Fullscreen HTML5Med Res (??MB)HTML4

Super Speedo Corrector By JOHN CLARKE & JULIAN EDGAR Get your electronic speedo reading accurately T HESE DAYS, having an accurate car speedo is vital if you’re to avoid fines and loss of licence points. But how do you correct the speedo if it is reading high or low? It’s easy with our Super Speedo Corrector which will work with any electronic speedo, either digital or analog. It allows you to alter the speedo reading in 1% increments, either up or down. Before you can use the Speedo Cor- rector, you’ll have to find and identify the speedometer sensor output wire or the speed signal output wire from the ECU. In some cars that’s easier said than done, so make sure you have a wiring diagram and that you can physically access the speedo input wire which is normally at the back of the instrument cluster. If you can’t find the right wire, you won’t be able to install the Speedo Corrector. Speedo Corrector: Main Uses • • • • Correct inaccurate speedos in standard cars. Correct inaccurate speedo caused by changed differential or gearbox ratios. Correct inaccurate speedo caused by changed tyre diameters. Correct tachometers 72  Silicon Chip This project is a development of the Speedo Corrector first published in the SILICON CHIP book “Performance Electronics for Cars”. Advantages over the original project include an automatic set-up procedure where the Super Speedo Corrector calibrates itself to suit the speed signal output characteristics, an on-board status LED that flashes to show correct operation and an AC output signal that will work with Nissan speedometers. Circuit description The circuit (Fig.1) is based on microcontroller IC1 which is programmed to alter an incoming frequency by a set amount. The exact amount is set using two rotary switches, which alter the frequency in 1% steps. The speedometer signal is applied to the input of the circuit that has the opsiliconchip.com.au Fig.3: all the clever stuff in this circuit is done by PIC microcontroller IC1. It takes the speedo signal and multiplies it by a factor set by two rotary BCD switches (S1 & S2). The speedo signal frequency can be either increased or decreased in 1% increments – see text. tions of a 1kW pull-up resistor selected with transistor Q4 or a 1kW pull-down resistor selected with transistor Q2. By selecting either link LK1 or LK2, the pull-up resistor can be connected to either the +8.2V supply or the +5V supply. The input signal is then fed via a 10kW resistor to zener diode ZD2, which ensures that levels cannot go above +16V or below -0.6V. A parallel 10nF capacitor filters the signal which then drives transistor Q1 via a voltage divider consisting of 10kW and 6.8kW resistors. This 6.8kW resistor at the base of Q1 can be either connected to ground via the RA4 output of IC1 or left floating when the RA4 output is set as a highimpedance input. When the resistor is connected to ground, the signal level required to switch Q1 is about 2.5V. Alternatively, when this resistor is efsiliconchip.com.au At only 105 x 61mm, the Super Speedo Corrector is small enough to fit behind the dash. It’s easy to set up and simply intercepts the speedometer signal. D December ecember 2006  73 Mechanical Speedo? Features & Specifications The Super Speedo Corrector will work only on electronic speed­ o­meters – ie, those that don’t have a mechanical rotating cable driving them. If you have an older car with a mechanical speedo, then you won’t be able to correct it – at least not using this circuit. Main Features • • • • • Allows alteration of speedo reading so it reads faster or slower. Automatic or manual set-up of input signal detection. Three output signal types catered for. LED indication of valid speed sensor signal being received. LED indication of output operation. resistor and a 150W series resistor. A 1nF capacitor filters out any highfrequency voltage variations. Pin 6 includes an internal Schmitt trigger to ensure a clean signal for measurement. The rotary BCD switches (S1 & S2) are monitored via the RB1-RB7 inputs and the RA1 input. The RB inputs are normally held high via internal pullup resistors within IC1, while the RA1 input uses a 10kW resistor to ensure this input is high, unless pulled low via S2. The switches provide a unique BCD (binary coded decimal) value on these inputs for each setting. The output signal is at RA0 (pin 17). This drives the indicating LED (LED1) via a 1kW resistor and the base of transistor Q5. Q5’s collector is held