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Ever run out of petrol? This simple project reminds you to fill 'er up by lighting a 12V lamp when_ the fuel drops below a preset level. A low fuel indicator for your car By JOHN CLARKE & GREG SWAIN OST LATE-MODEL cars have a low-fuel warning light but any vehicle that's more than about five years old probably lacks this important feature. This means that you have to remember to check the fuel gauge regularly to avoid runn~ng out of petrol Well that's the general theory but if you drive a "gas-guzzler", it's all too M easy to get caught. Result - an empty tank and you're left stranded. Often, this is only inconvenient but running out of petrol can also be dangerous. That's why late-model cars have a low-fuel warning lamp. It's easy to neglect a gauge but a carefully positioned warning lamp ("idiot" light?) is much more visible and consequently harder to ignore. The Low Fuel Indicator described here is easy to build and can be installed in most vehicles. It lights a 12V warning lamp when the fuel in the petrol tank drops below a preset level. If you've ever run out of petrol or have come close to doing so, this project is well worth building. It could save you quite a few dollars , not to mention embarrassment and inconvenience. Types of sensors Apart from the lamp, there are just three external wiring connections from the Low Fuel Indicator: to +12V via the ignition switch, to vehicle chassis & to the fuel sensor lead. The warning lamp lights when the critical point is reached. 38 SILICON CHIP In most cars, the fuel sensor consists of a float-controlled rheostat which is wired in series with the fuel gauge - see Fig, 1. These two components are then connected to a regulated supply rail derived from the battery. As the fuel level varies, the resistance of the rheostat also varies and this in turn controls the current through the fuel gauge (ie, the fuel gauge acts as a milliammeter). Usually, the higher the float level, the lower the resistance of the rheostat and the higher the current through the fuel gauge. Some circuits work in the reverse sense to this though, so our clrcuit has been designed to accommodate both situations. All you have to do is change the location of a single wire link. B_e warned, however, that this circuit is not suitable for cars that have capacitance-type fuel sensors. In this type of sensor, the fuel acts as a dielectric between two parallel plates PETROL TANK METER SENDER r---------, +V FROM IGNITION '' I ______ ":' L _ ___ JI which form a capacitor. As far as we know, only the Ford Falcon (model XD onwards) uses this type of sensor but you should still check to ensure your car has a rheostat sensor before starting this project. How it works Let's now take a look at the circuit of the Low Fuel Indicator - see Fig.2. It works by monitoring the voltage across the rheostat in the fuel sensor. In greater detail, the voltage from the fuel sensor is first filtered by a 47kQ input resistor and a ZZ0µF electrolytic capacitor which combine to provide a 10-second response time. This 10s delay is built in to prevent false triggering near the preset level due to fuel sloshing about in the tank. IC1a, an LM339 comparator, compares the filtered sensor voltage on its pin 8 input with a reference voltage on pin 9. This reference voltage is derived from a regulated 9.1 V rail via a lOkQ resistor and trimpot VR1. It is fed via another lOkn resistor to pin 9 of IC la, to pin 7 of IC1 b and to pin 4 of IC1c. Note that because the LM339 has PARTS LIST Fig.1: in most cars, the fuel level sensor consists of a float-controlled rheostat. This controls the current through a meter which then indicates the fuel level. 1 PC board, code 05203931, 61 x46mm 1 12V panel mount indicator lamp 1 10kQ horizontal trimpot (VR1) 5 PC stakes 1 3mm machine screw and nut open collector outputs, pin 14 ofIC1a is fitted with an external 10kQ pullup resistor. Thus, depending on the fuel level, pin 14 ofICla will be either high or low. The lMQ feedback resistor provides the stage with a degree of hysteresis, so that it switches cleanly at the transition point and eliminates lamp flicker. ICla drives comparator stages IClb and IClc. One of these stages then drives Darlington transistor Ql, depending on whether link LKl or link LKZ is installed. If the fuel sensor voltage increases with decreasing fuel level, LKl is installed and Ql is controlled by IClb. Conversely, if the sensor voltage falls with decreasing fuel level, LKZ is installed and Ql is controlled by IClc. Let's assume initially that the sensor voltage rises with decreasing fuel level (ie, LKl must be installed). Thus, when the tank is full , the sensor voltage will be low and pin 14 ofICla will be high (ie, at +9.lV). Pin 1 of IClb will thus be low and so Ql and the warning lamp will both be off. As the fuel level falls, the voltage on pin 8 of ICla steadily increases. +V FROM I G N I T I O N D - - - - - - - - - - - - - - - - - - - - - - - ---<11--- - - - . SWITCH ZD1 9.IV 1W 100 + 18VW+ 10k 1M 10k .,. FROM FUEL SENSOR 2.2k FUEL LEVEL SET VRI 10k 47k 220 + 18VW+ 47k CHASSIS~ 12V LAMP SW MAX