MX-5 Fuel Warning Light Circuit Diagram - DOC by tbp20087


									                          MX-5 Fuel Warning Light Circuit Diagram



                                   R1                                                                     LED1

tank                                                                                                       TR1
sender                        C1



           Note. Where leads are shown crossing, there is NO electrical connection.

           ZD1 – 15v Zener Diode
           R1 – 1Mohm Resistor                                 Note that the “+” and “-“ marking at IC1
           R2 – 1Kohm Resistor                                 terminals 2 and 3 refer not to positive and
           C1 – 47uF Electrolytic Capacitor                    zero voltage but to non-inverting and
           VR1 – 10K Cermet Variable Resistor                  inverting inputs, although this term is fairly
           IC1 – LM311N Voltage Comparator                     meaningless when referring to voltage
           TR1 – BC337 Transistor                              comparators. (In comparison with
           LED1 – 12V LED                                      conventional operational amplifiers)

           Maplin Shopping List: (
            QF57M BZX C15                           1              £0.08
            WR42V Cermet 10k                        1              £0.44
            M1K          Min Res 1k                 1              £0.07
            M1M          Min Res 1M                 1              £0.07
            VN59P Ax 47uF 16V 105C                  1              £0.08
            QB68Y BC337                             1              £0.08
            QY09K LM311N                            1              £0.49
            CJ61R LED 12V Yellow                    2              £0.58
            KC91Y T2 Box                            1              £1.19
            BL17T         8 pin dil socket          1              £0.14
           Prices correct in February 2001       Sub Total         £3.22

           You will also need a piece of stripboard (Veroboard), 5 PCB terminal pins and some single
           strand equipment wire. I recommend using an 8 pin dil socket to mount the IC – this enables
           the IC to be plugged in after assembly, and enables easy replacement should the IC be
           damaged during assembly or testing. Ensure that you obtain 12v LEDs – ie ones with a built
           in resistor

                                               IC1           R1

From sender           ZD1      C1

                                            VR1                   LED1



                      Stripboard size: 17 holes by 14 strips – 1.9” x 1.6”

   A possible stripboard layout for the circuit. Cut the copper strips at the points marked „X‟

See Appendix 2 for a guide to circuit construction on stripboard.

Construction notes

The IC, transistor, capacitor, zener diode and led must be inserted in the correct orientation.
The IC is marked with a small cut out on one shorter edge, and this must be at the top on the
diagram. The pins are numbered, top to bottom left hand side 1,2 3,4 and on the right hand
side 8,7,6,5.

The capacitor must be inserted with –ve to the bottom on the diagram, and the transistor so
that it looks as shown in plan view from the top (transistor pin diagrams are normally shown
as a bottom view looking up). The zener diode must be inserted with the bar marked on the
cylinder toward the +ve input – top on the diagram. The LED must be inserted so that its
anode (longer lead) is to the top on the diagram, cathode to bottom. Any remote LED must
also be connected in the correct orientation – anode to the extra pin on the 12v rail and
cathode to the pin near the on-board LED. (The LEDs work with a live anode and switched

Connect five wires, one to each terminal pin, and feed them through a grommet in a hole
drilled in the case and terminate them with spade connectors.

The shopping list includes 2 LEDs – one should be mounted on the circuit board, the other
can be mounted remotely in a convenient position, or a normal type of dashboard light can be
used – the transistor will provide power up to about 5w.

Circuit Function description

The voltage comparator, IC1, compares the voltage fed from the lead between the fuel tank
sender and the gauge with a reference voltage of about 5.5v provided by the preset variable
resistor VR1. The sender unit is insulated from the circuit by the resistor R1. Capacitor C1, in
conjunction with R1, provides hysteresis to reduce the indicator reacting to fuel surges on
cornering, braking, etc. The zener diode, ZD1, protects the circuit against voltage spikes
produced, in particular during the engine starting procedure. R2 is required for correct
operation of the IC switching function.
When the voltage between sender unit and gauge is below 5.5v (ie adequate fuel in the tank)
the output of IC1 is low, and the LED is not lit. When fuel runs low the sender unit voltage
increases until it exceeds the reference voltage and the output of IC1 goes high, TR1
switches on and the LED (and any parallel indicator) is illuminated.

The hysteresis system causes the indicator not to light for a couple of minutes or so if the
engine is started when fuel is low. Be aware that the circuit may take several minutes to
stabilise when adjusting the switch on point by altering the setting of VR1.


