2007-03-11
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DIY CDI Trigger Article

 
  Extract of another SC article which deals with a simple programmable TCI with nearly the same trigger input circuits than DIY-CDI. Good for us, the trigger circuits are describted in more details than the DIY-CDI article.  
 

DIY-CDI Trigger Systems based on a similar TCI

This Article shows a variety of trigger inputs.

 
  The DIY-CDI will not only will work with traditional points but will also happily function with any type of trigger signal including those provided by factory and after-market reluctor, optical and Hall Effect distributors. It will even interface with an ECU ignition output trigger, making it a universal fit for all single coil cars, motorcycles and go-karts. It's the ideal upgrade for an old points ignition system or it can be used to replace a defective factory ignition module for as little as one-fifth of the price.  

Main Features

  • Operates from points, reluctor, Hall Effect and optical triggers, or 5V signal from engine management computer
  • 5-15V negative earth operation
  • Two points debounce periods
  • Special operation for poorly operating points
  • Optional inverted trigger signal operation
 

Input triggers

The way in which points work is easy enough to understand but what's all this about reluctor, optical and Hall Effect sensors?

  • Reluctor: a reluctor trigger comprises a coil wound around an iron core. A ring magnet with small externally protruding sections (teeth) is installed on the distributor shaft. As each tooth of the magnet passes the sensor, a voltage is developed in the coil. These voltage spikes provide the timing signal. Incidentally, in place of the reluctor, a magneto signal can be used as a suitable trigger signal for this project.
  • Optical: an optical trigger comprises a LED and a phototransistor or photo diode. The pair is incorporated within a package that allows the light from the LED to impinge on the photodetector. To switch the photodetector on and off, an opaque vane passes between the LED and its sensor. In addition to factory optical systems, this ignition caters for commercial optical ignition triggers such as those from Lumenition, Piranha and Crane.
  • Hall Effect: a Hall Effect trigger is a semiconductor device that switches its output on or off, depending on the presence or absence of a magnetic field. Generally, the magnet is included within the sensor package and so the sensor is easily triggered by passing an iron vane through the provided gap. The Hall Effect unit triggers when the iron vane is removed from the gap.
  • ECU: as described above, in single-coil cars with engine management, the ECU signals the ignition module when to switch off current to the coil. This signal is generally a 5V square wave.
     
 
 

New design features

  • Points Debounce: points debounce is needed because points tend not to open or close cleanly. When closing, points can bounce back open, just as a hammer does when hitting a steel plate, and this can cause a series of rapid openings and closings. When opening, the points can also bounce as the distributor cam wobbles, because of slight play in the distributor shaft.
    By setting the minimum spark duration at 1ms, the coil will fire cleanly as the points first open. This provides the full spark duration and by this time the coil will have discharged. However, if the coil is then allowed to charge up before the points close again, there can be a second spark produced if the points bounce upon closure. This second spark can produce ignition in one of the engine cylinders at the wrong time.

  • Voltage Level Sense: because of the large number of triggers that can be used, there is an option to change the voltage level sense that determines the firing point for ignition. For points, the firing point is always when the points just open, so in this case the voltage goes positive from 0V to 12V.
    For other sensors, the voltage sense may be different. For example with the Hall Effect or optical triggers, it depends on whether the ignition firing point occurs when the vane enters the sensor or leaves the sensor. So at the firing point, the voltage could be going from 0V to a more positive voltage, or from the positive voltage to 0V. A simple jumper change selects the required sense.
     

 
 

Circuit description

IC1 accepts its timing signal at the RB0 input (pin 6).
The RB0 input is protected from excess voltages by the 2.2kOhm resistor in series with this input. The protection resistor prevents excessive current flow in the clamping diodes that are internal to IC1.
Clamping occurs when the voltage goes below 0V or if it goes above the 5V supply (ie, clamping to -0.6V or +5.6V). The 1nF capacitor at the RB0 input shunts transient voltages and higher frequency signals, preventing false timing signals.

The three inputs at RA1, RA4 and RA5 (pins 18, 3 & 5) are for the linking options. Link LK1 selects whether the firing edge for the RB0 input is for a positive going voltage (standard selection) or for a falling voltage (inverted selection);  link LK3 selects normal or points operation.

Transistor Q3 provides a tachometer output and it is driven from the trigger input which also drives pin 6 (RBO) of IC1. Q3's collector is pulled up to 12V with a 2.2kOhm resistor when the transistor is off. The output at Q3's collector can be used to drive most tachometers. An impulse tachometer (now very rare) requires a different connection and should operate when connected to the coil negative.