high via a 1kW resistor which connects to either the +8.2V or +5V supply (via link LK1 or LK2). Q5’s collector provides the pull-up output signal and also drives Q6 which has a pull-down resistor at its collector to provide the pull-down output. Coupling the pull-down output via a 100mF capacitor provides an AC output. The 10kW resistor provides the discharge path, while links LK3, LK4 and LK5 select the pull-up, pull-down and AC outputs, respectively. An internal power-on reset for IC1 is provided using the MCLR input (pin 4) Specifications • • • • • • • • • • Output Rate: adjustable in 1% steps from 0 to 99%. Output: either faster or slower than the input rate. Input and output types: Pull up or pull down resistance or AC. Output swing: 0 to 8.2V or 0 to 5V or 8.2V peak-peak AC or 5V peakpeak AC. Minimum operating frequency: Adjustable from 1-16Hz. Maximum input frequency to maintain 1% change resolution: 1.2kHz. Maximum input voltage: 50V RMS. Minimum input sensitivity: 0.7V peak (on high sensitivity setting). Minimum input sensitivity: 2.5V peak (on low sensitivity setting). Power 9-15V at 25mA. fectively out of circuit, the sensitivity is lowered to around 0.7V peak. The RA2 output of IC1 is used to select the pull-up resistor. When this output is at 5V, it switches on transistor Q3 and this in turn switches on Table 1: Capacitor Codes Value 100nF 10nF 1nF mF Code IEC Code 0.1mF 104 .01mF 103 .001mF 102 EIA Code 100n 10n 1n0 transistor Q4. Q4 then connects the 1kW pull-up resistor connecting from the input to Q4’s collector. This then connects the pull-up resistor to the +8.2V or +5V supply rail. If RA2 is at 0V, Q3 and Q4 are off and there is no pull-up resistor in circuit. The RA3 output selects the pulldown resistor when its output is at 5V. This output drives transistor Q2 to connect the 1kW resistor at its collector to ground. When RA3 is at 0V, the pull-down resistor is out of circuit. Q1’s collector inverts the signal and drives pin 6 of IC1 via a 10kW pull-up Table 2: Resistor Colour Codes o o o o o o o o No. 11    1    5    1    1    1    1 74  Silicon Chip Value 10kW 6.8kW 1kW 470W 220W 150W 10W 4-Band Code (1%) brown black orange brown blue grey red brown brown black red brown yellow violet brown brown red red brown brown brown green brown brown brown black black brown 5-Band Code (1%) brown black black red brown blue grey black brown brown brown black black brown brown yellow violet black black brown red red black black brown brown green black black brown brown black black gold brown siliconchip.com.au Tacho As Well? The Super Speedo Corrector will also work with electronic tachos that take their feed from the ECU (ie, all cars with engine management). The configuration procedure is the same as for use of the device as a speed interceptor, except that the “speed sensor” becomes the tacho output signal from the ECU. This application is particularly suited to engine and gearbox swaps. Fig.2: when assembling the PC board, take care with the orientation of the BCD switches, the PIC and the other polarised components. Use this diagram and the photos of the completed project (below) to help you in your assembly. 24V Operation The Super Speedo Corrector can be used on 24V vehicles if the following changes are made: change ZD1 to 33V 1W; change the 220W 0.5W resistor that feeds ZD3 to 1kW 1W; change the 100mF 16V capacitor at the input to REG1 to 100mF 35V. Non-Linearity? Corrections are easy to dial-up – just set the two rotary switches to give the up or down percentage correction that’s needed. Here, the Super Speedo Corrector is configured to reduce the speedo reading by 3%. which is connected to the 5V supply via a 1kW resistor. This keeps the IC reset until the power supply voltage is correct. IC1 operates at 20MHz using crystal X1. This frequency was chosen so that the software program runs sufficiently fast to operate with speedometer signals up to 1.2kHz. Note that the Super Speedo Corrector will operate with speedo signals above these frequencies, however the accuracy of speedometer correction will be reduced. Power for the circuit is applied via diode D1 which provides reverse polarity protection. Zener diode ZD1 and the 10W resistor provide transient protection to protect the input of REG1. The 100mF capacitor at REG1’s input provides a further degree of transient voltage suppression. A 10mF siliconchip.com.au The Super Speedo Corrector will not compensate for non-linear errors. In other words, if the speed reads 10% high