Hi Olin,

Just shooting from the hip eh? Wow, wish I could just shoot from the hip
like that! That looks to be exactly what I need, many, many thanks.

Graham

> Since you need to see 2V pulses, you will have to include some gain.
> Unfortunately that will reduce your noise immunity.  What is the source
> impedence of the pulses?  I'm not familiar with fuel injector
requirements,
> but I'm going to assume the pulse has a fairly low source impedence.  I
will
> further assume the pulses are short and the average duty cycle is small,
> like 10% or less.  Again, I could be wrong on these assumptions, but I
need
> to start somewhere.
>
> The biggest problem in a car is noise.  There is no such thing as a clean
> signal or clean power supply unless you go to some trouble to make them.
>
> So here goes shooting from the hip because there is too little information
> to do this right.  Make a resistor divider chain with three resistors.
The
> top end is connected to the pulse signal and the bottom end to ground.
The
> top two (R1 and R2) resistors are 500ohm 1/2W and the bottom one (R3) is
> 1Kohm 1/4W.  Put a diode from ground to the top tap.  This will clip
> negative spikes (yes, there will be negative spikes).  I would also add a
> cap accross the diode, but the value depends on the minimum pulse width
and
> height which we don't know.  I'm thinking about 1nF, but that could be way
> off.  The purpose of the cap is to filter out some of the high frequency
> crap, like pulse bounces and the like, but to not filter out whole pulses.
> Next connect the base of a small signal NPN transistor to the lower
divider
> tap, the emitter to ground, and the collector to +5V thru a 1Kohm
resistor.
> Almost any small signal NPN will do fine.  I would use a 2N4401, but
that's
> only because I have a few 100 of them in stock.  The collector of the
> transistor is the output and is suitable for connecting directly to a PIC
> input.  It will normally be high and pulse low when a pulse is detected.
>
>
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