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Is this the best car computer . . . ever? By Robert Priestley We believe this car computer is right up there with the very best commercial units – and is probably better. We say this because we have yet to find any car computer – commercial or otherwise – which will do as much as this. Would you believe it can even time quarter-mile drags? Just as important is that it is attractively housed and also very small – so it won’t look out-of-place in your car. 86  Silicon Chip M ost car computers can measure the distance travelled, speed, elapsed time, fuel used and engine RPM. Likewise the OzTrip Computer – but it’s how this information is interpreted and presented that makes this computer unique. It has to be the most comprehensive system ever available for the “ordinary” car user (as distinct from motor racing teams with millions of dollars to play with. Then again, with the features it offers, it’s a fairly safe bet that some OzTrip Computers will find their way into race and rally cars!). Want to know the amount of fuel used for a trip? How about the trip cost? How about speeds – current, average or peak? And then there’s fuel usage – current consumption, fuel left, distance until empty, and so on. OK, so any car computer worth its salt can handle many, if not most of these tasks. It’s the extra things that the OzTrip Computer can do that makes this one worth building – even if you already have a car computer. There are in fact 27 different functions available (or 81 if you count the three different quantity display modes!). It’s not limited to just a car computer, either. It can be used as a sprint timer (accurate to tenths of a second over any distance). Think about that for a moment: standing 400m (“quarter mile”) timing from inside the car – no dragstrip timing beams and tele-metry needed here! If you have FEATURES •  27 Functions covering distance, speed, fuel, engine RPM & time. •  3 display formats – metric, US & imperial (km/miles, litres/US   gallons/Imperial gallons) •  8-LED function display •  Sprint timer over any distance accurate to one-tenth of a second •  3 trip meters •  1 count down meter •  Programmable speed alarm •  EFI and fuel flow sensor compatible (software selectable) •  4-digit 7-segment display with day/night brightness control •  Small low cost unit (140 x 110 x 36mm) •  Simple 4-key user interface •  Audible alarm •  Diagnostic functions •  Optional serial data interface for telemetry and control •  PC software available for virtual dashboard data logging •  Can be used as a car or rally computer •  Can be used as a boat fuel computer •  Can be used in many general applications for counting or measuring the closed-off road or other suitable track, the OzTrip Computer will settle any arguments! It can be used as a rally computer – or even a boat fuel computer. It has diagnostic functions, an optional serial data interface for telemetry and control and there is even PC software available for virtual dashboard data logging. It could even be used as a general-purpose data logger not even related to vehicle use. When we say the OzTrip is comprehensive, we mean comprehensive! Details of all the functions of the OzTrip Computer are listed in Table 1. Each of the 27 functions has three readings – metric, US and Imperial. (Just in case you didn’t know, there is a difference between Uncle Sam’s gallons and good Queen Bess’s gallons – 1US gallon (3.785l) = 0.833 imperial gallon (4.546l)). Every time a new function is selected, a brief message appears on the display indicating the Function Num-ber selected. Physically, the computer is assembled on either two or three small PC boards. The third board is only required if input from other than an EFI engine is needed and/or a March 2000  87 fuel-flow sensor is wanted. The two (or three) boards mount back-to-back, connected by either wire links or resistors. All boards are housed in a small (140 x 110 x 36mm) case which can be mounted wherever practical. Because of its size, the OzTrip Computer doesn’t look out of place even in a sub-compact. A screen-printed, red acrylic front panel completes the project, hiding all LEDs and LED displays underneath until they are lit. The four pushbuttons used to select the various functions emerge through the front panel. A small number of connections are required to the vehicle but these should not cause any significant problems. We’ll examine these more closely later. Block diagram Despite its versatility, the OzTrip Computer contains relatively few components, most of the hard work being undertaken by a Motorola 68H705C8 microcontroller. This 40pin one-time-programmable chip is perfect for this application. It has 4 88  Silicon Chip x 8-bit input/output (I/O) ports, 384 bytes of RAM, 8K EPROM, 16-bit internal timer, serial port, interrupt pin and one Timer Input Capture pin. Just Fig.1: despite its versatility, the OzTrip Computer can be broken down into just a few elements. about every resource of this controller is used in this application. We will not attempt to describe what goes on inside the microcontroller; suffice to say that it manages the data presented to it and presents it in an understandable form. Perhaps the best way to understand circuit operation is to refer to the block diagram, Fig.1. On the left are the inputs to the microcontroller: the distance input and the fuel input. It is this raw data that the microcontroller uses to give you the various output functions on the right: the tone generator with its piezo buzzer (used to acknowledge inputs and also to warn you that you are travelling faster than your preset speed, among other things); the status LEDs and 4 x 8 digit LED displays, which of course give