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Digital speedom & fuel gauge 74  Silicon Chip meter Update your car’s dashboard to this fancy electronic display. It gives digital readouts of speed and the fuel remaining, and includes a 6-position overspeed alarm as well. Pt.1 – By JEFF MONEGAL Many modern cars have digital instrument panels and these are preferred by some people because they are easy to read at night. Not only that but they look fancy as well. If your car’s dashboard could do with an update, this electronic version will do the job. To simplify things as much as possible, the circuit is based on a Motorola 68705P3 microprocessor. This accepts inputs from a speed sensor, the fuel tank sender (via an A-D converter) and an overspeed switch and provides outputs to drive the dis­plays, alarm buzzer and warning lamp. The speedo display consists of a 3-digit 7-segment LED module which directly indicates the speed in km/h (kilometres per hour). An identical 3-digit display module is used for the “fuel gauge” and can display the fuel remaining in the tank in either litres, gallons or as a percentage (set during the calibration procedure). Both the speedo and fuel displays are automatically dimmed at night, so that they are not too bright. A dash-mounted “low-fuel” warning lamp lights when the reading drops below 9 (ie, below 9 litres, 9 gallons or 9%). As the amount of fuel in the tank hovers around 9, the lamp will slowly switch on and off as the fuel sloshes around in the tank. There are six overspeed alarms and these are selected by the driver using a simple rotary selector switch. The alarm speeds chosen are 62, 72, 82, 92, 102 and 120km/h. These figures were chosen to allow the driver to sit comfortably on the speed limit while still providing sufficient warning if the limit is exceeded. If the preset speed is exceeded, the circuit immediately sounds a buzzer and flashes the speedo display at a 1Hz rate (ie, once every second). This continues for as long as the preset speed is exceeded but, if necessary, the buzzer can be silenced for 30 seconds by pressing an “Alarm Mute” button. The display continues to flash even after the Alarm Mute button has been pressed, unless the speed drops back below the warning threshold. How it works Fig.1 shows the main circuit details of the Digital Dash­board. As indicated earlier, most of the action takes place inside IC2, the 68705P3 microprocessor. Crystal X1 (3.58MHz) and capacitor C8 (27pF) are the external clock components, while Q2 and IC1 are used to generate the interrupts. This circuit works as follows. When power is first applied, C1 charges via a 1MΩ 10-turn trimpot (VR1). As the voltage across C1 rises, the voltage on pin 2 of comparator IC1 eventually exceeds the 4V bias voltage on pin 3 (set by R4 & R5) and the output at pin 6 switches low. This, in turn, forward biases D4 and provides an October 1995  75 +12V +5V R4 1.5k R2 1.5k R8 47k IC1 2 TL071 4 CAL. VR1 1M PA7 2 6 S1 6 S1 1 : 62 2 : 72 3 : 82 4 : 92 5 : 102 6 : 120 C2 22 R5 6.8k 6 7 VPP TMR/BT 26 PA6 RN2 D4 1N914 7 3 C1 0.47 ALARM MUTE S2 R3 10  5 1 20 2 4 21 3 22 23 24 25 C INT 18 +5V PA1 17 5 R36 10k PA3 PA4 PA5 4 FUEL GAUGE A-D CONVERTER START CONVERSION 12 COUNT 13 END CONVERSION 14 Q3 BC548 7 E R18 100k R17 10k Q4 BC548 C B +12V B C Q5 BD679 E C10 100 LOW FUEL 12V +5V PB5 XTAL PC0 8 9 PC1 10 PC2 D6 1N914 CLK SPEED DISPLAY RESET LATCH BRIGHTNESS EXTAL R15 10k PB0 CLK PB1 RESET PB2 PC3 11 FUEL DISPLAY LATCH BRIGHTNESS 1 10 D9 1N914 PLASTIC SIDE B E E C B +12V VIA IGNITION SWITCH V+ O/P DIGITAL SPEEDO AND FUEL GAUGE C IN OUT D3 1N4004 C12 470 R7 10  ZD1 15V GND IN C3 2200 C4 2200 IC5 78L08 IC4 7805 GND OUT C13 10 C14 0.1 OUT C5 22 +8V TO FUEL GAUGE +5V C6 0.1 CHASSIS Fig.1: the circuit is based on IC2 which is a 68705P3 microprocessor. It accepts pulses from a speed sensor and the fuel gauge A-D converter and drives the speed and fuel displays. It also drives an overspeed