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Chapter 9 Check your fuel injectors with this: The Digital Duty Cycle Meter uses a 2-digit LED display to show the realtime duty cycle of the injectors. Duty Cycle Meter Digitally monitor fuel injector duty cycles and also switch devices on and off at different engine loads! A LL THE TIME that you’re driving along, the fuel injectors under the bonnet are rapidly clicking open and shut – opening to allow fuel to squirt into the ports behind the valves and then closing until their next turn for spraying action comes around. The proportion of time that each injector is open determines how much fuel gets added to the intake air – ie, the injectors precisely meter the fuel. This Digital Duty Cycle Meter allows you to actually see in real time how long the injectors are open for. For example, at idle they might be open for only 2% of the time. Put your boot into it and let the revs rise and you might find that the injectors are open for as much as 80% of the time! The most that they can ever be open for is 100%, so if you’re driving a modified car and the injector duty cycle (the Specifications Display resolution .................................................................................... 1% Display range ....................................................... 0-99% (100% shown as "- -") PWM display polarity ....................................... positive or negative selectable Output switch threshold ............................................... adjustable from 1-99% Output relay .........................................rated at 10A with NO and NC contacts Output switch ........... triggered on rising or falling PWM percentage (selectable) Hysteresis of switching .............................. adjustable in 1% steps from 0-99% Maximum PWM input voltage ........................................................... 50V RMS Maximum input frequency .........................10kHz (equivalent to 600,000 RPM) Minimum input frequency ........................................................5Hz (300 RPM) Display dimming ...................................... adjustable from full brightness to off Supply voltage .............. 9V <at> 120mA (relay on) to 15VDC <at> 140mA (relay on) 50 PERFORMANCE ELECTRONICS FOR CARS term for how long they’re open for) is getting close to 100%, then larger injectors (or an increase in fuel pressure) will be needed if you want even more power. As well as displaying injector duty cycle, this meter also allows you to turn things on and off on the basis of duty cycle. In other words, it can act as a sophisticated load switch. The relationship between load and duty cycle is very strong – much better than using a boost pressure switch in a turbo car to turn on a water spray or a throttle microswitch to trigger nitrous injection. You might want to turn on water injection with a load equivalent to 45% duty cycle, for example; or perhaps an intercooler water spray at 55% duty cycle. At the other end of the scale, perhaps you want to switch out an electronic modification at very light loads – eg, when there’s a duty cycle of less than 5%. This project allows you to monitor the actual duty cycle of the injectors and also allows you to switch devices siliconchip.com.au Fig.1: this shows where each of the components is placed on the main PC board. Use this diagram, the photos of the completed board and the parts list to help you assemble it correctly. Fig.2: this version has the two LED displays mounted on it, if you don’t want to use the external display board. Be sure to get the orientation of the pushbutton switches correct. on or off on the basis of load. It will work equally effectively with factory or aftermarket engine management. Construction Depending on how you choose to build it, the Digital Duty Cycle Meter will use two or three PC boards. One is the main board, another is used if the digital display is mounted remotely and the third is for the PWM generator. This last board is a pulse generator with a variable duty cycle so that it’s easy to test that the Digital Duty Cycle Meter is working correctly. When assembling the main PC board, follow the overlay diagram and photos closely. Make sure that the pushbutton switches are orientated with the flat side as shown and be careful to get the polarised components the siliconchip.com.au Follow this photo and the parts overlay diagrams when constructing the main PC board. If you are a beginner, it’s easier to mount the LED displays on this main board rather than remotely. PERFORMANCE ELECTRONICS FOR CARS 51 How It Works The circuit for the Duty Cycle Meter is based on microcontroller IC1 which monitors the pulse signal applied to fuel injector solenoids, etc. It displays