[Diy_efi] LM1815 problem

Bevan Weiss kaizen__ at hotmail.com
Mon Jul 8 13:14:24 GMT 2002


Please excuse me if this isn't applicable, I'm not very familiar with the
LM1815...

If the 10k resistor (or cabling) is running next to anything that will
experiance a high current then it will get a voltage induced across it, this
may be very short and not have enough energy to trigger the one-shot,
however it will still cause the output of the LM1815 to go high.  I'm
thinking of the LM1815 as a kind of pulse stretcher of some kind, like a
monostable.
If this is the case then shielding the cable (and resistor) would help to
decrease the energy transfer.  Moving the cable (and resistor) away from
high current sources (such as ignition coil) might be another way to
decrease the effect.

The datasheet (just looking at it now) states that if the signa is less than
30mV peak, the one shot is guaranteed to not trigger.  However it doesn't
state that the output will be held at 0V potential...

I would check the inputs to the LM1815 in operation, and make sure that the
instantaneous voltage is never greater than or close to 30mV.

Other suggestions...
Use a lower external resistor value, this should place the voltage at the
peak detector above the noise present in the system.
Check that the supply is correctly bypassed, a slight fluctuation in ground
potential could cause a mistriggering.
Make sure that the Input selector is correctly tied either low or high
(depending on the situation)


----- Original Message -----
From: "Arnoud van der Wel" <A.P.vanderWel at utwente.nl>
To: <diy-efi at diy-efi.org>
Sent: Tuesday, July 09, 2002 12:35 AM
Subject: [Diy_efi] LM1815 problem


> Good afternoon all,
>
> I am new to this list and though my question is not really EFI, it is
engine
> related and DIY...
>
> For a 1987 Moto Guzzi V75/4, I am building an electronic ignition. Nothing
> very hi-tech but simple, cheap and reliable.
>
> My design has four main parts:
>
> - 5 Vdc supply
> - VRS preamp with an LM 1815
> - Variable delay generator (Atmel 90S2313 microcontroller)
> - Coil drive (HGTP14N40F3VL)
>
> The prototype is up and running but there are still some unresolved
issues.
>
> In particular, the LM 1815 is not really doing what it should be. I use it
> in the adaptive mode (pin 5 open), and it is (mostly) working fine.
However,
> with input noise, I get short pulses at the output.
>
> I can understand that if I produce a trigger condition at the input, that
> the one-shot triggers and produces an output pulse (this would be my own
> fault), but this is input noise that apparently is *not* sufficient to
> trigger the one-shot but *still* generates some junk (pulses far shorter
> than the programmed one-shot duration) on the output. I don't like this:
as
> long as I don't trigger the one-shot, I feel there should be *no* pulses
at
> the output.
>
> Supplies are checked and clean.
>
> I have a 10k series resistor between VRS and LM 1815.
>
> I have now 'solved' the problem using a small capacitor over the output
(2.7
> nF/5.6 kOhm) but this introduces a delay into the system that I would
rather
> not have (though it is known and can be compensated for in software).
>
> Does anyone have similar experience using the LM1815? solutions? better
> workarounds? Do they all do this?
>
> I would be most interested in any ideas.
>
> Kind regards,
>
> Arnoud van der Wel (Enschede, The Netherlands).
>
> _______________________________________________
> Diy_efi mailing list
> Diy_efi at diy-efi.org
> http://www.diy-efi.org/mailman/listinfo/diy_efi
>

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