https://github.com/dawsonjon/speedometer
AVR based GPS speedometer with retro LED display
https://github.com/dawsonjon/speedometer
Last synced: 4 months ago
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AVR based GPS speedometer with retro LED display
- Host: GitHub
- URL: https://github.com/dawsonjon/speedometer
- Owner: dawsonjon
- Created: 2013-07-13T20:13:51.000Z (almost 12 years ago)
- Default Branch: master
- Last Pushed: 2018-05-18T20:29:21.000Z (about 7 years ago)
- Last Synced: 2024-12-01T01:13:18.827Z (6 months ago)
- Language: C
- Homepage:
- Size: 7.01 MB
- Stars: 4
- Watchers: 4
- Forks: 2
- Open Issues: 0
-
Metadata Files:
- Readme: README.rst
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README
==========================================================
Simple GPS Speedometer
==========================================================
Jonathan P Dawson
2013
Requirements
============
You need the following packages to build the speedometer:
+ avr-gcc
+ avr-libc
+ uisp
Build
=====
Compile the source code:
> make
Program
=======
Set the AVR fuses:
> ./set_fuses
Program the AVR:
> make load
Components
==========
=== =========== =================================
ref designation description
=== =========== =================================
1 IC1 ATMEGA16_16PU AVR Microcontroller
2 T1 P55NF06L 10A MOSFET
3 T2 P55NF06L 10A MOSFET
4 C1 22pf ceramic capacitor
5 C2 22pf ceramic capacitor
6 C3 1nf ceramic capacitor
7 C4 1nf ceramic capacitor
8 X1 16 MHz xtal
9 R1 10K resistor
10 GPS 1 GPS Module
=== =========== =================================
Circuit
=======
.. image:: https://raw.github.com/dawsonjon/speedometer/master/circuit.png
An Atmel AVR ATMEGA16 forms the basis of this project. The 16PU part is
available as a 40 pin DIP allowing easy prototyping using strip-board. The
Microcontroller setup is about as minimal as you can get, using a 16 MHz
crystal as a clock, and a pull-up resistor for the reset.
Programming is achieved using a 40 pin DIP test clip, connected to a PC
parallel port.
I am using a two digit display, but a third digit could be added quite easily.
Since the ready made displays I had available were rather too small, I formed
the display from low current 3mm LEDs, using 3 LEDs in series to form each
segment. Since the LEDs are connected in series, there is no need for a current
limiting resistor. The IO pins on the ATMEGA16 seem to be capable of powering a
single segment, but the common cathode (driving up to 7 segments) uses a power
MOSFET to avoid damage to the pins. The 10A MOSFET is overkill, but they
may be driven directly from an IO pin, and I had several available in my junk
box.
A GPS module is used to form the speed sensor. At first I had considered using
a hall sensor attached to the prop-shaft, but GPS modules turn out to be cheap
and easy.
.. image:: https://raw.github.com/dawsonjon/speedometer/master/hardware.jpg
Firmware
========
Again the firmware is pretty simple.
The timer and UART drivers are taken from `AVR lib
`_. The source code
needed to be modified to use a newer naming convention for interrupt events. The
UART code also needed some modification to make reads from the UART blocking.
The application is divided into:
+ seven_segment.c - Seven segment display driver
+ speed.c - Extract a speed in km/h from NMEA messages from the GPS module.
+ speedometer.c - The main application.
The main application reads the speed from the GPS module, and converts to mph.
A safety factor of 6% is added to the indicated speed. This is because the GPS
module gives the speed in a horizontal direction, which would cause the
speedometer to under read. A 6% safety factor more than compensates for the
effect of a 1 in 3 gradient. I consider a 1 in 3 gradient to be the steepest
slope that a car is likely to encounter.
When the device powers up, a flashing pattern is displayed to indicate that a
speed has not yet been received. This could take as long as 30 seconds.
LCD Version
===========
The initial tests of the speedometer proved to be a little disappointing. The
speedometer seemed to work well, but the readout was almost impossible to read
in bright sunshine. A replacement display was needed.
A 16x4 character LCD display was chosen. A back-lit display was chosen so that
it could be read in the dark and in bright sunshine. I chose a green LCD
because I liked the colour, red and blue ones are also available! A 4 row LCD
allows the large numerals to be formed by grouping together user defined
characters. The display only has space for 8 user defined characters, so it
takes a bit of imagination to find a set that will form all the digits from
0-9. The new LCD display was wired into the existing circuit.
.. image:: https://raw.github.com/dawsonjon/speedometer/master/circuit_lcd.png
The two digits for the speed use up the first 6 columns of the display. The
display could have been extended to three digits to allow for higher speeds, or
a kph representation. For my purposes, two digits is quite adequate, and this
leaves another 10 x 4 characters to play with.
Of all the information that can be extracted from the GPS, I decided that the
date and time were the most useful. The date and time require a little
manipulation, since the GPS provides the time in UTC. To allow for daylight
saving (British Summer Time), I added a function to work out from the time of
year whether GMT or BST should be used. This required a function that
calculates the day of the week, since the clocks change on the last Sunday of
March and October. After working out whether to add an hour, a check is
performed to see if the date needs to change (which could
happen in the hour before midnight).
.. image:: https://raw.github.com/dawsonjon/speedometer/master/lcd_readout.jpg
To keep all the bits together, I went for a simple ABS enclosure. To power the
microcontroller, I found a cheap car phone charger. The guts of the charger
make an economical power supply. The charger makes use of a switched mode
regulator which is much more efficient than a 7812 linear regulator.
.. image:: https://raw.github.com/dawsonjon/speedometer/master/assembled.jpg
The finished unit:
.. image:: https://raw.github.com/dawsonjon/speedometer/master/boxed.jpg