Simple circuit that project the car's speed on the wind screen using 7-segment LED displays, a GPS and a microcontroller.
Update December 27, 2016
I had more or less forgotten about this project until someone posted a comment recently which led me to discover that I never published the software. This is corrected now, the Arduino sketch is available below. The project in its current state consists of an Arduino Uno, a TM1638-based, 8-digit, 7-segment display (JY-LKM1638), and a GPS module (EasyGPS from mikroElektronika). It requires the TinyGPS and (modified) TM1638 Arduino libraries (included in the download). The TM1638 library was modified to allow characters to be displayed upside down for proper mirroring in a windscreen.
Update June 5, 2012
Thanks to the many votes this project received I will continue to develop this project here. It is not too difficult. As an MCU platform I will use the Elektor Platino board with some ATmega AVR on it. I do not need lots of memory so I guess I can get away with an ATmega168 that I have lying around.
The display will consist of a few 7-segment LED bricks, maybe 4 or 6 so you can also display time or something else. I have some of these too. The characters must of course be displayed mirrored, that will be a software thingy.
Power supply should be the car battery, I will start with the cigarette lighter output.
A block diagram is available at the bottom of this article.
For the car speed I am less sure. I will start with a GPS but it would be better to connect directly to the car's speedometer. I have access to three cars amongst which an Opel Corsa B without any electronics. I have been told that you can buy adapters to transform "mechanical" speedos into electronic ones, but I haven't yet looked into that. Any advice on this would be greatly appreciated.
If I can use the car's speedometer the device will need some way of setting it up so that it will convert pulses or whatever into the right speed. Also an option for km/h and mph will be needed.
Update June 19, 2012
I have decided to build a prototype out of parts avaible on DealExtreme. They have a very cheap serial 8-digit 7-segment display board with 8 two-color LEDs and 8 pushbuttons for which someone published an Arduino library. I also got a Chinese Arduino clone from them. This all works fine, I tried it last week.
For the power supply I ordered a DealExtreme cheapo cigarette lighter adapter but it has not arrived yet.
The GPS may be a brandnew ultraminiature model from Origin. I am waiting for a sample. It will need a 1V8 power supply though.
Update July 4, 2012
The DealExtreme parts have arrived in the mean time and my proto-prototype is finished, time to try it out in a car. I have modified the display library to mirror the font in such a way that the speed will be readable in the windshield. The setup consists of a Chinese Arduino Uno clone, a Chinese 8-digit 7-segment display, a Chinese cigarette lighter USB power supply, an EasyGPS module from MikroElektronika and a bit of prototyping board to hold and wire everything together. The GPS antenna is a magnetic one that I put on the roof of the car.
The speed (speed over ground) being displayed was very stable and I don't see a reason to connect the device to the speedometer. A GPS will do fine and will work in every car. The display itself could be sharper as characters are double, probably due to the thickness of the windshield. Maybe a sticky slightly opaque film may solve this.
The speed is clearly readable as long as it is shown against the road. I suppose that a short driver who may have to look up may not be able to distinguish the speed against the sky. The weather was not too good today, but the grey sky may be an advantage.
The software uses one of the push-buttons of the display to change between kmh and mph.The units are not very useful, also because kmh and mph are difficult to print with a 7-segment display. Only three digits are needed for speed display, I thought of using some other digits to set for instance a maximum speed. When you pass it the LEDs may flash. Another idea would be to show the remaining distance (birds view) to the destination or ETA.
Photos are below. I will draw up the schematic soon and post the software too.