Fullscreen HTML5Med Res (??MB)HTML4

Knocking can cause serious damage to an engine. This simple circuit warns you when engine knock is occurring, so that you can ease up and avoid costly engine damage. Do you drive an old car? If so, build this . . . Knock indicator for leaded-petrol engines By JOHN CLARKE D RIVERS OF OLD CARS are facing an increasing problem. With the progressive decrease in the lead content of super grade petrol, many older engines are starting to “ping” (or knock) when called on to deliver the goods. This pinging effect typically occurs when the engine is under load (eg, when lugging up a hill), or during periods of moderate to heavy accel­eration. Even fairly light engine loads can cause pinging in severe cases. The reason for this is that the reduced lead content in super grade petrol has lowered its octane rating. And that in turn means that the fuel is more disposed to pre-detonation, particularly in high-compression en- gines. Modern engines designed to run on lead-free petrol avoid this problem by running lower compression ratios than the old leaded engines. In addition, modern engines use devices known as knock sensors. These sensors typically screw into the engine block and listen for the onset of knocking. If knocking is detected, they feed a signal to the engine management system which then retards Fig.1: block diagram of the Engine Knock Indicator. Signals picked up by the knock sensor are amplified, filtered and fed to a rectifier to derive a DC voltage. This voltage is then fed to a LED bargraph display, which indicates the knock severity. 72  Silicon Chip Fig.2: the final circuit diagram. IC1a, IC1b & IC1c are the amplifier and filter stages, D1 is the rectifier and IC2 is the LED bargraph display driver. IC1d and Q1 ensure that the circuit only “listens” for engine knock while the coil is firing. the ignition timing so that knocking ceases. On older cars, knocking can sometimes be alleviated by retarding the static ignition timing and/or by altering the weights in the distributor to change the centrifugal advance curve. On some leaded cars, however, the ignition timing was controlled electronically and could not be altered, so this is not option. The VK Commodore is one such example. Another problem with older cars is that most are now well past the 100,000km mark and are no longer carefully maintained. Often, the ignition system will be in need of adjustment or the head could do with a decoke. The build up of carbon deposits on the head of an old engine can be a major cause of pinging, because it gets hot and pre-ignites the fuel. Stopping an old engine from ping- ing is usually easier said than done. Although it’s sometimes possible to have the engine modified, such modifications are usually expensive and not re­garded as economically viable. As a result, drivers of older cars either ignore the problem or, if they are aware of it, drive so that engine knock is minimised. More often than not, however, the problem is one of igno­rance. Many drivers do not know what pinging is and just com­ pletely ignore the characteristic noise coming from the engine. Unfortunately, this can April 1996  73 The LED bargraph display was mounted with its top surface 27mm above the PC board, so that it would protrude through a matching slot in the lid of the case. Note that shielded cable is used to connect to the knock sensor. cause severe engine damage and lead to costly repairs. Pinging can cause piston and valve damage, blown head gaskets, excessive bearing wear and overheating (which in turn can distort the head). In severe cases, holes can even be burnt through the piston crowns. Knock indicator Although it cannot stop an engine from pinging, this simple Engine Knock Indicator can warn a driver when pinging is occur­ring so that the appropriate action can be taken. This can be as simple as easing off on the accelerator or changing back a gear to reduce the engine load. As in modern cars, the circuit monitors the output of a piezoelectric knock sensor which is attached to the engine block. This sensor connects to a dash-mounted unit that carries a bargraph display. When pinging occurs, the bargraph display indicates the severity of the problem on a scale of 1-10 (minor to severe). In addition, the unit sounds a buzzer 74  Silicon Chip to provide an audible warning when the bargraph reaches step 6. This sort of easily understandable feedback allows the driver to quickly