Power for the circuit is derived from the ignition switch. This 12V supply is also directly used for other parts of the circuit. For example, it is used for the points trigger circuit and the 100Ohm base resistor for Q1.

The supply is regulated to 5V using 3-terminal regulator REG1. This is a low-dropout device that continues to deliver 5V even when its input is very close to 5V. This is useful in our application, as we want a regulated 5V supply to be maintained even when starting, when the voltage on the car battery can drop well below 12V.

The regulator is also protected from transients with internal protection clamping. The 100µF capacitors provide supply decoupling.
 

 
 

Trigger inputs

The Electronic Ignition is configured for the appropriate trigger input during construction. The six possible input circuits are shown in Fig.3.

Fig3

Fig.3: the six input trigger circuits: (a) points triggering; (b) Hall effect (and Lumenition) triggering; (c) triggering from an engine management module; (d) reluctor pickup; (e) Crane optical pickup; and (f) Piranha optical pickup.
 
 


The points input shown in Fig.3(a) comprises a 100Ohm 5W wirewound resistor connected to the 12V supply. The resistor provides a "wetting" current for the points to ensure there is a good contact between the two mating faces when they are closed. This wetting current is sufficient to keep the contacts clean  burning off oil resides, for example but not so high so as to damage them.

The Hall Effect input at Fig.3(b) uses a 100Ohm supply resistor to the 12V rail to feed the Hall sensor. This resistor limits current into the unit should a transient on the supply go above its internal clamping diode level. The 1kOhm resistor on the output pulls up the output voltage to 5V when the internal open-collector transistor is off. The voltage is at 0V when the internal transistor is on. The same circuit can be used for the Lumenition optical module.

The engine management input circuit is shown in Fig.3(c) and is quite simple its 5V signal connects to the trigger section of the main circuit in Fig.2.

Reluctor sensors produce an AC signal and so require a more complex input circuit, see Fig.3(d). In this case, transistor Q4 switches on or off, depending on whether the reluctor voltage is positive or negative.

Initially with no reluctor voltage, transistor Q4 is switched on via current through VR2 and the 47kOhm resistor.

 
 


Fig4
 

Fig.4: this oscilloscope view shows a reluctor signal (top) and the output of the ignition coil, as measured at the collector of Q1 (bottom). The reluctor signal has a larger voltage excursion than other trigger sensors and the negative-going edge triggers the firing of the coil. The primary voltage of the coil (lower trace) is clamped at around 332V by the four series 75V zener diodes.
 


Fig5
 

Fig.5: the yellow trace at top shows the reluctor signal, while the lower trace (blue) shows the base switching signal to transistor Q1. The coil fires each time the base voltage goes to ground. Note that the period for which the base signal is positive (ie, 6ms) is the dwell time and this is the charge period for the coil (ie, when energy is being stored in the magnetic circuit of the coil).

 
 
The voltage applied to Q4's base is dependent on the 10kOhm resistor connecting to the top of the reluctor coil and the internal resistance of the reluctor. VR2 is included to provide for a wide range of reluctor types. Some reluctors have a relatively low resistance, while others have a higher resistance.

In practice, VR2 is adjusted so that Q4 is just switched on when there is no signal from the reluctor. The 10kOhm resistor provides a load for the reluctor, while the 470pF capacitor filters any RF or hash signal that may have been induced. The 2.2nF capacitor ensures that Q4 quickly switches off when the reluctor signal goes negative.

Optical pickup circuits are provided for two different types of modules. One is for a module that has a common 0V supply connection [eg, Crane Fig.3(e)] and the other for a module that has a common positive supply [eg, Piranha (Fig.3(f)] . In each case, current for the LED is supplied via a 120Ohm resistor and the photodiode and a 22kOhm resistor are connected in series with the 5V supply.
 
 


Click for larger image

Fig.6: at top is the signal at the trigger input of the circuit - ie, the signal that is monitored by the RB0 input of IC1 via the 2.2kOhm resistor. This signal is typical of a points, Hall Effect and optical triggering. The lower trace is the base drive to transistor Q1. This shows the 6ms dwell occurring just before firing.


Fig7

Fig.7: the top trace (in yellow) is a high RPM signal (in this case, 6000 RPM for a 4-cylinder 4-stroke engine). The lower trace (in blue) shows the resulting switching signal fed to the coil. Note how the dwell is now 3.98ms instead of the standard 6ms, while the spark duration is 1ms.

