Hi David.

How about RC filtering togthe
with shmitt triggers on inputs.


WBR Dmitry.


David Duffy wrote:
> 
> Dwayne Reid wrote:
> 
> > At 06:54 AM 5/5/2005, David Duffy wrote:
> >
> >> Hi everyone,
> >> What are people using to drive small lamps and solenoids
> >> in automotive applications? Are logic level FETs the way?
> >> I need to interface a PIC with an auto-transmission ECU
> >> and the gear select switch. The current needed to drive
> >> the ECU and the associated gear indicaor lamps looks to
> >> be in the range of about 500mA but I'm aware of the nasty
> >> spikes that can occur on automotive wiring so I'd like a
> >> device that will take it.
> >
> >
> > Most any small mosfet will work here - even the older parts like
> > ZVN2001 and similar. But I don't use these small parts much these days
> > and go straight to something like a IRF3708 (most of my loads are 1 to
> > many Amps).
> >
> >> Another (related) question is about the inputs to the PIC
> >> from the auto wiring, etc. Does anyone have a proven way
> >> to stop all the hash on the inputs? I was thinking of RC
> >> and/or LC filtering on every input. (as well as power in)
> >
> >
> > Keep your logic and power sections separate. For the logic side power
> > supply, I most often use a simple zener regulator (10K & 1n5234)
> > feeding an emitter follower (2n7052). This gives excellent transient
> > rejection even in the presence of severe load dump conditions. If Vdd
> > needs better accuracy than a zener, I use the same circuit but with a
> > 8V2 zener, then feed a LP2950 low drop out regulator.
> >
> > Logic inputs are simple: resistive voltage dividers with added shunt
> > capacitors for filtering. Typical values for a 12V system are 10K
> > series / 3K3 & 100n shunt. This assumes you are feeding standard TTL
> > inputs on the PIC: logic 1 threshold is about 1.4V. These values are
> > not appropriate for ST inputs on the PIC or for inputs on CMOS 4000
> > series logic (shift registers, etc) and would need to be adjusted.
> 
> Hi Dwayne,
> The divider with a cap is what I had in mind but I was thinking
> of a zener to clamp any spikes as well. Another thought was to
> use NPN transistors with the R/C inputs and the weak pullups
> on the PIC thereby avoiding the spike issue with the PIC.
> ie. input > R divider & cap > transistor > PIC with pullup.
> David...
> 
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