Some factors can impact VR trigger quality.

The amplitude of tooth pulses is not equal

This is a common error with homemade trigger wheels that have a long deep gap at the missing tooth (this results in a higher amplitude around the missing tooth). Professional wheels have a continuous depth-change at the missing tooth, full depth is only reached at the middle. Examine a 60-2 Bosch wheel.

triggerwheel is not centered on the shaft

Sine wave will modulate the VR signal amplitude, so the arming threshold becomes important, especially at high RPM.

If in doubt, record VR signal with scope or your notebook soundcard.

Variation in individual tooth-height just makes the problem worse. Mik was the first to experience such problem when he patched his original (quite exotic) 18-1 + 18-1 crankwheel with a surgery for normal 36-1 tooth. The added tooth had smaller amplitude than the original teeth.

Soundcard can be used to record VR signal

See ElectronicDesign/SoundRecorder for better description

WhatEverYouDoYouDoItAtYourOwnRisk !!! (take care of your soundcard)

Since the VR signal amplitude can be quite high (especially at high-RPM, often >30V), the line-in input should be preferred to the mic-input. With mic-level input a much higher divider can be used, eg. 100k and 1k.

Example problematic wheel signal


Note that this signal is inverted because of the soundcard. It was verified with other means that v3.x sees proper signal polarity (either with InputTrigger/TriggerLog, or the engine does not even start with the other VR polarity). Here we focus on the amplitude problems.

shows that

The trigger occur when the VR signal fall down past 0v (which is the horizontal line in the middle of the signal, not marked so use your imagination). The trigger is armed for the next trigg after going higher than the arming threshold.

The arming threshold decays after every peek with 220nF * 1M Ohm = 220msec time constant. This decay is fast enough at cranking RPM and could allow very badly constructed triggerwheels to work during cranking but at idle rpm the arming threshold is pretty stable already and will not track the wheel inregularities.

A second problem is that the difference between the normal amplitude and the amplitude in the gap increase with rpm. This is the most common multitooth problem we have. In a few cases we have had cars that run nicely at low rpm but up toward 3000 rpm they start to missfire when the ECU start to see trigger errors and cut fuel and spark. (above 3000 RPM for MembersPage/EmilMalmsten/AudiS who provided the "beautiful" example: thanx!)


Note that the 40% safety margin should eliminate any runout problems on a sane multitooth triggerwheel, but if the teeth for some reason result in a 40% amplitude difference there will be trigger problems !

Should we measure this characteristics of each LM1815 during board testing ?

If someone want to waste some time on simulating this better:

Proposed solution

Easy to do cleanly with the following step:


Note that a similar solution is the standard factory-populated setup since v3.3 - no modification needed. As a side-effect, this decreases the maximum tolerable noise somewhat, but it's fine with any reasonable quality VR signal and is a big help for those with irregular wheels.

See also