adjust his driving technique so that engine knock ceases. So if you own an old “bomb” and you suspect that it is pinging, take a close look at this circuit. It could save you a packet in engine repairs. There’s just one proviso here – this circuit is designed to pick up engine knock under everyday driving conditions. It will not reliably detect Main Features • LED bargraph shows knock intensity • • Preset sensitivity control • Knock severity depends on repetition rate and intensity Audible warning when bargraph reaches threshold level engine knock at very high revs or on a high-performance engine that makes a lot of noise. In these situations, the noise from the engine simply swamps out the knock frequencies that this circuit is designed to detect (note: some modern cars get around this by using special filtering techniques plus a second sensor that’s specially tuned to detect knock at high revs). What is knock? Before we take a look at the circuit, let’s take a closer look at what causes engine knocking. In simple terms, knocking is caused by the irregular burning or explosion of the fuel-air mixture in the combustion chamber of the engine. The result is widely varying cylinder pressures that vibrate the engine components. By contrast, a correctly burning mixture within the combustion chamber produces a smooth pressure that causes a steady increase in the acceleration of the piston. When an engine knocks it does so at a particular frequency and this can be calculated as follows:      F = 900/πr where F is the frequency in hertz and Fig.3a (right): the parts layout on the PC board. Make sure that you don’t get ZD1 and ZD2 mixed up and note that they face in opposite directions to each other, as do the ICs. Fig.3b (far right) shows the full-size etching pattern. r is the cylinder radius in metres. For most cars, this equates to a frequency somewhere between 800Hz and 5kHz. In addition, the major knock sounds become audible from 0-60° after top dead centre. Designing an engine knock indicator can be difficult since it must be able to discriminate between knock and all the other noises produced by the mechanical action of the engine. These noises include those produced by the valve operation, chain drives, pumps, camshaft and crankshaft, plus any other mechanical noise makers which can mask the knock. One way to filter out these unwanted sounds is to only “listen” for knock during the time that it occurs. suffi­cient level and then fed to highpass and low-pass filter stages. These effectively select only the frequency band of interest (800Hz to 5kHz). Following the filters, the signal is rectified and fil­tered. It is then fed to a LED bargraph display. The number of lit LEDs in the bargraph depends on the knock intensity and repetition rate. The audible warning is provided when LED 6 on the bargraph lights. This is detected by Q2 and Q3 which in turn drive a buzzer. Block diagram Fig.1 shows a block diagram of the circuit arrangement. The knock sensor consists of a piezo element which is attached to the engine block. The resulting signal is first amplified to a TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  1 ❏  2 ❏  3 ❏  1 ❏  8 ❏  1 ❏  1 ❏  1 ❏  1 ❏  2 ❏  1 Value 1MΩ 100kΩ 27kΩ 18kΩ 15kΩ 12kΩ 10kΩ 9.1kΩ 6.2kΩ 2.2kΩ 1.2kΩ 1kΩ 10Ω 4-Band Code (1%) brown black green brown brown black yellow brown red violet orange brown brown grey orange brown brown green orange brown brown red orange brown brown black orange brown white brown red brown blue red red brown red red red brown brown red red brown brown black red brown brown black black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown red violet black red brown brown grey black red brown brown green black red brown brown red black red brown brown black black red brown white brown black brown brown blue red black brown brown red red black brown brown brown red black brown brown brown black black brown brown brown black black gold brown April 1996  75 Fig.4: basic detail for a do-it-yourself knock sensor. The piezo element is scrounged from a crystal earpiece. The piezo element is removed from the earpiece by first carefully cutting the housing at the glued joint. Schmitt trigger stage IC1d monitors the ignition coil prim­ary to provide a dwell gate signal for the rectifier/filter stage. This ensures that the rectifier/filter stage only receives signal from the low pass filter during the time that the ignition coil is firing; ie, when there