 
 


Fig8

Fig.8: this shows the points mode where the input points signal at top is followed by the output signal (lower trace). The debounce period is set at 2ms, as shown by the 2ms pulses that follow the main pulses.
   
 

Construction

Depending on the type of trigger input, there are six different component layouts for the PC board  choose the one that is applicable to your car's trigger sensor. For example, if your car has reluctor distributor, follow the component layout of Fig.9. If it has a Hall Effect device or Lumenition distributor (same thing), use the layout of Fig.10.

Q1 is mounted at full lead length, with its metal flange toward the edge of the PC board.

 
 

Fig9
 

Fig.9: follow this parts layout diagram if your car's distributor has a reluctor pickup.


Fig10

Fig.10: this is the layout to follow if the distributor uses a Hall Effect device or a Lumenition module. Take care with component orientation during assembly.

 
 


Fig11

Fig.11: this is the points version. Secure the 100Ohm 5W resistors to the board using silicone, to prevent them from vibrating and fracturing their leads and/or the solder joints.


Fig12

Fig.12: the engine management trigger version requires no additional input conditioning circuitry. In this case, the ECU trigger signal goes straight to pin 6 of IC1 via a 2.2kOhm resistor.

 
 


Fig13

Fig.13: build this version if your distributor has been fitted with a Crane optical pickup.
 


Fig14

Fig.14: the Piranha optical pickup version is similar to the Crane version but note the different locations for the 22kOhm and 120Ohm resistors.
 
 

Installation

Next, set VR1 fully anti-clockwise, then switch on the ignition and check that there is 5V between pins 5 & 14 on the IC socket.

Reluctor settings

If you are using the reluctor circuit, adjust VR2 fully clockwise and measure the voltage at pin 6 of IC1. If the voltage is close to 0V, wind VR2 anti-clockwise several turns until the voltage goes to 5V. That done, wind it about two turns more anti-clockwise and leave VR2 at this setting.

If the voltage is 5V when VR2 is fully clockwise, rotate VR2 fully anti-clockwise and start to wind it clockwise until the voltage goes to 5V again. Then wind it two more turns clockwise.

That done, switch off the ignition and connect Q1's collector wire to the ignition coil's negative.

Starting

Now try to start the engine. If it doesn't want to start, the sensor signal may be inverted. This can happen with Hall Effect sensors and optical sensors if the output voltage goes low at the point of firing. In this case, change link LK1 to the "invert" position.

The reluctor circuit is designed to fire the coil when its output voltage swings negative. If the engine doesn't start and you are using a reluctor, try swapping the reluctor connections.
 
   

Converting From Points To A Hall Effect Sensor
 

 
 

FigD1 This photo shows how the slotted Hall Effect sensor is rivetted to the vacuum advance plate inside the distributor.

 

 

You can replace your existing points with a Hall Effect sensor - but be warned, it takes quite a lot of precision work! All the details are shown in Fig.12.

First, rotate your engine so that the rotor button in the distributor is facing the high-tension outlet for cylinder number 1. Also note the direction that the rotor button moves when the engine is turned in its correct direction. Set the timing mark on the flywheel to the number of degrees before Top Dead Centre specified in the workshop manual and indicated by the engine block timing marks.

Now place a mark on the edge of the distributor body to show where the timing mark on the rotor button arm is positioned. This sets the alignment for the Hall Effect modification. The distributor can now be removed from the engine

The Hall Effect sensor is designed to be used with a rotating vane that passes through the gap incorporated in its housing. The Hall sensor is mounted on the distributor advance plate and secured using the rivets incorporated on its housing. The rotating vane needs to be made so that it spins with the distributor shaft and its vanes pass through the sensor gap.

 
 
For this to happen, the rotating vane needs to be cup-shaped. The horizontal face has a hole to allow it to be placed on the distributor shaft and locate with the rotor button. The vertical section needs to have slots cut in it to appropriately trigger the sensor.

The number of slots on the vane equals the number of spark-plugs for which the distributor caters. So a 4-cylinder car with four spark plugs will use four slots. These slots need to be evenly spaced around the circumference of the rotating vane. It is essential to be accurate here, as a 1° difference between slots represents 2° on the engine.

A 4-cylinder engine will have each slot positioned 90° apart. 6-cyclinder and V8 cars will require slots spaced 60° and 45° apart, respectively.

 

 

 


Fig15D2

Fig.15: these diagrams and the accompanying photos show how to replace the points with a Hall Effect sensor and make the rotating vane assembly. Note that the slots in the vane must be accurately positioned - see text.
 
