[Diy_efi] LM1815 problem

[Arnoud van der Wel]

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).

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