is a high voltage on the switched side of the ignition coil primary. This measure effectively restricts the “listening” time of the circuit to the coil firing period, when knock is most likely to occur. At other times, signals from the low pass filter are “blocked”, to prevent false alarms which may be generated during the remainder of the ignition cycle. Circuit details Refer now to Fig.2 for the full circuit details. There are two ICs, 10 LEDs, 76  Silicon Chip This close-up view shows how the piezo element is mounted on the baseplate. The cover comes from a 16mm pot. Use shielded cable to make the connections to the knock sensor before the cover is fitted. three transistors, a regulator and a few other minor parts. IC1 is an LM324 quad op amp package which performs the signal processing. IC1a amplifies the signal generated by the piezo transducer. Its gain can be varied from one to 201, as set by 200kΩ trimpot VR1 and the 1kΩ resistor on pin 9. Its frequency response is rolled off below about 600Hz by the associated 0.27µF capacitor, while the 120pF capacitor across VR1 restricts the high frequency response. The output from IC1a appears at pin 8 and is fed to high-pass filter stage IC1b. This stage rolls off frequencies below 800Hz, as set by the RC filter network on the input. The signal is then fed to 5kHz low-pass filter stage IC1c. As a result, IC1b & IC1c together form a bandpass filter which passes signals only in the range from 800Hz to 5kHz. Note that IC1a, IC1b and IC1c are all biased at about half supply using common 12kΩ and 10kΩ voltage divider resistors. This bias voltage is filtered using a 100µF capacitor. The bandpass filtered signal appears at pin 1 of IC1c and is rectified and filtered using diode D1 and its associated 1µF capacitor. The charging time is set by a 1.2kΩ resistor which prevents transient signals from providing false indications on the meter. IC1d and Q1 provide the gating signal. In operation, the ignition coil input is fed to a voltage divider network and clamped to 6.8V using zener diode ZD2. The ignition coil signal is then fed to pin 6 of IC1d. Op amp IC1d is wired as an in- This commercial knock sensor is from a Daihatsu Mira and worked quite well with the circuit described here. verting Schmitt trigger. This means that when the ignition coil input is at ground (ie, when the points close or the coil switching transistor turns on), IC1d’s pin 7 output is high. This turns on transistor Q1 which then shunts the signal output from IC1c to ground. Conversely, when the ignition coil is firing, pin 6 of IC1d is high (+6.8V) and so pin 7 goes low. Transistor Q1 is now off and so the signal from IC1c is fed to the rectifier and filter stage. The output from the rectifier/filter stage is fed to IC2, a 10-LED dot/ bargraph display driver wired here in bargraph mode (pin 9 high). This device provides a linear output for signals ranging from RLO (ie, approximately half supply) to RHI. In other words, the voltage between RLO and RHI sets the full-scale vol­tage of the display. In operation, the REF OUT voltage (pin 7) sits 1.25V above the voltage at REF ADJ (and RLO). The voltage on RHI is then set by an internal 10kΩ resistor string (to RLO) and the external 15kΩ resistor. As a result, RHI sits about 0.5V above RLO which means that the display has a full-scale voltage of 0.5V. The 2.2kΩ resistor between pin 7 and ground sets the LED brightness. Transistors Q2 and Q3 monitor pin 14 (LED 6) of IC2. When LED 6 lights, pin 14 goes low and Q2 turns on. This then turns on Q3, which drives the buzzer to provide an audible warning. D2 protects Q3 from high back-EMF voltages when the buzzer turns off. Power for the circuit is derived via the ignition switch. The +12V supply is fed to 3-terminal regulator REG1 which provides an 8V rail for the ICs. The buzzer is powered from the +12V rail at the input of REG1. ZD1 and the 10Ω resistor protect the PARTS LIST 1 PC board, code 05302961, 102 x 59mm 1 plastic case, 130 x 67 x 43mm 1 self-adhesive front panel label, 123 x 60mm 1 10-LED bargraph display (LED1-LED10) 1 12V buzzer 1 200kΩ miniature trimpot (VR1) 1 3mm screw and nut 6 PC stakes 1 large grommet regulator against high voltage transients which may be pres­ent on the ignition supply. Construction The prototype Engine Knock Sensor was built on a PC board coded 05302961 and measuring 102 x 59mm. This board clips neatly into a standard plastic case (130 x 67 x 43mm). Fig.3a