• Digital speedometer
• Over-speed alarm
• Fuel economy meter
• Distance and time to destination
• Digital clock with GPS accuracy
• Altitude in metres
• Heading and compass
• Current latitude and longitude
• A USB 2.0 interface so that you can use it with your laptop for navigation or uploading firmware changes...
This is the result. It is small enough to sit on the dash or mount in the radio-sized (DIN) blank panels that many vehicles have.
It is also relatively simple to construct and uses just a few active components on a single PC board with no other wiring. There is no setup or calibration required – just plug it in and away you go!
Fig.1: conceptually, the GPS Car Computer is quite simple. The microcontroller takes data from the GPS module, monitors a number of external signals, formats the data for display and sends it to the LCD. While doing this it can also manage communications with your laptop via USB.
It is designed for use in a moving vehicle, where your attention should not be distracted from the road for any longer than is necessary. Therefore data, such as your speed, is displayed in easily-readable 14mm high digits, while a button press will show other data as needed.
You can also set it to automatically step through the data screens for complete hands-off operation.
A key component is the graphic LCD which allowed a great deal of design flexibility. Three fonts are used ranging from a jumbo font to a small detailed font, while some areas also use graphic symbols.
The display uses a green LED backlight, with the brightness automatically changed between day and night settings, which can be easily configured.
An USB interface allows you to plug the device into a laptop for a complete navigation experience with your position accurately pinpointed on a map. The USB interface also allows you upload new firmware and re-program the device without any special hardware.
Fig.1 shows a block diagram of the unit. The central item is a 40-pin microcontroller which does most of the work, including receiving data from the GPS module, driving the LCD and communicating via USB.
The GPS module used is the EM-408 which we have used in a number of previous projects. Once every second this transmits your current speed, position, etc as a serial data stream to the microcontroller.
When a complete set of data has been collected the microcontroller will calculate parameters such as the fuel economy and format the data for display. This data is then sent at high speed to the LCD so that the display updates very quickly – in the blink of an eye!
The microcontroller also monitors a number of inputs, the three pushbutton switches, a light dependent resistor (or the car’s headlights) for night/day detection and a signal from the fuel injector system used in measuring fuel economy. It also controls the backlight brightness of the LCD and communicates over USB to a laptop.
The only other items of note are two voltage regulators which supply 5V for the microcontroller and 3.3V for the GPS module. The device can be powered from the vehicle 12V battery or from a computer’s USB interface.
The main PC board is very small at 123 x 43mm and piggybacks onto the LCD readout – the complete display/PC board stack is about 25mm high. The dimensions were deliberately kept small so that the complete unit could be mounted in many places in addition to sitting on top of the dashboard.
Fig.3 shows the full circuit diagram of the GPS Car Computer. As discussed before, it is not very complex and is dominated by the microcontroller, a Microchip PIC18F4550. This is mostly concerned with driving the graphic LCD via an 8-bit bus (D0 to D7) and seven control lines.