It is a good idea to test the circuit and set the reference voltage without the IC plugged into
place. Power the circuit from a power supply set to about 14.2 volts (you can use your car
with the engine running – take care, and use the blue connector point behind the lhs headlight
and a convenient earth point. No connection to the signal input pin is necessary for this
operation) Use a multimeter to set the voltage at pin 3 to 5.5 volts. Note that the LED will be
illuminated at all times that power is applied to the circuit without the IC in place.

Now test the circuit. Disconnect the power supply. Plug the IC into place, reconnect power
and apply a varying voltage to the signal pin. This can be obtained from another variable
resistor. Test the circuit by applying a voltage ranging from 0v up to about 7v. The LED
should illuminate as the applied voltage passes 5.5v.

This will provide approximately 10 litres reserve on an early car with the 45 litre fuel tank. You
may adjust the variable resistor on the board. A lower voltage setting at IC pin 3 will provide
an earlier switch-on point, and a higher one a later switch-on. My fuel tank is completely
empty at 6.75 volts or so. There may be variations from car to car, and for later cars with the
larger fuel tank.

Fitting to the car

To fit the unit to the car, first disconnect the battery. Remove the steering wheel shroud and
instrument cowl. Remove the two plugs in the back of the instrument panel (you may need to
remove the four securing screws and ease the panel forward slightly). Use “Scotchlok”
connectors to connect wires to the following leads:

1) 12v from Black/Yellow (second connection in on right hand plug)
2) 0v from Black (third connection in on right hand plug)
3) Fuel level signal from Yellow (end connection on left hand plug)

Feed the wires down the back of the instrument panel, and connect them to your new warning
light unit. Mount the second LED somewhere convenient (mine is in the face of the steering
wheel shroud) and connect the leads to it so that the LED is in parallel with the LED mounted
on the board.. Mount the new unit somewhere convenient out of sight. You may need to make
small adjustments to the trimmer on the circuit board to suit your requirements for the switch-
on point of the LED.

Similar, but not identical circuits can be used to provide low oil pressure and high water
temperature warning lights. For details, see Appendix 1.

Use this circuit at your own risk. Check the circuit operation carefully
before connecting it to your car. Be particularly careful that the
connection between the sender and resistor R1 has no chance of
shorting to 12v. Check that there is no more than 0.025 volts on the
sender terminal pin with 12v and earth applied to their respective pins
BEFORE connecting the unit to your car.

                                       APPENDIX 1
As mentioned in the main text, similar but not identical circuits can be used to provide warning
lights for High Water Temperature and Low Oil Pressure.

The circuit variations are necessary because slightly different criteria apply. Firstly, the
capacitor is not required, because no damping of the input signal is required – indeed it is
desirable that the circuit should react instantly to changes in the input signal.

The High Water Temperature detection section needs to react to a voltage falling from the
normal levels of about 9.8 volts (cold) through 7.5 volts (normal running temperature) to
about 7.0 volts, which would indicate a fault situation.

The Low Oil Pressure detection section needs to react to a voltage rising from normal levels
of about 3.0 volts (normal running pressure) through about 3.7 volts (pressure at idle) to about
4.0 volts indicating a fault situation.

The circuit changes necessary are reflected in the stripboard layout shown (note in particular
that the connections to the IC pins 2 and 3 are reversed in the water temperature section).

The reference voltages should be adjusted with the appropriate cermet trimmers to:
Water Temperature section: 7.0 volts at IC pin 2.
Oil Pressure section: 4.0 volts at IC pin 3.

The stripboard layout shown has one warning light shared between both Water Temperature
and Oil Pressure warning circuits – separate warnings could be provided by an additional
LED and slight circuit variation, but I feel that one warning light is sufficient to draw the
driver‟s attention to the gauges.

On-board LEDs are not strictly necessary, but they do help for testing and setting purposes.

Testing is similar to that described for the Fuel Warning system. I recommend that you test
the Water Temperature circuit before completing the Oil Pressure circuit – the use of one LED
can be very confusing if there is a fault in one circuit!

Note that the warning LED will illuminate at ignition switch-on (no oil pressure). It should
extinguish fairly quickly after starting the engine as oil pressure builds.