Semiconductors 1 LM339 quad comparator (IC1) 1 BO679 NPN Darlington transistor (01) 1 9.1 V 1W zener diode (ZD1) Capacitors 1 220µF 16VW PC electrolytic 1 100µF 16VW PG electrolytic Resistors (0.25W 1%) 1 1Mn 2 47kn 310kW 1 2.2kn 1 150!1 1 10Q 5W Miscellaneous Automotive hook-up wire, automotive connectors. When the fuel drops below the preset level, pin 14 ofICla switches low and pin 1 of1C1b is pulled high by a z.zkn pull-up resistor. This high turns on Ql and lights the warning lamp. The lamp then remains lit until the fuel tank is filled above the preset level (or until the ignition is switched off). Now let's consider the circuit action where the sensor voltage falls Fig.2: the circuit uses IC1a to compare the voltage from the fuel sensor with a reference voltage set by VR1. When the fuel in the tank drops below the critical level, pin 14 ofIC1a changes state. This then switches on Q1 & the lamp, either via IC1b & LK1 or via IC1c & LK2. Q1 B0879 .,. .,. PLASTIC SIDE rn E B C LOW FUEL INDICATOR .,. FEBRUARY 1993 39 FROM F SENSO 0 c.n ro CHAS 0 w ....w '° Fig.3: install LKl if the sensor voltage rises as the fuel level drops. Alternatively, install LK2 if the sensor voltage falls with decreasing fuel level. Fig.4: check your PC board for defects against this full size etching pattern before mounting any of the parts. with decreasing fuel level (ie, LKZ 05203931 and measures 61 x 46mm. must be installed). In this case, pin 14 Fig.3 shows the assembly details. of IC la is normally low and thus pin 2 Before starting construction, you first of IClc is also low. have to test the sensor to discover If the fuel level now drops below whether link LKl or link LKZ should the preset level, pin 14 switches high be installed. To do this, locate the and so pin 2 of IClc is pulled high by gauge terminal that connects to the the 2.ZkQ pull-up resistor. This then fuel sensor and short it to chassis via turns on Ql via link LKZ and lights a l0Q 5W resistor with the ignition the lamp as before. on. If the fuel gauge reading goes up, The level at which the circuit trig- . install LKl. Conversely, if the reading gers is set by adjusting VRl. Normally, goes down, install LKZ. this would be set so that the lamp The remaining parts can be installed comes on just before the fuel gauge in any order but make sure that ICl, ZDl and the two electrolytic capacineedle dips into the red danger zone. Power for the circuit is derived from tors are correctly oriented. We used the car's battery via the ignition PC stakes at the five external wiring switch. This +12V (nominal) supply points but these can be considered powers the indicator lamp directly to optional. ensure maximum brightness when Ql Ql should be mounted flat against turns on. the PC board with its plastic side facZDl and its parallel lO0µF filter ing upwards. Bend its leads at rightcapacitor provide the regulated 9.1 V angles so that they pass through the supply rail. This supplies ICl and the holes in the PC board before securing voltage divider for the reference volt- it using a screw and nut. Trim away age, and ensures that the circuit is any excess lead lengths after solderunaffected by fluctuations in the bat- ing. tery voltage. ZDl also protects ICl by clipping any voltage spikes that may Installation Because the PC board is so small, it be generated by the electrical system. can be easily wrapped in foam insulaConstruction tion and hidden under the dashboard. All the parts for the Low Fuel Indi- The warning lamp can be mounted cator (except the lamp) are mounted directly on the dashboard and conon a small PC board. This is coded nected to the PC board via flying leads. Alternatively, the board could be fitted inside a small plastic case, along with the warning lamp, and the entire assembly then mounted on top of the dashboard. The remaining connections are to +12V via the ignition switch, to chassis, and to the fuel sensor lead. If you have trouble identifying the sensor lead, check the colour code in the manual or check the lead at the fuel sensor. Use automotive connectors to interface to the vehicle wiring and solder the other ends of the leads to the PC stakes on the circuit board. Calibration The best way to calibrate the unit is to wait until the fuel gauge reaches the critical level and then adjust VRl so that the device just triggers. This critical level will normally be just above the red zone. If want to calibrate the unit immediately, disconnect the fuel sensor lead at the gauge and connect a lO0Q potentiometer between the gauge and chassis. Adjust this potentiometer until the gauge indicates the desired critical fuel level, then adjust VRl on the PC board until the lamp just comes on. That's it. Now there's no excuse for ever running out of petrol. SC RESISTOR COLOUR CODES 0 0 0 0 0 0 40 No. Value _4-Band Code (1%) 5-Band Code (1%) 1 2 1MQ 47kQ 10kQ 2.2kQ 150Q brown black green brown yellow violet orange brown brown black orange brown red red red brown brown green brown brown brown black black yellow brown yellow violet black red brown brown black black red brown red red black brown brown brown green black black brown 3 1 1 SILICON CHIP