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Feedback on the Programmable Ignition System By ANTHONY NIXON The programmable ignition system featured in the March 1996 issue has created quite a lot of interest from motoring enthusiasts. Now the designer has some follow-up information to enhance its operation. Since the original article was published in the March 1996 issue, a reluctor version of the circuit was published in the Circuit Notebook pages of the May 1996 issue. Apart from that, I have come across some problems which may affect the processor due to electrical noise finding its way back into the inputs. This causes the micro to operate in an erratic manner and upsets the engine operation. Fig.1 shows suggested modifications to give better electrical isola80  Silicon Chip tion between the ignition circuit and the Programmable Ignition board. “Method 1”, shown at the top of Fig.1. shows the use of 4N28 optocouplers for the three connections to the PIC microprocessor. “Method 2” employs zener diode clamping to prevent any serious voltage transients which may otherwise affect the micro. The software has been upgraded and now allows the user to program a two-stage advance curve instead of the original single stage curve. This is shown graphically in Fig.2 while the effect on an 8-cylinder car is shown in Fig.3. The new software allows the user to switch between the two data settings while the engine is running. Also the Rev Limit feature has been changed and it now misses every second spark instead of retarding the timing. The main concern with users was the fact that you could set the advance for one data set, say 20 degrees, but you could not program more advance into the Fig. 1: two methods of minimising noise in the microprocessor circuitry. Fig. 2: with new programming the system now allows the use of a twostage advance curve. Fig. 3: the timing diagrams for the two-stage advance curve on an 8-cylinder engine. September 1996  81 Fig. 4 (above): connecting the Knock Sensor (SILICON CHIP April 1996) may be done using an LM311 comparator. It connects to the Vacuum Advance input on the microprocessor. Fig. 5 (right): a rotor button with a “lagging” tail piece added. This can prevent misfiring problems caused by the rotor button being at the wrong position relative to the relevant spark lead post. second data set, say 30 degrees. This was due to the fact that the timing was retarded from the advance point as set by the distributor. At low revs, the software retards the timing by 45 degrees and will give advance to that set by the user as the RPM rises. In this way, more advance can be programmed for the other data set. This is needed to correctly set up timing for a change from petrol to gas, for example. This upgrade is available for the cost of return postage to anyone who has purchased either the micro direct from myself, or to those that have bought a kit from Jaycar which may have the original micro supplied. The upgrade also includes documentation. There have also been enquiries about using the Knock Sensor (published in the April 1996 issue of SILICON CHIP) in conjunction with the Programmable Ignition. Fig.4 shows how the knock sensor is connected to the Vacuum Advance input to the micro. The vacuum advance mechanism is left connected to the distributor as normal. The filtered output from the knock sensor is fed to an LM311 comparator. When this voltage goes higher than that preset on the inverting input pin 3, the output at pin 7 will go high. When the micro detects this high, it will retard the ignition by an amount set by the user. In effect it works in the opposite manner to which it was intended. As the output of the LM311 comparator is open collector, it provides com82  Silicon Chip patibility between the 8V circuitry of the Knock Sensor and the 5V supply of the Programmable Ignition board. Note: this circuit arrangement has not been tested on a vehicle). Modified rotor button Having address­­ed all of the problems that have been presented so far, one still remained, which I also had trouble with at times on my vehicle. The engine was misfiring especially while starting. I finally traced it to the shape of the brass contact on top of the rotor button. From my observations, the relative firing position of the rotor button to the spark lead posts in the rotor cap does not change even when the timing is retarded or advanced by the normal action of the advance springs. It does change though, when the vacuum advance mechanism is functioning. When the ignition is controlled by the micro, it has the same effect as changing the timing the way that the vacuum advance mechanism does, ie, it also alters the relative position of the rotor button to the spark lead post. As the micro is capable of delaying the spark by 22.5 degrees on the distributor shaft, the rotor may rotate past the correct spark lead post and send the spark on to the next one, thereby causing the engine to misfire. To counter this, I made up a new brass top for the rotor button with a “lagging” tail piece added and I also trimmed off the leading tip. This is shown in Fig.5. This diagram can only be used as a guide as each vehicle has a different distributor setup. I had to look at a few of the newer types of rotor button available and some of these also had a “lagging” edge. These are used with factory electronic ignition systems that still employ distributors. I have designed a new board which incorporates the method 2 protection mode mentioned previously. It also allows Jaycar keypads and LED displays to be used directly and has provision for the optical timing module. The board dimensions are still the same. SC