Now this is from a long time ago...

We used to use a liner pot with a wiper to end pad resistor to change it to
sort of log.
So take your base requirement of 40K mid point and 47K total you can get
something like that with a 51K fixed resistor and a 220K pot across it

Steve

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
IVP
Sent: 15 July 2012 05:39
To: Microcontroller discussion list - Public.
Subject: [EE] Reverse log pot

Hi all,

I have a 16F88 project in which the speed of instruction cycles
is controlled by a variable R in RC mode, as per the attached.
The R value as shown allows variation between about 0.5MHz
and 4MHz Fosc

What I would like is a reverse log or anti-exponential pot with a
curve such that the exponentiality of the RC is cancelled. IOW
the frequency appears to change linearly, so for example mid-travel
of the pot corresponds to the mid-point

SQRT(500*4000) =3D 1.4MHz

The other gang of the pot I intend to use so that an LED can be
lit in the region of the mid-point

Does anyone have a suggetion for making such a curve ? Tried
various combinations across the terminals but haven't yet found
what I'm looking for. For example the mid-travel usually comes
out at around 40k. What is needed is 47k-ish at maximum with
the majority of the travel for just a few k so the higher frequencies
can be set more accurately

I don't have to use any particular pot, single or dual. Examples
I've found on the web are for converting linear to anti-log taper,
something I've done myself in the past, and all use the 3 terminals

Possibly I could put the C to Vdd and R to Vss so that the 3rd
terminal can be part of the conversion. I've heard that C to Vdd is
acceptable for crystals, not sure about RC mode, haven't tried it

TIA

Joe

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