 

Making The Disk

Making the disk is easier if you can start off with something that is already preformed. We used the tin-plated backing from a high power potentiometer. A suitable one is the Jaycar RP-3975 15W potentiometer. This provides us with a cup that is 40mm in diameter. All that is required is to drill out a hole in the top for the distributor shaft and cut the slots in the side.

 
 


Mounting The Sensor

 
FigD3

FigD4

 
When this has been done, the Halsensor can be mounted on the distributor advance plate. The sensor needs to be located so that the centre of its slot is 20mm away from the centre of the distributor shaft. This will allow the 40mm diameter cup to spin without fouling the Hall sensor.

Drill the two holes in the distributor advance plate and countersink the holes on the underside of the plate. This will allow space for the rivets in the Hall sensor to be peened over. Before riveting, check that the Hall Effect wires do not foul against the points cam (this happened in the distributor we were modifying!). To prevent this, the wires were passed under the Hall sensor by filing a small channel beneath the sensor, so that the wires could be fed through to the other side. The wires were then fed through a grommet in the distributor's body.

 
 


Rotating Vane

 
The rotating vane should be placed over the distributor shaft and should sit on the top of the points camshaft. Check that there is sufficient clearance between the vanes and Hall sensor gap. If the cup needs to be higher than this, it can be placed over the rotor button shaft.

In this case, the rotating vane must be electrically connected to the distributor shaft to prevent static build up which may damage the Hall sensor. A small piece of tinplate soldered to the vane and bent so it passes up inside the rotor button to make contact with the distributor shaft is suitable.

When the Hall Effect sensor has been mounted, place the rotating cup over the distributor shaft and hold it in place with the rotor button. Check that the vane spins freely through the Hall sensor slot.

 


FigD5

The rotor button assembly fits over the distributor's camshaft, with the vanes passing through the Hall Effect sensor.

 
Now you are ready to align the disk. Rotate the rotor button to the alignment marks set previously. Remember, these indicate the centre position of the rotor button at Number 1 cylinder timing. Move the rotating vane relative to the rotor button so that the gap is just leaving the centre of the Hall Effect sensor.

Note that you must be turning the distributor in the direction that it travels when installed in the car. Mark the position on the rotating vane and rotor button using a marking pen (do not use a scriber on the rotor button or the high tension voltage may travel down this). We soldered in a couple of PC stakes inserted into holes drilled in the top of the vane, to align the vane position – these keyed into the locating slot in the rotor button.
 
 

Glueing The Vane

Finally, the rotating vane can be glued to the bottom of the rotor button using high-temperature epoxy resin. We used JB Weld epoxy steel resin, a 2-part epoxy. This is suitable for temperatures of up to 260°C. The quick-setting version can be used for temperatures up to 150°C.

 
     
 

Parts List - Trigger Systems

 
 

Semiconductors
1 MJH10012, BU941P TO-218 high-voltage Darlington transistor (Q1)
2 BC337 NPN transistors (Q2,Q3)
1 LM2940CT-5 low-dropout 5V regulator
4 75V 3W zener diodes (ZD1-ZD4)

Capacitors
3 100µF 16V PC electrolytic
1 10µF 16V PC electrolytic
1 100nF MKT polyester
1 10nF MKT polyester
1 1nF MKT polyester
2 33pF ceramic

Resistors (0.25W 1%)
1 100kOhm; 1 1.8kOhm
2 47kOhm; 1 470Ohm
2 2.2kOhm; 1 100Ohm5W

Points version

1 100Ohm 5W resistor

Reluctor Version

1 BC337 NPN transistor (Q4)
1 2.2nF MKT polyester capacitor
1 470pF ceramic capacitor
1 100kOhm top-adjust multi-turn trimpot (VR2)
1 47kOhm 0.25W 1% resistor
2 10kOhm 0.25W 1% resistor
1 1kOhm 0.25W 1% resistor
1 PC stake
 

Hall Effect Version

1 Hall Effect sensor (Jaycar ZD-1900) or Lumenition module
1 rotating vane using a 15W power potentiometer backing (eg, Jaycar RP-3975 - not required for Lumenition module)
1 small quantity of high-temperature epoxy (eg, JB Weld Epoxy Steel Resin)
1 1kOhm 0.25W 1% resistor
1 100Ohm 0.25W 1% resistor
2 PC stakes

Optical Pickup Version

1 optical pickup (Piranha, Crane, etc)
1 22kOhm 0.25W 1% resistor
1 120Ohm 0.25W 1% resistor
2 PC stakes

Miscellaneous Angle brackets for mounting, automotive connectors, self-tapping screws etc.

 
   

 

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