Jonathan P Dawson
2013
You need the following packages to build the speedometer:
- avr-gcc
- avr-libc
- uisp
Compile the source code:
> make
Set the AVR fuses:
> ./set_fuses
Program the AVR:
> make load
| 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 |
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.
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.
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.
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).
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.
The finished unit:
