Input Trigger/Analog Side - VEMS wiki www.vems.hu

InputTrigger/AnalogSide (2006-03-16 04:43:08)

This page is about the LM1815 circuit we use for InputTrigger on GenBoard/VerThree

The aim of the circuit is to allow a huge input amplitude voltage range for easy installation and support for VR type engine(or wheel) phase(which has RPM info of course) sensors.

Look at the [waveform generated from VR] type InputTrigger

History: the former inferior optocoupler circuit

GenBoard/VerTwo used the same optocouple based circuit that was used in motorola-megasquirt. Even though it went through several iterations in the ancestors, the circuit is not very good.

An input trigger needs:

hysteresis
better if the hysteresis increases with input signal amplitude (adaptive hysteresis)
protection from input spikes

The optocoupler could also provide total isolation, but it's not required, nor used here: the input and output GNDs are actually connected. Protection from input spikes is not a big deal, it can be solved with well sized resistors, caps and diodes, which are needed anyway.

GenBoard v3.0 early revisions (never manufactured) had 2 selectable solutions:

optocoupler based circuits like the original, but two of them (with a nice 8-pin SMD part with 2 darlington optocouplers)
and 2 generic OPA based circuits with hysteresis that could be configured as amplifier as well.

After comparisons, the optocoupler based circuit was dropped, since it cannot cope with the basic requirements: no hysteresis. Even though it definitely works with difficult installation and software filtering, but why not do better?. Read below.

The solution

So a professional solution with the LM1815 chip was implemented, which can handle any type of InputTrigger (although the resistor-values depend on type). 2 channels of them, of course.

After reading the datasheet and inspecting proven applications like

We put 2 instances of a similar circuit on GenBoard/VerThree PCB.

The LM1815 is not very cheap:

footprint: Jorgen can make an ECU with beyond motorola megasquirt functionality in about a DB37 case, which could only host about 4 LM1815 circuits.
price ~ 3 Euro for 1 chip (1 channel) + external components + assembly + PCB footprint etc..

But it's a very good and proven solution. Something similar is used in EDIS module for EDISIgnition (so those using EDIS will not need to use the LM1815 on GenBoard/VerThree).

The input amplitude range is amazing in any case: it can clamp upto +-3mA, the external resistos must be sized according to this (but never belive me, read datasheet).

That means that without the external voltage divider it can trigger on a 200mV .. 50V peak2peak signal. 200 mV is small enough: you can acquire it with a few turns around a HV wire.

I have a wire with 2*470k resistors in it, in series. I can plug into the wall which is in Europe 230V effective (+-400V peak) and trigger GenBoard/VerThree with it: 50Hz reads as 1500 RPM (at least for 4 cyl, since this depends on config).

High Voltage input: ignition transformer primary (not secondary!) side

LM1815 based VR circuit can be used for coil-setups (the wire going to the transformer primary of an alien ignition) too: it is recommended to use an external (or onboard) voltage divider with a 900..1000k (eg. made from 2*470k in series) and a 100k to GND. This divides the max 400V (rather 200V normally, when sparkplug is there on the secondary) peak2peak signal to about 36V peak2peak (1/11).

applications:

batch-injection: engine phase is not very interesting
capture the factory-based ignition-advance map against a position signal

Triggering on falling edge not ideal for directly tapping transformer primary

Unfortunately the falling edge is triggered, but this is not very good: the rising edge would be much better, since there is only one rising edge, with significant amplitude. Normally there is a smaller and bigger falling edge, and the smaller wants to be neglected (and is in fact, due to the adaptive histeresys, but not ideal). Look at the textual scope snapshot for the why there are 2 falling edges:

12V => small falling edge (that should be neglected by trigger)
2V (IGBT pulls down voltage to charge the coil - this is the dwell period as you know) appr. 2V => big (and only) rising edge
250V (or more, if sparkplug is disconnected, and 380V clamping takes effect) => big falling edge
12V back to original resting state (and the cycle starts over, the small falling edge is next!!!)

Therefore some people feel more comfortable to wind a few turns around the coil wires (primary or secondary is better?) and trigger from there. A scope comes handy.

VR sensing without the LM1815 chip

This is only for the curious, since the LM1815 circuit is very nice and proven, and used all over the project, eg. there is 2 channels on GenBoard/VerThree PCB, 2 channels on InputTrigger/AudiTrigger (and on small round WideBand meter unit).

Earlier we tried smaller footprint OPA circuits with success (ask Dave), although we did not do very intensive tests.

A very simple basic comparator circuit built from OPA (with Schmitt-trigger histeresys input, any analog introduction books have the circuit example):

using with HALL sensor

use the jumper to bypass the LM1815 circuit (no need to populate the LM1815 in fact, but doesn't hurt). The HALL output must be pulled up to 5V not 12V!
OR use an input cap from the HALL output (C88, C89 10uF non-polarity 35V sound fine) so the signal seen by LM1815 goes below zero. Note that the information about the rising edge is lost this way.

See GenBoard/VerThree schematic