Configure the LS20031 GPS receiver baud rate and other settings using the MiniGPS 1.4 utility.
- Connect the LS20031 GPS receiver either in real-time or batch-mode (import NMEA log file) to Google Earth.
LS20031 GPS RECEIVER SPECIFICATIONS
The LS20031 is a bread and butter GPS receiver. It's very simple to operate like most serial GPS receivers. It's a decent receiver for the money (appx. $50). This receiver is made by LOCOSYS Technology.
- Model: LS20031
- Chip: MediaTek MT3329
- Voltage: 3.3V
- Frequency: L1 1575.42MHz, C/A code
- Channels: Support 66 channels (22 Tracking, 66 Acquisition)
- Update rate: 1Hz default, up to 10Hz
- Hot start: (Open Sky) < 2 seconds (typical)
- Acquisition Time: Cold Start (Open Sky) 35 second (typical)
- Autonomous 3m (2D RMS)
- Position Accuracy: SBAS 2.5m (depends on accuracy of correction data)
- Datum: WGS-84 (default)
- Max. Operating Altitude: < 18 Km
- Max. Operating Velocity: < 515 m/s
HARDWARE PARTS LIST
It's possible to use a 3.3V FTDI interface to wire the LS20031 directly to the PC without the 5V to 3.3V level-shifting, but I am planning to use this prototype for my GPS logger which is controlled by a 5V Arduino Uno. Also, most of the FTDI interfaces in circulation in my geek group (PIN13 Protospace) were purchased from Ebay and they tend to be 5V only.
I tried using resistors as voltage dividers to shift from 5V to 3.3V but the circuit was unstable, especially when an SD card was part of the circuit So I opted to use a 3.3V regulator and a quad bus buffer to drop the FTDI's 5V power pin and the TX signal from 5V to 3.3V. The performance of the final prototype is stable.
- LS20031 GPS Receiver: $50 (Ebay, Pololu, SparkFun, Adafruit)
- Basic FTDI (5V) Serial over USB: $7 (Ebay)
- SN74AHC125 Quad Buffer used as 5V to 3.3V level shifter/converter: $0.5 You can user the popular CD4050 if you prefer. They are not pin compatible but plenty of wiring examples on net. (Mouser).
- LM1117T-3.3V Regulator TO-220: $0.5 (Futurlec, Ebay)
- Electrolytic capacitors 10uf X 2: $0.5
- Right-angle male header 2.54mm/0.1in. Only 5-pins are needed to be soldered to the GPS receiver so we can plug it into the breadboard.: $0.5 (Ebay)
- Jumper wires
- MiniGPS - To configure and debug the GPS receiver
- Google Earth - To map the data tracked by the GPS receiver
- TeraTerm - You can use this simple terminal software to also receive and send data to/from the GPS receiver. In this guide I will use it as a detection tool to find out which COM port the FTDI adapter is connected to.
Hazim Bitar (techbitar)
techbitar at gmail dot com
This is a short video to help visualize the process of exporting the NMEA coordinates from MiniGPS to a log file. Also, the video shows how to connect Google Earth to the GPS receiver to receive NMEA coordinates and other data in realtime as well as how to import the NMEA log file generated by MiniGPS into Google Earth.
Step 1: Wiring the circuit
WIRING THE LS20031 GPS RECEIVER
- VIN......PIN2 (3.3V) which is the middle pin of the LM1117T regulator
- RX.......PIN3 (1Y) of SN74AHC125 IC
- TX.......RX pin of FTDI adapter
- GND...GND rail of breadboard
- GND...GND rail of breadboard
WIRING THE FTDI (5V) ADAPTER
- GND.....GND rail of breadboard
- 5V ........5V rail of breadboard
- RX........TX pin of the LS20031 GPS receiver
- TX.........PIN2 (1A) of the SN74AHC125 IC
WIRING THE SN74AHC125 IC
- PIN1 (1OE) ........GND
- PIN2 (1A)............5V PIN of the FTDI adapter
- PIN3 (1Y) ............RX PIN of the LS20032 GPS receiver
- PIN7 (GND)........GND rail of breadboard
- PIN14 (VCC)......3.3V pin which is the middle pin of the LM1117 regulator
- PIN1 (GND)......GND of breadboard rail and to negative pins of the two 10uF electrolytic capacitors.
- PIN2 (3.3V).......VIN pin of the LS20031 and PIN14 of the SN74AHC125 IC and to the positive pin of the first 10uF electrolytic capacitor
- PIN3 (VIN).........5V pin of FTDI adapter and to the positive pin of the second 10uF electrolytic capacitor