Date sent:              Tue, 3 Oct 2000 19:30:13 +1000
Send reply to:          staff@blackrobotics.com
From:                   Roman Black <fastvid@EZY.NET.AU>
Organization:           BlackRobotics (Fastvid)
Subject:                Re: [EE]:Resistive circuit
To:                     PICLIST@MITVMA.MIT.EDU

> Ruben Jvnsson wrote:
> >
> > Hello all,
> >
> > I have a circuit with a potentiometer (value 1kohm) in it. This
> > potentiometer is to be set when the equipment is first installed in
> > its environment. I am looking at a way to replace this potentiometer
> > with an automatic setting, done every time the circuit is powered
> > up. I thought I had a solution with a digital potentiometer in
> > reostat configuration (2 terminal) and a PIC to control it. The
> > problem is that the wiper current of these circuits have to be lass
> > than 1 mA (I need about 10mA).
> >
> > So my question is, does anyone know of a two terminal circuit (plus
> > power and control) that acts as a variable resistor where the
> > resistance is set by a voltage level or a variable resistor with
> > less current through it?
> >
> > I'm sure there must be a nice litle OP-amp or transistor curcuit
> > that does this but I just can't seem to figure it out.
> >
>
>
> Sounds like you to need to establish WHAT the pot normally connects
> to. If it is used in some type of resistive divider to set a voltage
> you can probably use a transistor when you know more of the spec of
> the target circuit, ie whether it sinks or sources current.

It is used as a current damper in the feedback winding in a
oscillator for a metal detector. The pot is adjusted until the
oscillator stops and then turned back (about one turn) until the
oscillator starts again. If a metal object is now put into the field
of the inductors the oscillator stops again, indicating metal. The
problem is that the pot has to be set when it is mounted (in a
conveyor belt) because the surrounding metal affects the detector,
otherwise making its sensitivity very poor.

This is, since long, a proven design which works very well and I was
thinking that I could improve its functionality by removing the
trimming in a simple way. Every metaldetector unit contains between 3
and up to 9 oscillator circuits, I think, depending on the width of
the belt, and it is very easy to get a poor sensitivity if the pots
aren't set accordingly.

>
> As a curiosity, many modern cheap hifi units have a motorised
> volume control, you may have seen these. You use the remote control
> and the volume knob on the hifi winds itself up or down. Pretty nifty!
> I have a couple of these I have pulled from dead hifi units. I always
> thought they might be handy for times when I have no option but to use
> a pot, and it needs to be electrically isolated etc. They are usually
> 500k or 250k. And log probably, being volume pots and all. :o) -Roman
>
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==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================

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