shows the parts layout on the board. Before starting the assembly, check the board carefully for any defects in the etching pattern. This done, install PC stakes at the six external wiring points, then install the links and resistors. Table 1 shows the resistor colour code but it is also a good idea to check each value on a digital multimeter, as some colours can be difficult to decipher. The diodes and zener diodes can go in next. Note that ZD1 and ZD2 face in opposite directions and that they have different values, so be careful not to mix them up. Similarly, note that D1 is a 1N4148, while D2 is a more rugged 1N4004 type. Take care when installing the ICs, as they also face in opposite directions (pin 1 is adjacent to a notch or dot in the body of the IC – see Fig.3). Once the ICs are in, the capacitors and transistors can be installed. Note that Q2 is a BC558 PNP type, while the others are BC338 NPN types. The 3-terminal regulator (REG1) is mounted with its metal tab flat against the PC board and is secured with a screw and nut. Bend its leads through 90°, so that they pass through their designated holes. This done, fit trim­pot VR1 to the board. The LED bargraph array must be installed with its anode (A) adjacent to the 1MΩ resistor – see Fig.3. It should be mounted so that the top surface of Sensor 1 crystal earpiece, DSE Cat. C-2765 1 cheap TO-3 transistor or equivalent baseplate (to make sensor) 1 16mm pot (for sensor cover) 1 solder lug 1 3mm screw and nut Semiconductors 1 LM324 quad op amp (IC1) 1 LM3914 10-LED bargraph driver (IC2) 2 BC338 NPN transistors (Q1,Q3) 1 BC558 PNP transistor (Q2) 1 7808 regulator (REG1) 1 16V 1W zener diode (ZD1) 1 6.8V 1W zener diode (ZD2) 1 1N4148 signal diode (D1) 1 1N4004 diode (D2) Capacitors 2 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.27µF MKT polyester 3 .015µF MKT polyester 1 .0047µF MKT polyester 1 .0015µF MKT polyester 1 .0012µF MKT polyester 1 120pF ceramic Resistors (0.25W 1%) 1 1MΩ 1 9.1kΩ 1 100kΩ 1 6.2kΩ 1 27kΩ 1 2.2kΩ 2 18kΩ 1 1.2kΩ 3 15kΩ 2 1kΩ 1 12kΩ 1 10Ω 8 10kΩ Miscellaneous Automotive hook-up wire, shielded cable, tinned copper wire, heat­ shrink tubing, bullet terminals, solder, etc. April 1996  77 1 2 3 4 5 6 7 8 9 10 MINOR SEVERE ENGINE KNOCK INDICATOR Fig.5: this full-size artwork can be used as a template when making the notch for the LED bargraph display. the display is 27mm above the board, so that it will later fit into a matching slot cut into the lid of the case. Once completed, the PC board can be installed inside the case and flying leads connected to the power supply, ignition coil, buzzer and knock sensor wiring points. These leads pass through a grommeted hole drilled in one end of the case. The slot in the front panel for the bargraph display is made by first attaching the label and then using this as a drilling template to give a rough knockout. The slot can then be carefully filed to shape. Knock sensor The easiest way of obtaining a knock sensor is to scrounge a commercial unit from a wrecking yard. The commercial knock sensor shown in one of the photos is from a Daihatsu and this worked quite well with the circuit. Alternatively, you can make your own knock sensor. We made ours using a piezo transducer taken from an earpiece. This was mounted on a TO-3 transistor baseplate and clamped in posi­tion using the rear enclosure from a 16mm pot. If you don’t have a transistor baseplate, or don’t want to destroy a perfectly good transistor, you can make up your own baseplate using 3mm steel or brass. Fig.4 shows the details of our home-made sensor. The pot cover is secured by soldering its lugs to the TO-3 baseplate. The transistor package is modified by first cutting the cap off the baseplate using a hacksaw. The two leads are then removed by breaking them 78  Silicon Chip Fig.6: basic scheme for connecting multiple coils to the ignition input. An extra diode should be added for each additional coil. off with pliers and the baseplate filed to a smooth finish. Warning – transistors can use dangerous materials inside. Use rubber gloves during this process and a facemask and goggles when cutting and filing the baseplate. Wash both the transistor baseplate and your hands after the work has been completed. Next, one of the transistor mounting holes is enlarged to accept the mounting bolt (the prototype sensor was mounted on the edge of the rocker cover using an existing bolt into the head). The piezo element is removed