the duty cycle as a percentage. In operation, the unit measures the time between two positive edges of the pulse waveform and also the time from the positive edge to the negative edge. Through a series of calculations, these measurements are converted to a percentage which is shown on the 2-digit display. The meter can display the percentage duty cycle for positive referenced signals or for ground referenced signals. All this means is that positive referenced signals have a 0% duty cycle that when the signal is always positive and a 100% duty cycle when the signal is always at ground. This type of signal is normal for fuel injectors which are switched to ground to open them. Conversely, ground referenced signals have 0% duty cycle when the signal is continuously at 0V (ie, off) and 100% duty cycle when the signal is fully on. The pulse signal is applied to pin 6 of IC1 via a network consisting of two 10kΩ resistors, zener diode ZD2 and a 1nF capacitor. Internal to pin 6 is a Schmitt trigger which ensures a clean signal for measurement. The display segments are driven The PWM Generator is included to allow the Duty Cycle Meter to be tested. It uses a 7555 timer (IC1) which charges and discharges a 100nF capacitor connected to pins 2 and 6 via trimpot VR1 and diodes D2 & D3. When the VR1’s’ wiper is close to D3, the 100nF capacitor charges quickly and discharges slowly, giving a pulse train output at pin 3 with a short high duration and a long low period; ie, low duty cycle. Alternatively, when VR1’s wiper is close to D2, the 100nF capacitor charges slowly through D3 for a long high output and discharges quickly through D2 for a short low output time; ie, high duty cycle. Thus VR1 allows the duty cycle to be adjusted from 1% to 99%. Fig.3: this shows where each of the parts is placed on the PWM generator test module. Use this diagram, the two photos and the parts list to help you assemble it correctly. 52 PERFORMANCE ELECTRONICS FOR CARS from IC1’s RB1-RB7 and RA0 outputs via 150Ω current limiting resistors. The displays are multiplexed, with each digit’s common anode driven separately via a transistor (Q1 and Q2). Q1 is switched on when the RA3 output goes low and so DISP1’s display segments are driven by RB1-RB7 and RA0. Similarly, transistor Q2 is turned on when RA2 is low to drive DISP2. The displays are driven alternately at a fast rate so that they appear to be continuously lit. Dimming is achieved using LDR1, op amp IC2 and transistor Q3. In bright light, the LDR is a low resistance and so pin 3 of IC2 is held close to +5V. This turns Q3 fully on to supply full current to the emitters of Q1 and Q2. This allows the displays to operate at full brightness. If the ambient light drops to a low level, the resistance of LDR1 increases and the voltage at pin 3 of IC2 falls. The lower voltage at pin 3 is reproduced by Q3 and the display is dimmed. Trimpot VR1 sets the brightness of the display. Switches S1-S3 are monitored via RA4 (pin 3). This pin is normally held high via a 10kΩ pull-up resistor. When this input is pulled low, it means that one of the switches has been pressed. The program inside IC1 decides which of the three switches has been pressed by checking if the RA2 and RA3 outputs are low or not. Output RA1 (pin 18) drives transistor Q5 which in turn drives the relay connected to the 12V supply. When the relay is powered, the common (C) and the normally open (NO) contacts are closed. When the relay is off, the common and normally closed (NC) contacts are closed. Transistor Q4 performs a power-on reset for IC1 to ensure that pin 4 is right way around. Use an IC socket for IC1 and remember that both ICs must be orientated correctly. As indicated above, the LED displays can be remotely mounted on a separate display PC board and connected to 5-way pin headers on the main board using rainbow cable. Alternatively, the displays can be siliconchip.com.au Fig.4: this is the circuit for the Duty Cycle Meter. It’s based on a specially programmed PIC16F84-20P microcontroller (IC1) driving two 7-segment LED displays. only switched high when the supply is above about 3.5V. For voltages below this, the microcontroller is held in the reset state (ie, quiescent). IC1 is operated at 10MHz using crys- tal X1. This enables the program within IC1 to perform fast measurements of the duty cycle at up to 10kHz. Power for the circuit comes via diode D1 which provides reverse polarity pro- mounted directly on the main PC board in the holes provided. If you use the remote-mount option, be sure to install the wire link on the display PC board between DISP1 and DISP2, before actually mounting the displays in place – see Fig.5. The LDR (which controls the auto-dimming function) can also be mounted on long leads – alternatively, drill a hole in the box to allow ambient light to shine on the LDR. The test PWM generator has only a handful of components but be careful with those that are polarised. siliconchip.com.au Testing Connect the output of the test PWM tection. IC1 is powered from +5V which is derived from REG1, an LM2940CT-5 regulator designed specifically for automotive applications. The 10Ω resistor and 100µF capacitor at REG1’s input provide a degree of transient voltage suppression. Zener diode ZD1 protects IC2 from voltage spikes. generator to the Digital Duty Cycle meter input. Using a 12V power supply or the car battery, apply power to both the Digital Duty Cycle Meter and the test PWM generator. The display should spring into life and as the trimpot on the generator is rotated, the numbers on the display should also change. PERFORMANCE ELECTRONICS FOR CARS 53 Parts List 1 main PC board, code 05car021, 122 x 61mm 1 display PC board, code 05car022, 30 x 28mm 1 plastic case, 130 x 68 x 44mm (Jaycar Cat. HB6014 – optional; not supplied with kit) 1 28 x 28 x 2mm red transparent Perspex or Acrylic sheet 1 12V 5A relay with DPDT contacts (Jaycar Cat. SY4052; Relay 1) 1 DIP18 IC socket for IC1 2 5-way (or 6-way) pin headers 2 5-way (or 6-way) header sockets (CON1, CON2) 1 LDR (Jaycar Cat. RD3480 or equivalent) (LDR1) 1 10MHz parallel resonant crystal (X1) 1 500kΩ horizontal trimpot (VR1) 6 6.3mm PC-mount spade connectors with 5mm pin spacing 5 6.3mm female spade connectors 3 click-action pushbutton switches (S1-S3) 2 M3 x 6mm tapped standoffs 2 M3 x 6mm countersunk screws 2 M3 x 6mm machine screws 2 3mm washers 2 1.5m lengths 5-way rainbow cable 1 2m length of heavy-duty red hookup wire 1 2m length of heavy-duty green hookup wire 1 2m length of heavy-duty black hookup wire 1 150mm length of 0.8mm tinned copper wire Semiconductors 1 PIC16F84A-20/P microcontroller programmed with dutycycl.hex (IC1) 1 LM358 dual op amp (IC2) 1 LM2940CT-5 low dropout automotive regulator (REG1) 2 common anode displays (DISP1, DISP2) (Jaycar Cat. ZD1857) 3 BC327 PNP transistors (Q1-Q2, Q4) It’s important to have the system working before you install it in the car. If there are problems, switch off immediately and inspect the board very closely, looking for solder bridges between tracks, dry joints or components either in the wrong way around or in the wrong place entirely. When everything appears to be 54 PERFORMANCE ELECTRONICS FOR CARS RESISTOR COLOUR CODES Value 4-Band Code (1%) 5-Band Code (1%) 220kΩ 39kΩ 22kΩ 10kΩ 2.2kΩ 680Ω 150Ω 10Ω red red yellow brown orange white orange brown red red orange brown brown black orange brown red red red brown blue grey brown brown brown green brown brown brown black black brown red red black orange brown orange white black red brown red red black red brown brown black black red brown red red black brown brown blue grey black black brown brown green black black brown brown black black gold brown 2 BC337 NPN transistors (Q3, Q5) 2 1N4004 1A diodes (D1, D2) 2 16V 1W zener diodes (ZD1, ZD2) Capacitors 1 100µF 16V PC electrolytic 3 10µF 16V PC electrolytic 1 100nF MKT polyester (code 104 or 100n) 1 1nF MKT polyester (code 102 or 1n) 2 22pF ceramic (code 22 or 22p) Resistors (0.25W 1%) 1 220kΩ 1 39kΩ 1 22kΩ 5 10kΩ 1 2.2kΩ 2 680Ω 8 150Ω 1 10Ω PWM Generator 1 PWM generator PC board, code 05car023, 40 x 28mm 1 500kΩ horizontal trimpot (VR1) 3 6.3mm PC-mount spade connectors with 5mm pin spacing 3 6.3mm female spade connectors 1 2.2kΩ resistor (0.25W, 1%) Semiconductors 1 7555 CMOS 555 timer (IC1) 1 1N4004 1A diode (D1) 2 1N4148 diodes (D2,D3) Capacitors 1 470µF 16V PC electrolytic 2 100nF MKT polyester (code 104 or 100n) working correctly, use the pushbuttons to try out the various functions of the meter. The meter is initially set having a ground-referenced reading so that a high voltage (eg, +12V) will show 100% and a ground voltage (0V) at the input will show 0%. The relay output is set so that it will switch on when the duty cycle exceeds 50%. It will switch off when the duty cycle drops below 45%; ie, the hysteresis is set at 5%. To change these settings, press the Mode switch and the display will show “P.”. The “P” stands for polarity and can be changed by pressing the up or down switch so that the display shows “P.-”. This setting means that the display will show 0% when the input is high and 100% when the input is at ground. The polarity setting switches between a “P.” and “P.