I would only like to add that I would use a zenner diode instead of an
ordinary diode as this will limit the the max voltage as well as
clipping the negative spikes

Peter Cousens
London UK

> -----Original Message-----
> From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]
> On Behalf Of Olin Lathrop
> Sent: 27 April 2002 13:41
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: How do I condition a high voltage pulse for a PIC
to
> read
>  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.

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