from the earpiece by first carefully cutting around the outside of the housing at the glued joint. This done, carefully prise the element from the plastic housing using a knife. You should leave the wire attached to the top of the element intact and remove the wire from the larger lower plate. The piezo element is now centred on the baseplate (larger plate down) and secured using the pot cover – see Fig.4. Be sure to pass the lead under the pot enclosure and protect it with heatshrink tubing before soldering the tangs of the pot cover to the baseplate. Finally, bolt a solder lug to one of the baseplate mounting holes and connect a suitable length of shielded cable to the transducer, so that is can be wired back to the circuit board. We used heatshrink tubing to help secure the wiring. Testing To test the circuit, first apply power and check that pin 4 of IC1 and pin 3 of IC3 are at 8V. If this is correct, switch off and connect the knock sensor wire to the sensor input on the PC board. You should also connect the case of the sensor to the GND terminal (via the shielded cable braid). Next, short the base and emitter terminals of Q1 using a clip lead, set VR1 fully clockwise and apply power. If you now lightly tap the knock sensor with a screwdriver, the LEDs in the bargraph display should light. Adjust VR1, so that the display just reaches the 10th LED each time the sensor is tapped. Assuming everything is operating correctly, remove the short between the base and emitter of Q1. Installation Be sure to install this unit in a professional manner. The display should be mounted where it can be easily seen by the driver, while the buzzer can be either mounted inside the case (drill a few holes to let the sound out) or installed under the dashboard. The GND connection can be made via an eyelet lug screwed to the chassis, while the +12V ignition supply rail should be de­rived from the fusebox using automotive connectors. Make sure that this rail is fused and only goes to +12V when the ignition is switched on. In most cases, the only wires passing through the firewall will be to the ignition coil and to the piezo sensor. Be sure to connect the ignition coil lead to the switched side of the coil (ie, to the negative terminal). Do not connect to the coil lead to the EHT terminal. If your car uses multiple-coil ignition, use the circuit shown in Fig.6 to make the connections (add an extra diode for each extra coil). The PC board clips into a standard plastic case and the leads brought out through a grommeted hole. These leads go to the negative side of the ignition coil, to the power supply (+12V & ground), to the buzzer and to the sensor. Important: the ignition coil lead will have up to 500V on it when the coil is firing and so must be well insulated from the chassis. It would also be wise to insulate the ignition coil terminal on the PC board to prevent accidental contact. The piezo sensor is best mounted on the engine block using an existing bolt. As a second preference, it can be attached to the head. As mentioned above, we secured our sensor using one of the rocker cover securing bolts. Once the unit has been installed, start the engine and adjust VR1 so that the display is just off for all engine revs while the car is in neutral. This effectively provides maximum sensitivity for knock signals without also detecting normal engine noise. Finally, the unit can be tested by deliberately provoking engine knock on the road (don’t overdo this though). This can be done by lugging up a steep hill in a higher gear than normal. If the unit fails to respond to knocking or is overly sensitive, then it’s simply a matter of slightly adjusting VR1 for the correct response. Now you will always be warned when engine knock is occurring, regardless of how loud your kids are screaming or how far your sound sysSC tem is cranked up. 20 Electronic Projects For Cars $8.9s5 plu $3 p&p Yes! Please send me ___ copies of 20 Electronic Projects For Cars Enclosed is my cheque/money order for $­________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­________________________ Card expiry date_____/______ Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail the coupon to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. Name _______________________Phone No (_____)____________ Street PLEASE PRINT _________________________________________________ Suburb/town _____________________________ Postcode_________ April 1996  79