G.9.4) GPS & Fishfinder v3

The third version of this device was placed in a slightly larger box than v2. This allowed more space for wiring to allow the engine RPM to be recorded and the tachometer gauge of the boat to be driven.

This unit is connected to the boat via a stainless steel 12-pin plug and socket and by a Toslink fibre-optic connector.

The diagram below shows the logical connections between the Propeller chip and the various peripherals. It is not intended to be a wiring diagram.

Yep, that's nineteen I/O's from one 10 mm x 10 mm chip. Try that with an Arduino...  ;-)

Fishfinder and GPS

The Fishfinder and GPS are connected to the box via an RS232 link, with the information in NMEA-0183 format.

The Fishfinder operates at 4800 baud, the GPS at 9600 baud. Each has a 'cog' dedicated to receiving the information an passing the data to global variables for use by the main block of code. This allows each to operate asynchronously and, again, without the need for any interrupts.

Fuel-flow sensor

Via the 'harness' the box supplies 12V to the fuel flow sensor, together with a ground an a connection to the output of the sensor. The stream of pulses is counted by a dedicated 'cog. Every pulse is counted, enabling the total fuel used and flow rate to be measured and parameters such as flow rate (w.r.t time, w.r.t. speed, w.r.t distance) to be calculated.


Determination of engine RPM is achieved as detailed elsewhere and a dedicated 'cog' counts every pulse arriving at the optical receiver, allowing r.p.m. to be precisely determined.

Digital Tacho

Generation of signals to drive the digital tacho is an adjunct to the measurement of r.p.m. Even at 6,500 rpm there are more than enough clock-cycles between pulses for the generation of this data.

2 Pole Momentary Switch

This switch allows the user to move between the different types of information displayed on the LCD. Each pole of the switch moves the display mode in different directions.

GPS Date, Time & Lat/long
GPS Speed & Heading
Fishfinder Depth
Fuel Used and Fuel Flow Rate

Fuel Efficiency (l/nm)
Speed History Graph
Maximum speed
The number of samples recorded 
in the current file
Fuel flow rate history graph
Current RPM
RPM History Graph

Single Pole NO Switch

In normal operation the LCD will not show anything than RPM / fuel data: if GPS data is available that will be shown; if FF data is available that will be shown. This switch overrides these constraints.


The 4 row x 20 column LCD is the means by which information is presented to the user.

µSD Card.

The µSD card records the logged data. It also contains the cailbration constant for the fuel flow sensor and the compiled binary file for the software. The propeller chip contains in its EEPROM an image of an SD Bootloader program from the Parallax OBEX. When the propeller boots it loads this into RAM, executes it and uploads the binary file from the µSD card.

This makes development nice and easy since there is no need to open the enclosure to change the software.

Indicator LEDs

The LEDs indicate:
  • GPS data being received
  • Valid GPS data read and available
  • FF data being received
  • Valid FF data read and available
  • Data being read to the SD card.
  • RPM pulse received
  • Power available