you the information in an understandable form. Not mentioned yet is the four-button keypad which you use to select the various functions of the OzTrip Computer and also the optional serial interface (bottom left) which is used if you really want to get serious and input and/or extract data from the computer. A typical application here would be a laptop computer for diagnostics or perhaps even a radio data link – maybe back to the pits? There is also a 5V power supply – actually, two 5V power supplies. One powers the microcontroller and most of the circuitry while the second The OzTrip Computer is assembled on two small PC boards which slot into a tiny plastic case (the third PC board shown here is for pulse conditioning in non-EFI vehicles). A red acrylic panel hides the components but allows the LEDs and LED readouts to shine through. We’ll cover full construction, testing and fitting details next month. gives a reduced output if the vehicle headlamps are turned on, thus dimming the LED displays (both individual and 7-segment) for night driving. We’ll take a much closer look at these various functions a little later. Circuit description As mentioned above, the two main inputs to the microcontroller monitor the speed of the vehicle and the amount of fuel being used. Both of these are “real time” measurements – that is, they present the microcontroller with a continually updated reading of both speed and fuel use. For the moment, we won’t concern ourselves with how this data is read, only what is done with it. The Speed Input conditioning circuits consists of R2, C1, ZD1 & R1 which are used to protect the input to Schmitt trigger IC3f, which produces a clean digital signal to the Interrupt input (pin 2) of the controller, IC4. Similarly, the Fuel Input conditioning circuit consists of R4, C2, ZD2 & R3 and is identical to the Speed Input protection. Two Schmitt triggers are used, IC3e and IC3d, so that the pulse is not inverted. The output of IC3d is connected to the Timer Capture Input, pin 37 of the controller. The microcontroller oscillator circuit consist of C3, C4, a 4MHz crystal (X1) and R5. The microcontroller RESET and Electrical Specifications Characteristic Typical Supply Voltage 12VDC Supply Current Operating 150mA Switched Off 11mA Speed Input Trip Voltage 5V Injector Trip Voltage 12-0-12V Accessories sense circuitry is formed around R12, R26 and ZD3. Because of the likelihood of noise coming in from the ignition wiring, these components protect the inputs to the controller by clipping any voltages above about 5V. D5 & D6 provide additional protection while R13 and C14 form a delay network to the input of the RESET pin. When the accessories are switched off, the RESET pin is at 0V holding the controller in a low power RESET state. When the accessories are switched on, the voltage at the RESET input pin is pulled high by R12 after a short delay while C14 charges. Eventually C14 is charged to +5V taking the controller out of RESET. The controller uses PB5 pin 17 to hold the RESET pin high. When PD3 senses the Accessories have been switched off, the controller executes a shut down procedure and clears PB5, March 2000  89 90  Silicon Chip March 2000  91 causing the voltage at the reset pin to fall to 0 and placing the controller in RESET. If the accessories input was used to directly control the RESET input then correct controller shut down could not be guaranteed and data could be lost. Moving now to the controller’s output ports (there are four of them), we can see that portA is used to drive the individual segments of the four 7-segment displays via transistor buffers Q5-Q12. The controller multiplexes all of the segments. To switch a segment on, the controller drives the output pins PA low. Port B0-B3 is used to address the appropriate 7-segment displays via driver buffers Q1-Q4. PortB (B4) also drives the audible tone generator, formed around IC3a,b & c and a piezo buzzer. When IC3a input is pulled low by PB4, the three inverters hold the piezo input high. But when PB4 goes high the output goes low, allowing the piezo transducer to sound. PortC is used to drive the eight indicator LEDs via transistor buffers Q13-Q20. Eight 1kΩ resistors are used for current limiting of the LED indicators. These resistors are connected between the two PC boards, not only forming the circuit elements but also providing some mechanical rigidity. PortD 7, 5, 4, 3 is connected to the four pushbuttons or “keys” (S1-S4). Each input is normally pulled high by a 10kΩ resistor and pressing a key pulls its input line low. The controller samples the keyboard inputs 200 Project Details This project and software is Copyright to Oztechnics Pty Ltd. A full kit can be purchased from Oztechnics. You can place your order on-line from the Oztechnics secured WEB server or make inquires via email. Visa, MasterCard and Bankcard accepted. All components, case and laser cut front panel filter are included in the kit. Oztechnics Pty Ltd PO BOX 38 Illawong NSW 2234 Phone: 02-9541 0310 FAX: 02-9541 0734 WEB: www.oztechnics.com.au Email: info<at>oztechnics.com.au 92  Silicon Chip times per second, or every 5ms. This is much faster than anyone can press and release a push button. PortD 0,1 provide the RX & TX serial communi- Table 2: the eight indicator cations. This sec- LEDs are split into two tion of the circuit columns. Here are their is optional – IC6 functions. Table 4: four pushbutton and C16-20 – and switches enter data to the is only required Table 3: the ranges of computer. The table at right if serial commu- values displayed for the (Table 6) shows the various combinations of keys. nications will be various functions. required. If fitting as a standalone unit to a vehicle, don’t bother fitting any of these components. The power supply is split into two. A permanent +5V supplied by IC1, a 78L05 regulator, is used to supply the controller and logic while IC2, a LM317 Display values variable regulator is used to supply While the computer has only a the variable display voltage. 