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Parts have started to arrive so I’ll list what I’m using and some options

ADXL345 Accelerometer

Used to measure acceleration, calculate speed, but more importantly to calculate tilt as the compass doesn’t handle tilt and needs adjustment to its readings to allow for tilt

HMC5883L Digital Compass Module 3 Axis Magnetoresistive Sensor

The compass itself, not tilt compensated

Nokia 5110 monochrome LCD screen with backlighting

For development work I’m going with a nice cheap graphics capable screen, for release I may replace with a nice OLED screen which will be much brighter, and much more expensive

LiPo battery charger

Mini-USB battery charger

LiPo battery, 3.7v 1000mAh

Or may just go with a nice standard PP9 9v battery

Battery level warning board

I think the test LED I popped in to test the board may be a tad too large at 10mm!

3.3 volt to 5 volt 500ma converter board

Could use this with battery above and feed both 3.3 volt and 5 volt as appropriate to the sensors and board as some require 3.3 volt and some 5 volts

BMP085 Digital Barometric Pressure Temperature Sensor

To give me a pressure feed for depth, and a temperature feed which I may or may not use

Logic level converters

Easy way to use 3.3 volt and 5 volt devices together

MC14490P Hex Contact Bounce Eliminator

Switch debouncer to use with the input switches as I’ll be using normal switches rather than the Piezo ones I normally use - on a cost basis

GPS - 66 channel 22 satellite, with 10 Hz updates - MTK3339 chipset with inbuilt data logging

This is a nice to have that is only any use on the surface so may just get dropped but it would be useful to georeference all bearing and distance data

Real time clock

Used as a date and time source for logging and to calculate the speed data

One option as the controller; an Arduino Nano

Might use one of these or maybe a Mini or Pro-Mini

The Boarduino RBBB (Really bare bones) kit and board

I’ve used several of these and they are a really good alternative to the genuine boards, but there is currently no great price difference between them and the Nano

One I made earlier, I have a few of these both built and in kit form

I have loads of options using the parts above and the bits in my boxes and whilst investigating I spotted, and purchased, a 10DOF 9-axis Attitude Indicator L3G4200D ADXL345 HMC5883L BMP085 Module which combines the three-axis gyro, three axis acceleration, tri-axial magnetic field, and pressure sensor on a single board

I will update once some more bits arrive and I get building and coding