alarm buzzer (via IC3) and a low-fuel lamp via Q4 and Q5. interrupt signal to the microprocessor (IC2) which then executes an interrupt routine in its software. IN GND VIEWED FROM BELOW I GO HALL DEVICE (SMOOTH FACE) GND 76  Silicon Chip C PB4 16 RESET 28 C7 10 R14 56k C11 0.1 R11 4 10k B +5V +5V 9 R13 1k R16 10k C8 27pF +8V IC3c 14 PB6 X1 SPEED SENSOR 8 6 IC2 68705P3 V+ HALL SENSOR O/P D7 D8 1N914 R12 CAR 1N914 15k LIGHTS D5 1N914 C9 10 PA2 RN9 +5V 5 R9 82k PA0 19 PB7 GND IC3b 3 ALARM BUZZER E E MAGNETS 2 R10 33k PB3 15 R6 10k B 1 RN1 10k RN3-RN8 Q2 BC548 27 IC3a 4093 During this interrupt routine, pin 18 (PB6) of IC2 briefly goes high and turns on Q2. This discharges C1 and thus resets the interrupt timebase. VR1 sets the timebase frequency to provide calibration of the speed display, while C2 decouples the bias voltage set by R4 & R5. A Hall Effect device is used as the speed sensor. It provides a 5V signal +8V A LED2 YELLOW R19 470  SET EMPTY VR2 1k R20 33k 2 3 TO FUEL SENDER R22 10k R26 470  6 LDR  A C16 100  LED1 +8V Q6 BC548 R25 10k B R24 100k E B R28 2.2k Q8 BC558 2 C18 100 R33 10k END OF CONVERSION R34 10k D10 1N914 7 IC7 CA3140 R31 22  C19 10 C E 3 K C +5V C R27 680k C17 4 0.47 C15 10 B R29 1k 7 IC6 CA3140  K R23 100k R21 100k R30 1k D11 1N914 6 4 SET FULL VR3 50k 4 Q9 C BC548 B 8 IC8 555 6 2 1 C21 .01 R32 10k Q7 BC548 COUNT 5 C20 .01 E R35 1k 3 START CONVERSION E +8V R19 820  SET EMPTY VR2 1k B 3 TO FUEL SENDER 2 7 IC6 CA3140 4 6 A C17 0.47 K E C VIEWED FROM BELOW R23 100k R20A 33k COMPONENTS FOR POSITIVE SENDER FUEL GAUGE A-D CONVERTER Fig.2: this circuit converts the analog output of the fuel sender to a digital signal that can be applied to the microprocessor. IC6 functions as an amplifier and this drives comparator IC7 which, in turn, controls oscillator stage IC8. at its output each time a magnet passes its sensitive area. In practice, two magnets are used and these are secured to the drive shaft of the vehicle, with the Hall Effect device mounted nearby – see Fig.7. The output from the Hall Effect device is fed to pin 17 (PB5) of IC2. Note that the output is normally pulled low via a 10kΩ resistor to ground. The signals to drive the speed display module appear at pins 8-10 (PC0-PC2) of IC2. These signal lines are labelled Clk, Reset and Latch. Note that the same Clk and Reset lines are also applied to the fuel display module. Only the Latch signals are different, the fuel display module being driven from pin 11 (PC3) of IC2. Speed buzzer & dimming Pin 27 (PA7) of IC2 is the speed alarm output. This output switches high when the vehicle’s speed exceeds the overspeed setting, as selected by switch S1. Depending on its position, S1 simply pulls one of the PA0-PA5 inputs (pins 20-25) to +5V. The remaining inputs are normally held low by 10kΩ resistors RN3-RN8 (part of a resistor array). When the set speed is exceeded and pin 27 goes high, it activates a Schmitt trigger oscillator based on IC3a. R9, R10, D5 & C9 set the oscillator frequency to about 3Hz, with the output appearing at pin 3. This drives transistor Q3 via inverter stage IC3b to pulse the buzzer on and off. IC3c is also connected as a Schmitt trigger oscillator but in this case is used as a brightness control for the two display modules. This oscillator is permanently enabled since pins 8 & 9 of IC3c are connected together. When the car’s lights are off, the duty cycle is about 50:1, as set by R13 and D9 in the feedback path. The output appears at pin 10 of IC3c and drives the blanking input (pin 4) of a 4511 display driver in each display module. If, however, the lights are turned on, D8 becomes for­ward biased which means that R12 is effectively connected in parallel with R14 each time pin 10 of IC3c goes high. This reduc­es the duty cycle to about 12:1 and this in turn considerably reduces the brightness of the displays. D7 is necessary to protect IC3 against excessive voltage (+12V) from the lights circuit. It does this