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D. MOHAN KUMAR

FUEL RESERVE INDICATORFOR VEHICLES

Here is a simple circuit for moni-toring the fuel level in ve-hicles. It gives an audiovisual

indication when the fuel level dropsalarmingly below the reserve level,helping you to avoid running out ofpetrol on the way.

Nowadays vehicles come with adash-mounted fuel gauge meter thatindicates the fuel levels on an analoguedisplay. The ‘reserve’ level is indicatedby a red marking in some vehicles, butthe needle movement through the redmarking may be confusing and notprecise. This circuit monitors the fueltank below the reserve level and warnsthrough LED indicators and audiblebeeps when the danger level is ap-proaching.

The fuel sensor system consists of atank-mounted float sensor and a cur-rent meter (fuel meter), which are con-nected in series. The float-driven sen-sor attached to an internal rheostatoffers high resistance when the tank isempty. When the tank is full, the resis-tance decreases, allowing more currentto pass through the meter to give ahigher reading.

The fuel monitoring circuit works

by sensing the voltage variation de-veloped across the meter and activatesthe beeper when the fuel tank is al-most empty. Its point A is connectedto the input terminal of the fuel meterand point B is connected to the bodyof the vehicle.

The circuit consists of an op-ampIC CA3140 (IC1), two 555 timer ICs(IC2 and IC3) and decade counterCD4017 (IC4).

Op-amp IC CA3140 is wired as avoltage comparator. Its inverting in-put (pin 2) receives a reference volt-age controlled through VR1. The non-inverting input (pin 3) receives a vari-able voltage tapped from the input ter-minal of the fuel meter through resis-tor R1.

When the voltage at pin 3 is higherthan at pin 2, the output of IC1 goeshigh and the green LED (LED1) glows.This condition is maintained until thevoltage at pin 3 drops below that atpin 2. When this happens, the outputof IC1 swings from high to low, send-ing a low pulse to the trigger pin ofthe monostable (usually held high byR3) via C1. The monostable triggersand its output goes high for a prede-termined time based on the values ofR5 and C2. With the given values, the

‘on’ time will be around four minutes.The output of IC2 is used to power

the astable circuit consisting of timer555 (IC3) via diode D2. Oscillations ofIC3 are controlled by R6, R7, VR2 andC4. With the given values, the ‘on’ and‘off’ time periods are 27 and 18 sec-onds, respectively. The pulses from IC3are given to the clock input (pin 14) ofdecade counter CD4017 (IC4) and itsoutputs go high one by one.

When the circuit is switched on,LED1 and LED2 glow if your vehiclehas sufficient petrol in the tank.When the fuel goes below the reservelevel, the output of IC1 goes low, LED1turns off and a negative triggeringpulse is received at pin 2 of IC2. Theoutput of IC2 goes high for aroundfour minutes and during this time pe-riod, clock pin 14 of IC4 receives theclock pulse (low to high) from theoutput of IC3.

For the first clock pulse, Q0 outputof IC4 goes high and the green LED(LED2) glows for around 50 seconds.On receiving the second clock pulse,Q1 goes high to light up the yellowLED (LED3) and sound the buzzer foraround 45 seconds. This audio-visualsignal warns you that the vehicle isrunning out of fuel. On receiving the

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CIRCUITIDEAS

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third clock pulse, LED3 and the buzzergo off. There is a gap of around two-and-a-half minutes before Q5 outputgoes high.

By the time Q5 goes high and thered LED (LED4) glows, four minuteselapse and the power supply to IC3 iscut off. The output state at Q5 will notchange unless a low-to-high clock in-put is received at its pin 14. Thus LED4will glow continuously along with thebeep. The continuous glowing of thered LED (LED4) and the beep from thebuzzer indicate that the vehicle willrun out of fuel very shortly.

Q6 output of IC4 is connected toits reset pin 15 via diode D3. Thismeans that after ‘on’ state of Q5, thecount will always start from Q0. Ca-pacitor C5 provides power-on reset toIC4 when switch S1 is closed. The out-put of IC1 is also connected to resetpin of IC4 via diode D1 (1N4148). Sowhen your vehicle is refueled abovethe reserve level, LED2 glows to indi-cate that the tank has sufficient fuel.

IC5 provides regulated 12V DC forproper functioning of the circuit evenwhen the battery is charged to morethan 12V.

The circuit can be assembled on aperforated board. Adjust VR1 until thevoltage at pin 2 of IC1 drops to 1.5V.When point A is connected to the fuelmeter (fuel gauge) terminal that goesto the fuel sensor, green LEDs (LED1and LED2) glow to indicate the nor-mal fuel level. VR2 can be varied toset the ‘on’ time period of IC3 ataround 20 seconds.

Enclose the circuit in a small caseand mount on the dashboard using ad-hesive tape. The circuit works only invehicles with negative grounding ofthe body.