MX-5 Fuel Warning Light Circuit Diagram 12v ZD1 R2 IC1 R1 LED1 Signal from tank TR1 sender C1 unit VR1 0v 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: (www.maplin.co.uk) 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 12v R2 IC1 R1 From sender ZD1 C1 VR1 LED1 TR1 0v 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 cathode) 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. Testing 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. E.O.E. 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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