by clamping the inputs of IC3c (pins 8 & 9) to the +5V rail – ie, pins 8 & 9 of IC3c can never rise above 5.6V. Low fuel lamp Q4 and Q5 control the low fuel lamp. When the microproces­sor detects low fuel (via an A/D converter), pin 15 (PB3) switch­es low. This turns Q4 off and so C10 slowly charges via R18. As the voltage across C10 rises, the voltage on the emitter of Darlington transistor Q5 also rises and so the lamp gradually turns on to full brilliance. October 1995  77 +5V 3 16 Q0 12 CLK 13 MR 10 LE CLK RESET LATCH Q1 Q2 Q3 7 9 7 1 6 2 5 6 4 IC1 4553 4 C1 .001 3 C1A C1B DIS 11 A B IC2 4511 C D BI LE 5 DS3 DS2 16 LT 8 15 R1-R7 DIS3 68  7 a a 12 6 b b a 11 4 c c f g b 10 2 d d c 9 1 ee e 15 9 f d f 14 10 g g COM 3,8 R9 120  B DIS2 3,8 3,8 E Q2 BC558 C 1 B E Q3 BC558 C B DS1 2 8 DIS1 D1 4x1N914 BRIGHTNESS Q4 BC558 C +5V D2 B D3 E E Q1 BC558 C B E C VIEWED FROM BELOW D4 R8 27k SPEEDOMETER/FUEL GAUGE DISPLAY Fig.3: the display driver circuit is based on a 4553 3-digit counter (IC1) and a 4511 BCD to 7-segment decoder (IC2). The displays are multiplexed by using IC1 to switch driver transistors Q2, Q3 and Q4 on and off at the appropriate times. Diodes D1-D4 and transistor Q1 provide leading zero blanking. Conversely, if the microprocessor detects more than 9 (gal­ lons, litres or percent) in the fuel tank, pin 15 goes high. This turns on Q4 which discharges C10 and the low fuel lamp dims to off. R18 and C10 set the lamp dimming time constant to about 10 seconds. As well as ensuring that the lamp gradually comes up to full brilliance at the low fuel point, it also prevents the lamp from rapidly fluctuating between on and off as the fuel sloshes around in the tank. Power supply Power for the circuit is derived from the car’s battery via the fusebox. D3 provides reverse polarity protection, while R7 and ZD1 provide protection against any abnormally high voltage spikes that may be present. The resulting +12V rail is then fil­tered using C3 and C4 and fed to 3-terminal regulator IC4 which provides a +5V rail. This +5V rail is used to power the ICs, the timebase cir­ cuitry and the 78  Silicon Chip LED display modules. In addition, a second 3-terminal regulator, IC5, is used to provide a +8V rail to power the A/D converter circuitry. The low-fuel lamp driver circuit (Q4 & Q5) and the buzzer driver circuit (Q3) are powered from a +12V rail derived from the input to IC4. A/D converter Fig.2 shows the fuel gauge A/D converter circuit. This circuit is necessary to convert the analog output of the existing fuel sender in the car to a digital signal that can be applied to the microprocessor (IC2). The sender in most cars consists of a rheostat with the movable arm connected to some sort of float arrangement. When the tank is full, the resistance is at minimum. Conversely, maximum resistance is obtained when the tank is empty. However, some vehicles have fuel senders that work in the opposite sense. This type of sender is catered for by making a few minor changes to the input circuitry, as shown on Fig.2. Note, however, that the circuit will not work with cars that have capacitive type fuel senders. To our knowledge, the only vehicle that uses this type of sender is the Ford Falcon range from model XD and on. Tests showed that the resistance of most senders varies from 0Ω when full to 2kΩ or more when empty. As the resistance varies, in response to a changing fuel level, the voltage applied to the inverting input (pin 2) of IC6 varies accordingly. IC6 is wired as an inverting op amp with a gain of three, as set by R23 and R20. R21 and R22 bias its non-inverting input to about 0.7V, while the amplified signal output appears at pin 6. As the fuel level falls, the voltage at pin 6 also falls and vice versa. Following IC6, the signal passes via a filter network (R27 & C18) to pin 3 of comparator stage IC7. This filter network provides a long time constant (68s) to prevent short-term fluc­tuations in the reading