-” at a 0.5-second rate while one of the Up or Down switches is pressed. If, when connected to the idling car, the display shows (say) 98% instead of 2%, alter the polarity with this function. The remaining mode functions are for the relay output switching. Pressing the Mode switch again will show a “d.H” on the display which means that the relay will switch on when the set duty cycle is exceeded. Pressing the Up or Down switch will toggle the display to the “d.L” setting which means that the relay will be switched on for duty cycles below 50% and will be off for duty cycles above 55%. This 55% off setting is due to the 5% hysteresis. The next pressing of the Mode switch will show “50.”. This is the relay switching threshold setting. It can be changed by pressing the Up or Down switches. Press the Up switch to increase the setting and the Down switch to decrease the setting. The next pressing of the Mode switch will show the Hysteresis setting which is initially 5. It can also be changed using the Up and Down switches. Now press the Mode switch again and the display will return to showing duty cycle as normal. Any changed settings will be permanent unless changes are made again to the alternate settings or values. Pressing the siliconchip.com.au Mode switch to cycle through the settings will not alter the values. Adjust trimpot VR1 so that the display brightness is sufficiently dimmed in darkness – set it clockwise for maximum display brightness. Fitting Fitting the Digital Duty Cycle Meter is straightforward – at its simplest, only three wiring connections need to be made. These are: ignition-switched +12V power, earth and an injector connection. This wiring is most easily performed at the ECU. No harm will come from connecting the input signal directly to +12V or earth, so don’t panic if you first back-probe the wrong ECU pin. The relay can be used to control external loads (up to 5A). It has three terminals: common, normally-open (NO) and normally-closed (NC). If you want to switch an intercooler water spray pump, for example, feed ignition-switched +12V to the common terminal and wire the pump between Peak/Hold Injectors? Some cars are fitted with what are known as “peak-hold” fuel injectors instead of conventional fuel injectors. You can still measure the duty cycle of this type of injector but, in this case, you have to connect the Duty Cycle Meter via the Peak-Hold Adapter described in Chapter 18. What are peak-hold injectors and how do you know if your car has them? Chapter 18 has the details. Fig.5: here’s how to assemble the display board. Make sure that the displays are orientated correctly, with the decimal point at the bottom. A piece of transparent red acrylic makes the LED displays more visible, as shown at right. the normally-open relay terminal and ground. That way, the pump will switch on whenever the designated duty cycle is exceeded (assuming that you have the Digital Duty Cycle Meter set to dH mode, of course). Devices that you can control with the relay include: (1). A “high duty cycle” warning light or buzzer (eg, operates at 90% and higher duty cycles, switches off at 85%). (2). An intercooler water spray (eg, operates at 50% and higher duty cycles, switches off at 45% – ie, operates at high loads). (3). An intercooler cooling fan (eg, operates at duty cycles of less than 2%, switches off at 5% – that is, operates only at idle or very slow speeds, although note that it will also trigger during injector over-run shut-off). (4). An engine management modification (eg, switches in modification only at loads above say 50% duty cycle, switches out modification at 45%). (5). An extra fuel pump (eg, switches in additional pump above say 50% duty cycle, switches out additional pump at 45%). Because the relay can be triggered with either a rising or falling duty Fig.6: this diagram shows how ribbon cable is used to connect the main and remote display PC boards together. siliconchip.com.au PERFORMANCE ELECTRONICS FOR CARS 55 Here the PWM generator has been connected to the Digital Duty Cycle Meter – it’s as easy as connecting the “OUT” terminal of the PWM generator to the “INPUT” terminal of the Digital Duty Cycle Meter, and making power and earth connections to both PC boards. Finished with the PWM Generator? After you’ve built and installed the Digital Duty Cycle Meter, you’ll have one PWM generator that’s surplus to requirements. But with just a few component changes you can turn this into a high performance LED flasher. At its simplest, all that you need to do is replace the 100nF capacitor near the output terminal with a different capacitor and then wire a LED between the output terminal and earth (with the long LED lead to the output terminal). The value of capacitor that is used will determine the flash rate, while the duty cycle (the relative length of on and off times) and LED brightness can still be altered by the pot. If you use a 2.2µF electrolytic capacitor (negative closest to the edge of the PC board), the LED will flash around 56 PERFORMANCE ELECTRONICS FOR CARS