at 25km/h and 4% high at 100km/h, you won’t be able to use the Super Speedo Corrector to get the speedo accurate at all speeds. However, most speedo errors are proportional and so can be dialled-out with the Speedo Corrector. filter capacitor is provided directly at REG1’s output and the 100mF and 100nF capacitors decouple the supply to IC1. An 8.2V supply is derived from the supply at REG1’s input via a 220W resistor and zener diode ZD3. This supply is for the pull-up resistors if required. even though they look the same. The BCD switches must be mounted with their dots positioned as shown on the overlay diagram (Fig.2). The BCD switch with 0-9 capabilities is S1 and the BCD switch with 0-F on it is S2. Construction Making adjustments The Super Speedo Corrector circuit fits onto a small PC board coded 05112061 and measuring 106 x 60mm. Fig.2 shows the parts layout. Construction is straightforward but be sure to correctly install the polarised components such as the PIC microcontroller, electrolytic capacitors and the diodes. Note that Q4 and Q6 are not the same as Q1, Q2, Q3 & Q5, The speed reading can be altered in 1% increments. This is most easily explained if you use a test speed of 100km/h. If the speedo is wrong by 5km/h at 100km/h, the adjustment needed is about 5%. S1 (the switch nearest the bottom when the PC board is orientated with the connections at the right) corrects the speedo reading in single units December 2006  75 Once located, the speed sensor wire must be cut. The wire that goes to the speed sensor connects to the Super Speedo Corrector’s “IN” terminal and the wire going to the speedo connects to the “OUT” terminal. Finding the speed input wire to the speedo can involve a dash disassembly job. In this Honda, the steering column had to be dropped, the dash fascia removed and the speedo cluster unbolted and pulled forward. Make sure you’re aware of the safety precautions that need to be taken if the car is equipped with airbags. and S2 changes the output in tens. So where you want a correction of 5%, simply set S1 to “5” and S2 to “0”. If the required correction is 16%, set S1 to “6” and S2 to “1”. Using the two BCD switches in combination allows the speedometer reading to be altered by as much as 99%, in increments of just 1%. The default output reduces the reading of the speedo. This default was picked because most speedos read fast (often by about 5%). Alternatively, if you wish to increase the speedo reading , set S2 to its F position and wait for a 2-flash acknowledgement from the LED. This needs to be done with the unit connected and powered up. You will need an accurate reference to set the speedo. This can be provided by a handheld GPS, another car with a known accurate speedo or even, if you ask nicely, a police car. Just make sure that you have an assistant to do the adjusting as you drive! You can also use the “speedo check” distances that are marked on some roads – although strictly speaking, this is intended for checking the accuracy of the odometer rather than the speedometer. Installation Now for the installation but first, a word of warning: if you need to pull the dash out to locate the speed input wire to the speedo, make sure you’re Table 3: Functions Of S2 Settings Switch Setting Function IC1 Pin Status A Autoset (automatically finds a suitable input setting) Pins 1, 2 & 3 change. Pin 17 goes from 0V to 5V to 0V at a 1-second rate to flash LED when automatic sensing is complete B Pull-up resistor (low sensitivity <at> 2.5V peak) Pin 1 <at> 5V, Pin 2 <at> 0V, Pin 3 <at> 0V C AC input (high sensitivity <at> 0.7V peak) Pin 1 <at> 0V, Pin 2 <at> 0V, Pin 3 open circuit D Pull-down resistor (low sensitivity <at> 2.5V peak) Pin 1 <at> 0V, Pin 2 <at> 5V, Pin 3 <at> 0V E (initial setting) No pull-up resistor or pull-down resistor (low sensitivity <at> 2.5V peak) Pin 1 <at> 0V, Pin 2 <at> 0V, Pin 3 <at> 0V F (default is slow) Fast or slow option (LED acknowledgement: 1 flash = slow, 2 flashes = fast) Note 1: in most applications, only the ‘A’ (automatic) setting will need to be used during set-up. Note 2: switch setting must be selected for a minimum of four seconds to initiate new function. 