as the fuel sloshes around in the tank. Q8, LED 2, R29 and R30 form a constant current source and this charges C19. The resulting linear saw­ tooth YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM This view shows the speed sensor assembly and the two magnets which are mounted on the tailshaft (or on a drive shaft). The sensor assembly is covered in heatshrink tubing and sealed with silicone sealant to make it waterproof. waveform is applied to pin 2 of IC7 and compared with the DC voltage across C18. When the microprocessor starts the conversion process, its pin 12 output (PB0) pulses high. This briefly switches on Q9 which discharges C19. As a result, pin 6 of IC7 goes high and this starts an oscillator stage based on 555 timer IC8. C19 now charges via the constant current source (Q8). When the voltage on pin 2 of IC7 rises above that on pin 3, pin 6 switches low and stops the oscillator. At the same time, it pulls pin 14 (PB2) of the microprocessor low via D10 to signal the end of conversion (EOC). Note that D10 and R33 provide 8V to 5V level translation for the microprocessor. During the conversion process, the microprocessor counts the pulses at the pin 3 output of the oscillator (IC8). This count is then processed and the resulting information used to indicate the amount of fuel in the tank. VR2 provides the zero calibration when the tank is empty, while VR3 adjusts the fre­quency of the oscillator and allows the reading to be correctly set when the tank is full. The circuit based on Darlington pair Q6 and Q7 is used only at power on. Because of the long time constant formed by C18 & R27, the fuel readout would not otherwise be accurate for several minutes when the ignition is first turned on. This problem is solved as follows. When power is first applied, C16 pulls the base of transistor Q6 high via R25. This switches on the Darlington pair (Q6 & Q7) and lights LED 3. This LED is positioned against the face of an LDR connected to pin 6 of IC6. As a result, when the LED turns on, it lowers the resistance of the LDR to SATELLITE ENTHUSIASTS STARTER KIT WARNING! The fuel gauge circuit in this design derives its input from the car’s existing fuel sender. As a result, the existing fuel gauge in the car must be disconnected and is thus rendered inoperative. If you don’t want to do this, then you might consider building only the digital speedometer section of the design. Alternatively, you can install a 2-position switch (with break before make contacts) to select between the existing fuel gauge and the digital fuel display. Finally, readers are reminded that it is illegal to tamper with a car’s odometer. In particular, it should not be disabled or removed from the vehicle. YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● just a few hundred ohms. C18 can now charge up quite quickly via the LDR & R28 and so the correct fuel level is displayed almost immediately after the ignition is switched on. In the meantime, C16 charges via R24. After a few seconds, Q6, Q7 and LED 1 turn off and the resistance of the LDR rises to over 5MΩ. As a result, C18 now mainly charges via R27 and so the time constant is increased to over one minute to prevent fluctua­ tions due to fuel slosh as described previously. Fig.2 also shows the alternative circuit for fuel senders that work in the opposite sense to normal (ie, low resistance when the tank is empty; high resistance when the tank is full). In this case, IC6 is configured ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 October 1995  79 PARTS LIST MAIN MODULE 1 main PC board, 168 x 85mm 2 10-way ribbon cables with IDC sockets 2 10-way PC-mount IDC plugs 1 case to suit (not part of kit) 1 12V mini buzzer 2 button magnets 1 U-shaped heatsink to suit 1 6-position single-pole rotary switch (S1) 1 momentary contact pushbutton switch (S2) 1 knob to suit rotary switch 1 1MΩ 10-turn trimpot (VR1) 1 1kΩ 10-turn trimpot (VR2) 1 50kΩ 10-turn trimpot (VR3) 