once per second. This is an ideal flash rate for a car alarm indication and the beauty is that you can adjust the pot to make the “on” time very short, just as factory alarm LEDs operate. This also reduces the current draw enormously. Use a high-intensity LED together with the current limiting resistor already in the circuit and you could expect the LED to flash for literally weeks without flattening the car battery. Halve the capacitance and the flash rate will double – 1µF is excellent if you need a bit more urgency to your alarm flashing. Alternatively, a 470nF MKT polyester capacitor and suitably tweaking the duty cycle pot setting will give a fast flash – ideal as a dashboard warning or for a really attention-getting shift-light. cycle and because the hysteresis (the difference between switch-on and switch off values) is also adjustable, the switching side of the Digital Duty Cycle meter is very useful. In Use When first powered up, the display will stay blank for a moment or two, before settling at “0”. Start the car and the display will show a very small number – perhaps 1 or 2%. This is because at idle, the injectors are open for only a very small proportion of the available time. Blip the throttle and the number will race up. When you take the car for a drive you’ll notice that if you lift off the throttle at high revs, the display will show ‘0’. This is because on the over-run, the injectors are shut off completely to save fuel. If your car’s injectors are being pushed so hard that sometimes they’re continuously open, siliconchip.com.au Remote Mounting the Display The 2-digit LED display can be mounted either directly on the main PC board (Fig.2) or remotely, with the connection to the main board made via ribbon cable. We chose this “remote mounting” approach, placing the display in a housing that was then positioned inside a second glovebox. The housing was made from a small diameter plastic pipe blanking cap. A holesaw was used to make a hole in the end of the cap and then progressively finer sandpaper used to smooth and shape the resulting flange formed around the opening. A separate piece of plastic was cut to form the rear panel of the enclosure. The lens was made from smoked grey translucent plastic, salvaged from an old VCR. This was shaped into a disc that dropped into the flange from the rear of the holder. The lens was then masked from inside using four short, straight pieces of electrical tape, stuck to the back of the lens and creating a rectangular window for the LEDs to show through. A thin piece of clear orange-red acrylic was also placed between the LED display and the lens. Finally, the holder was spray painted black and mounted in place. When selecting the mounting location, keep in mind that the LED display – even behind grey plastic – won’t be able to be read with direct sunlight falling on it. Try to position it so that the display is shaded in most conditions. Portable Instrument To get more power out of your engine, you need to add more fuel. Whether or not the injectors can keep up with the new demands made on them will depend on what duty cycle they’re running – once they reach 100%, they’re fully open. This meter displays the real-time duty cycle of the injectors, so you can see how much latitude you’ve got left. the display will show “--”, meaning 100% has been reached. Other Uses While we have concentrated on measuring injector duty cycle, there are other automotive devices which are controlled with varying duty cycles. These include turbo boost control siliconchip.com.au solenoids, power steering flow control solenoids and automatic transmission flow and pressure control solenoids. The Digital Duty Cycle Meter can be used to display these duty cycles as well, allowing you to see (for example) the control behaviour of the factory boost solenoid. This information is very useful if you are modifying the The Digital Duty Cycle Meter will work from a 9V battery, allowing the unit to be mounted in a box and used as a portable diagnostic tool. In this form, the PC board must be mounted so that the mode switches are accessible and the LED display is visible. You would also need to add a power switch. The relay could be used to trigger an inbuilt buzzer, with the trip threshold set depending on the application (eg, as an “injector duty cycle too high” warning). To connect the instrument to the car, you’ll need to make only the earth and injector connections under the bonnet. system. In addition, you can use the relay to switch devices on the basis of these measured duty cycles. Conclusion Whether it is built to monitor injector opening percentages or to switch loads, the Digital Duty Cycle Meter is  a useful and effective tool. PERFORMANCE ELECTRONICS FOR CARS 57