EfiO2Meter Pulsed Rpm

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RPM Measurement Connection

The efiO2Meter is equipped with a 5 V to 24 V pulse counting input.
Since it is not always possible to have an engine revolution proportional signal available for the above specified Voltage range, the available signals need to be adapted to it.
The efiO2Meter does not have the pc-board space to accommodate any signals outside the specs and an external small adaptor is needed.

There are typically 2 different RPM signal sources which do not match the specs above.

  1. Ignition coil primary coil pickup at the breaker points signal. This signal typically has high voltage spikes up to and exceeding 300 volts across the points at opening time, when the energy stored in the magnet field in the core is being transferred into a voltage on the capacitor across the points (simple explanation).
    Since this voltage exceeds the pulsed RPM input voltage max of 24 V, an attenuator circuit as described below can be used to attenuate the ignition coil primary voltage down to 12 V.
  2. The second input source which needs external signal processing is from simple magnetic pickups. Depending upon the pickup coil and it's sensing circuitry, it might work or it needs a high input pulse shaping circuit, described below.

DIY High Voltage Ignition Coil Attenuator

The schematic below outlines a typical voltage divider circuit to attenuate any high voltage spikes exceeding 400 V down to 12 V

EfiO2Meter Ign Schema.png

Capacitor C1 is only necessary in case of a permanent high voltage presence on the primary coil, as in case of a Capacitor Discharge Ignition. This Cap blocks the DC component to have a constant drain current flowing through the resistive divider R1 over R2. The R1 / R2 voltage divider has a ratio of about 20 : 1. Therefore 250 V are being divided down to 12.5 V while 100 V still provide an output voltage of 5 V.
Anything above 250 V on the input side will be clamped down to 12 V on the output by Zener diode D1,
In case of the use of C1 to block a high DC voltage, the Cap is being discharged on the falling slope of the pulses through the Schottky Diode D2.
It would't hurt to have R1 100k as a 1/2 W resistor, just in case.

EfiO2Meter Ign Layout.jpg

Above is a sample layout for component placement on a pref. board or similar.

efiLabs is currently working on a more versatile and advanced attenuator dongle, but unfortunately it is currently not available.

High Voltage Attenuator Adaptor Dongle

The schematic below provides the schematic for the Voltage Attenuator Dongle. (soon to come)


Inductive Pickup Signal Conditioner

We tested the crank-shaft position sensor on a middle of the road KIA Sephia 1.8 L 16 valve and it produces a compatible signal with a positive amplitude of 5 V - 10 V at 800 RPM and a slightly smaller negative amplitude. This amplitude increases to about 20 V at higher RPM which is still acceptable.

The crank produces 60 pulses per turn at a period time of 1.25 msec at 800 RPM.


and here is the usual more to come :)


Note : All published design documentation is Copyright efiLabs.com and its use is without exception FOR NON COMMERCIAL PURPOSE ONLY. For commercial licenses contact efilabs.com ... try us, we're reasonable :)

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