1 12V panel-mount lamp & bezel 1 28-pin IC socket 1 14-pin IC socket 4 8-pin IC sockets Semiconductors IC1 – TL071/TL081 op amp IC2 – 68705P3 programmed microprocessor IC3 – 4093 quad Schmitt trigger IC4 – 7805 regulator IC5 – 78L08 regulator IC6,IC7 – CA3130 op amp IC8 – 555 timer Q2,Q3,Q4,Q6,Q7,Q9 – BC548 NPN transistor Q5 – BD679 Darlington transistor Q8 – BC558 PNP transistor D3 – 1N4004 silicon diode D4,D5,D6,D7,D8,D9,D10, D11 – 1N914 silicon diode ZD1 – 15V 1W zener diode LED1 – 5mm high brightness LED LED2 – 3mm yellow LED X1 – 3.58MHz crystal 1 Hall Effect sensor Capacitors C1 – 0.47µF MKT C2,C5 – 22µF 16VW electrolytic C3,C4 – 2200µF 16VW electrolytic C6,C11,C14 – 0.1µF monolithic C7,C9,C13,C15,C19 – 10µF 16VW electrolytic C8 – 27pF ceramic C10,C16,C18 – 100µF 16VW electrolytic C12 – 470µF 16VW electrolytic C17 – 0.47µF monolithic 80  Silicon Chip C20 – .01µF MKT C21 – .01µF monolithic Resistors (0.25W, 5%) R2,R4 – 1.5kΩ R13,R29,R30,R35 – 1kΩ R3 – 10Ω R5 – 6.8kΩ R6,R11,R15,R16,R17,R22,R25, R32,R33,R34,R36 – 10kΩ R7 – 10Ω 1W R8 – 47kΩ R9 – 82kΩ R10, R20,R20a – 33kΩ R12 – 15kΩ R14 – 56kΩ R18,R21,R23,R24 – 100kΩ R19 – 470Ω or 820Ω (see test) R26 – 470Ω R27 – 680kΩ R28 – 2.2kΩ R31 – 22Ω RN1-9 – 10kΩ resistor network 1 LDR (as supplied) DISPLAY MODULE (1 each required for speedo and fuel displays) 2 PC boards, 56 x 46mm ADVERT 4 12mm spacers 1 10-way PC-mount IDC plug 1 red perspex sheet 2 16-pin IC sockets Semiconductors D1,D2,D3,D4 – 1N914 silicon diode Q1,Q2,Q3,Q4 – BC558 PNP transistor IC1 – 4553 3-digit BCD counter IC2 – 4511 BCD to 7-segment LED display driver DIS1,DIS2,DIS3 – 7-segment LED display Capacitors C1 – .001µF ceramic Resistors (1/4W, 5%) R1-R7 – 68Ω R8 – 27kΩ R9 – 120kΩ Where to buy parts Kits for this design will be available from CTOAN Electronics and this company has retained copyright of the PC board designs. as a non-inverting amplifier instead of being an inverting amplifier. The remainder of the circuit is identical. Display modules Fig.3 shows the circuit for the two display modules (ie, the speedo and fuel displays). IC1 is a 4553 3-digit counter with multiplexed outputs. It counts the pulses on its clock input from pin 8 (PC0) of the microprocessor and outputs the resulting data in BCD form. This data appears on the Q0-Q3 outputs of IC1 and drives IC2 which is a BCD to 7-segment decoder. IC2 in turn drives the a-g segments of the LED displays via current limiting resistors R1-R7. The displays are multiplexed by using IC1 to switch driver transistors Q2, Q3 and Q4 on and off at the appropriate times. A crude form of leading zero blanking is used to blank the leading digit (DIS1) when ever its value is zero. This is achieved using diodes D1-D4 and transistor Q1. D1-D4 monitor the Q1-Q4 BCD outputs of IC1. When the lead­ing digit has a value of zero, the four BCD outputs will all be low and so D1-D4 will all be reverse biased. As a result, Q1’s base is pulled low via R8 and so Q1 turns on and Q2 turns off and blanks the leading display digit. For other leading digit values, one or more of the BCD lines from IC1 will be high. Because D1-D4 effectively form a 4-input OR gate, Q1’s base will also be high. Thus, Q1 will be held off and Q2 operates as normal. Note that no blanking has been applied to the second digit (DIS2), as this would add greatly to the circuit complexity. In any case, this digit only reads “0” on the speed display when the vehicle is travelling at less than 10km/h, and “0” on the fuel display when there is less than 9 litres (or gallons, or percent) remaining in the fuel tank. The clock, latch and reset signals for the display module come from the microprocessor (IC2 on the main board), while the brightness signal comes from pin 10 of oscillator stage IC3c as described previously. Next month, we shall give the full constructional details and describe the calibration procedure. Note that several kit versions will be available SC from CTOAN Electronics.