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Trigger
The plan was to use a wire connected to the negative pole of one of the ignition coil primaries.
The voltage at igncoil primary is a bit high (also, dangerous to your health and your equipment):
- +14V normally (battery +)
- drops to appr +2V, for appr 3 msec (dwell) before spark
- rises to appr +200V for a moment at spark
- actually, possibly upto +400V if the sparkplug high-voltage wire is removed from the sparkplug and left open (which is not a good idea)
- drops to +14V again
As you see, it has 2 falling edges, a small 14=>2V and a big edge +200V =>14V
Concerns
- a voltage divider is a must, like a 2x270k to 10k (appr 1/55 division)
- rising edge is preferred
- the voltage does not go below 0V (needed for VR setup), but even under 1V is not guaranteed (needed for HALL setup)
Suggestion
- divided signal: this is a low-voltage signal made from the high voltage signal:
- 270k throughole from the high voltage (~200V coil primary, not the secondary that would be fucking high voltage!) signal. Prefer throughole because of high voltage
- 4k7 to ground
- between divided_signal and controller_input we use a high-pass filter cap (that allows the signal through with a DC difference): 100..470uF (or higher) electrolytic cap (min 25V, the negative pin goes to controller_input) in parallel with a 10uF quick-cap: like the "big" 1206 (formerly 1210) cap in rescue-kit
- a bias pulldown resistor might be needed on the controller input towards GND or -8V (-8V available on yellow wire inside LCD flying-loom DSUB9 connector housing, not connected to any DSUB9 pin) so the signal goes low enough. This is a bit different with HALL and VR
- during debugging, it is very important to document the measured DC on the controller input (with the pulldown resistor you actually used)
HALL
With HALL setup, it's perfect if the signal goes below 1V, than above 3V. Should be rather easy. The 1/58.5 voltage divider from 270k and 4k7 is good, keep appr those values in case of HALL
- Try to achieve appr 100..500mV signal bias (this is well under 1V): start with a 1k pulldown to GND first
- at the high-pulse, the 200V / 58.5 should be enough to drive the signal above 3V easily
VR
With the controller primary_trigger=VR, the signal must go below 0V to trigger. Also, the falling edge is detected (not possible to detect the rising edge). Because there are 2 falling edges, this might cause some problem with unsuitable bias hardware. With good setup, the 14V=>200V will arm the adaptive-threshold circuit and only the 200V=>14V edge will be sensed. With VR I would try the following setup as a starting experiment:
- voltage divider: 100k and 10k (1/11 division, not 1/58.5)
- even 47k and 4k7 are possible (that is also 1/11 division but somewhat stronger signal to the controller)
- high-pass filter cap: 10uF + 220uF
- controller input:
- 1k pulldown to GND
- measured DC appr: +0..300mV
Capacitive coupling from sparkplug wire => VR input
With VR input (at least on suzuki swift of MembersPage/Gabor ), we've seen success with a few (3..5) turns around a High-Voltage spark wire (that is transformer secondary). No galvanic contact, of course. The "capacitive coupling" was enough to trigger the circuit reliably. Note that the signal is small (maybe just 50..100mV): it worked without primary trigger pullup resistor. Currently the R30= 18..22k internal pullup (now standard) of the VR input is in the way for this application (it's ment to make the input non-sensitive to noise, but in this application we want to sense "noise"). A strong 470 Ohm pulldown to GND or a weak (appr 39k, but this would need to be adjusted for near 0mV +- 40mV DC) towards -8V could work, but the internal pullup might need to be removed or the sensing wire might need to follow the sparkplug wire along a longer, 20..40cm section to get a stronger coupling. Experimentation might be needed (scope is almost essential)
Also, keep the sensing wire away from all other sparkplug wires (so only the chosen spark is detected without interference from other spark-wires).
Note:
- The trigger is just for RPM
- fuel injection control?
- there is no ignition control with this application as the stock Suzuki ECU takes care of that