The locations of the appropriate signal leads are as follows:

Water temperature: Black/Blue wire – position 1 in right hand plug.
Oil Pressure: Yellow/Red wire – position 11 in right hand plug.
FUEL   WATER                                OIL
                           ALDERLEY TECHNOLOGY GROUP

               Title               Warning lights for Fuel, Water, Oil

               Drawn by:           J R Cookson

               Date:               12/2/2001

               Scale:              1 grid unit = 0.1”

               Notes:          Fuel section detects rise thro‟ 5.5v
                               Water section detects fall thro‟ 7.0v
                               Oil section detects rise thro‟ 4.0v
                                                         PARTS LIST

           Type              Specification           Additional Specification   No. Off                     Comments

Resistor             Metal Film 0.6W           1K                                 3
Resistor             Metal Film 0.6W           1M                                 3
                                                                                          Value can be varied according to choice of
Capacitor            Electrolytic, Polarised   16V, 47uF                          1       damping vs sensitivity.
Zener Diode          1.3W                      15V                                1
Potentiometer        Single turn, Cermet       10K                                3
Voltage Comparator   LM311N                                                       3
DIL Socket           8 way                                                        3
Transistor           npn General Purpose       BC337                              3
                                                                                          Additional Superbright 12v LED suggested for
LED                  12V (Built-in resistor)   Red                                1       remote indicator.
                                                                                          Additional normal brightness 12v LED suggested
LED                  12V (Built-in resistor)   Yellow                              1      for remote indicator.
Terminal Pin         Veropin                                                       8
Veroboard            0.1" hole spacing         40 holes x 17 tracks                1
Box                  Black Plastic             110mm x 55mm x 20mm                 1
Equipment wire       Single strand                                                1M
                                             Appendix 2
Stripboard Circuit Construction

This method is probably the most suitable for d-I-y construction of electronic circuits. A diagram of the
stripboard layout is shown for the circuit suggested. Stripboard is made from insulating material, usually
resin-bonded plastic. It has parallel copper strips on one side, spaced on 2.54mm (0.1in) centres and
with lmm holes also spaced on 2.54mm centres, as shown in the diagram. Components and wire links
are mounted on the plain side of the board. The leads or wire links pass through the holes and are
soldered to the copper strips. A strip may have to be cut across if different sections of it are to be used
for different parts of a circuit.

Connections to components off the board, or between one board and another are made by wires
soldered to terminal pins. Special stripboard terminal pins are used. These are 1mm or 1.5mm in
diameter, and fit tightly into the holes (drilled out to size if necessary), after which they are soldered to
the strips.

Stripboard is available in a number of sizes, but these are generally too big and of the wrong shape, and
it will be necessary to cut an appropriately-sized piece.

     The sequence for building a circuit on strip-board is as follows:

1)   Cut the board to the required shape using a junior hacksaw· Smooth the cut edge with glass paper
     or a fine file. Check that no flakes of copper strip at the edges of the board are bent across so as to
     short-circuit adjacent copper strips.

2)   Use a stripboard cutter or twist-drill to cut the strips where shown in the layout diagram. When a
     strip has been cut, examine the cut ends carefully, using a magnifier if possible. It is very easy for a
     hair-thin bridge of copper to remain, still connecting the two sections· Flakes of copper strip may
     also lie across the gap between adjacent strips causing a short circuit. Look for and remove these.

3)   Solder wire links in place first. Single-stranded insulated wire is preferable· The insulation is
     stripped for a length of about 5mm at each end before inserting the ends through the holes and
     soldering the ends to the copper strips. As far as possible these wires should be short and pulled
     tightly to the board before soldering. If adjacent strips have to be connected, a blob of solder can be
     used to join them, instead of a wire link.

4). Solder resistors in position, after bending their leads carefully at right angles. Usually resistors are
     best mounted flat and as close as possible to the board. If leads have to be soldered to holes only 1
     or 2 strips apart, it may be necessary to mount the resistors in the upright position.

5)   Solder terminal pins and integrated circuit sockets in position. Insert terminal pins from the copper
     side of the board and push them down with the tip of a hot soldering iron. Normally the strips
     beneath an i.c. socket are cut, so as to isolate pins on opposite sides of the i.c.

6)   Mount and solder other components, such as transistors and capacitors. It is advisable to clip a
     heat-shunt to the leads of transistors and diodes (on the component side of the board) while
     soldering them.

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