4-digit display, it is capable of 6-digits When the headlights are switched resolution in many ranges. When a on, transistor Q21 is turned on via value exceeds the 4-digit display resD3 and the 10kΩ resistor. This effec- olution, the computer alternates the tively shorts the 2.2kΩ (R8) resistor, display between the first four digits which lowers the output of the LM317 and the last two digits on a 5:1 second voltage regulator. This has the effect of dimming the display for night-time driving. Provision has been made on the PC board for six components not used in this version of the computer: IC5, a 24C02 connected to PB6 and PB7; IC8, a 4020 divider and 4 x 1N914 diodes (D8-D11). Display interface The display consists of four multiplexed 13mm 7-segment displays and eight indicator LEDs. The 7-segment displays are used to display messages and values. The messages that can appear on the display are shown in Table 5. The eight indicator LEDs are split into two columns and indicate the current function being displayed, eg DIST REM for Distance Remaining of Journey. Table 5: here’s how to decode the various LED readout messages. March 2000  93 Reproduced life-size, this is the front PC board of the two (or three) in the OzTrip Computer. Two boards are required in EFI-engined vehicles, the third board required only for processing the output of a fuel-flow sensor (see below). ratio. The ranges that can be displayed are listed in Table 3. The LED indicators cover the main functions of the OzTrip computer. These functions are listed in Table 2. The ENTER LED lights when a numeric value is required to be entered into the computer from the pushbutton “keypad”. Keypad interface The keypad interface allows the user to enter all the data required to select the various modes of the computer and enter any required data. This is done through just four push-buttons or “keys”. Some actions require two keys to be simultaneously pressed. The key functions are shown in Table 4 while the various key combinations are listed in Table 6. Connections The computer requires a permanent +12VDC supply, an “Accessories” connection (ie, a +12V supply switched by the ignition switch), speed sender connection, fuel connection and a headlight connection so that the display can be automatically dimmed when the headlights are switched on. The speed sender connection to the computer can be taken from a number of sources. Many modern vehicles (most EFItypes) have an electronic speed sensor to drive the digital speedometer. This, or a speedo cable sensor can be tapped into on the back of the speedo A fuel flow sensor available from Oztechnics for those with carburetted or non-standard EFI vehicles. 94  Silicon Chip instrument panel. Alternatively, a wheel/tail shaft sensor can be installed to measure the vehicle’s speed. If the vehicle’s speed sensor is an analog (inductive) type then its output signal needs to be amplified and conditioned to drive the speed input to the computer. The optional PC board 3 has a high gain differential amplifier for this purpose. A typical speed sender unit produces eight pulses per wheel rotation. The engine type determines the fuel sender connection. Carburetted engines don’t have any fuel flow measurement and will require a fuel flow sensor to be fitted. Oztechnics have a low-cost fuel flow sensor available for this type of vehicle. It is an inductive type, which requires signal conditioning to drive the digital input to the computer. Signal conditioning for the flow sensor is also achieved on PC board 3. Entering values When a value is required to be entered into the computer the ENTER LED illuminates and the display clears to 0. The computer accepts the values entered according to the Function range selected; ie F1-F27 metric, F28-F54 US, F55-81 Imperial format. All values entered are converted back to metric and all calculations are performed in metric and displayed in the selected function range. Values are entered one digit at a time using the push-button “keypad”. There are four keys: a plus (+) and minus (-) key, a Set/Clear key and a Mode/Enter key. The + and - keys select the value of the digit (each time you press the + or key the value goes up or down by one, respectively). The Set/Clear key locks the current digit in and scrolls the display to the left to accept the next digit, while the Mode/Enter key either inserts a decimal point (first press) or acts as an Enter key (second press) and the value displayed on the screen is locked into the computer (see example below). If the Set/Clear key is pressed twice in succession within 0.3 second it clears the display ready for a new entry. Note that the computer will accept up to two decimal places. Enter any more and the computer will display the “Err” message and clear the display ready for another attempt. If no decimal places are required to be entered then the Mode/Enter key still has to be pressed twice to Enter the value. The first press inserts a decimal point, which has no effect on the value of the number entered and the second press of the Mode/Enter key acts as an Enter function. The computer can accept input values up to 999.99 even though the first digit scrolls off the display. For example to Enter “18.2” into the computer you would use the sequence of keys tabled below. This concludes the introduction to the OzTrip Car Computer. Next month we’ll conclude with the complete assembly, testing, installation and calibration procedures. SC