76  Silicon Chip siliconchip.com.au Digital Speedo Lag If the Speedo Corrector is fitted to a car with a digital speedo, some lag may occur in the action of the speedo. Typically, this is noticeable when abruptly coming to a stop from a slow speed (eg, 10km/h), where the speedo may keep displaying a number greater than zero for up to a second, even when the car is stationary. Lag may also make itself evident when moving away from a standstill, where the speedo initially shows 0km/h before then jumping to 15km/h or 16km/h. This problem can be overcome by the use of the special “digital speedo function” built into the Corrector. This function is enabled during set-up by setting S1 to a position other than 2 before selecting A on S1. Positions 1–9 on S1 vary the number of pulses for which the Speedo Corrector calculates the output frequency of the speedo sensor (odd switch numbers calculate over one pulse and even numbers calculate over two pulses) and the time delay before the corrector stops sending a signal to the speedo after the input signal ceases. The delays are: positions 0 & 1 – 1 second; positions 2 & 3 – 500ms; positions 4 & 5 – 250ms; positions 6 & 7 – 125ms; positions 8 & 9 – 62.5ms. If the speedo reading noticeably lags behind actual vehicle speed, try different positions of S1 before each time setting S2 to A and proceeding with the self set-up process described in the main text. The optimal setting is that which gives the shortest lag while still reliably operating the speedo. aware of the safety precautions that need to be taken if the car is equipped with airbags. In the vast majority of cars little set-up will be needed – the corrector will mostly work out for itself what configuration is required. These are the steps to follow: (1). Connect power (use an ignitionswitched source), ground, speedo “in” and speedo “out” (to the speedo). Position the corrector so that a passenger can observe the on-board LED. (2). Set S1 to 2. (3). Set S2 to A. (4). Install link LK2. (5). Drive the car for a minute (the speedo will not work). (6). Observe that the LED flashes at 1Hz when the car is moving. This shows that the Speedo Corrector has set itself for the type of speedometer signal that is present and is receiving a valid signal from it. If the LED doesn’t flash, install link LK1 (instead of LK2) and try again. (7). Set S2 to 0 (8). Set S1 to 0 (9). Try the link options LK3, LK4 or LK5 until speedo works (the speedo should read as it did with the car standard). (10). Set S1 & S2 to give the required correction (S1 is for single units, S2 for tens). siliconchip.com.au Table 4: Link Functions Link Function LK1 8.2V max. output LK2 5V max. output LK3 Pull-up output LK4 Pull-down output LK5 AC output (11). If the speedo reading needs to be corrected upwards rather than the default downwards, set S2 to F and then wait for the LED to flash twice. Then set S2 back to its required correction value. To return to downwards speed correction, again set S2 back to “F” and wait for a single flash acknowledgement. If the required settings are already known (eg, in the case of auto electricians fitting large numbers of the design to just one type of car), Table 3 shows how S2 can be used to manually set the input configuration, while Table 4 shows the output configurations achievable by the different link positions. Any changes to the switches will not be registered by the circuit until after about four seconds, so make sure you don’t switch off power during this Parts List 1 PC board, code 05112061, 105 x 61mm 1 UB3 plastic utility box, 130 x 68 x 44mm 2 2-way PC-mount screw terminal blocks 1 DIP18 IC socket 3 2-way 2.5mm jumper headers 1 3-way 2.5mm jumper header 2 jumper shunts 1 20MHz crystal (X1) 1 0-9 BCD rotary switch (S1) 1 0-F BCD rotary switch (S2) Semiconductors 1 PIC16F628A-I/P microcontroller programmed with speedcor.hex (IC1) 4 BC337 NPN transistors (Q1,Q2,Q3,Q5) 2 BC327 PNP transistors (Q4,Q6) 1 1N4004 1A diode (D1) 2 16V 1W zener diodes (ZD1,ZD2) 1 8.2V 1W zener diode (ZD3) 1 3mm high-intensity red LED (LED1) Capacitors 3 100mF 16V PC electrolytic 2 10mF 16V PC electrolytic 1 100nF MKT polyester 1 10nF MKT polyester 1 1nF MKT polyester Resistors (0.25W 1%) 11 10kW 1 220W 0.5W 1 6.8kW 1 150W 5 1kW 1 10W 1 470W time! This delay allows you to rotate the switches to the required position without any unwanted changes occurring. Conclusion Once the Corrector is working properly, it can be mounted in its UB3 box and tucked up behind the dash out of sight. But don’t then assume that your speedo is then always going to be dead accurate – accuracy depends on tyre diameter, which changes with wear and when new tyres are fitted. Of course, with the Super Speedo Corrector, it’s easy enough to then make the required speedo calibration change! SC December 2006  77