Voltage controlled resistor or low pass 1

[D101]

Hi
Can any one help I want to make a voltage controlled resistor or a voltage controlled low pass filter.
Many Thanks

 

[MMAshwin]

Hi,

What sort of resistance or frequencies are we talking and what is the power requirment ?.

And I take it we're talking about a voltage controlled potentiometer and no a Voltage dependent resistor (VDR).

If its the former then there are devices on the market such as DS180X series or DS1869, DS166 or MC3340 to name but a few see the following links for details

http://pdfserv.maxim-ic.com/arpdf/DS1807.pdf

http://pdfserv.maxim-ic.com/arpdf/DS1666-DS1666S.pdf

http://www.mil.ufl.edu/links/datasheets/more.datasheets/mc3340rev6.pdf

I suggest the last link being more suitable.

As for the voltage controlled low pass filter what are you controlling. The upper & lower frequencies, the bandwidth any gain?

Anyhow again there are various devices for configurable filters. The first that comes to mind is the UAF42AP see

http://focus.ti.com/docs/prod/folders/print/uaf42.html

http://www.informatik.hu-berlin.de/~hochmuth/bvp/uaf42.pdf

Any help ?, yes no let me know.

Regards

 

[D101]

Hi Laffalot
Thank you for your help so far
Really I need a voltage controlled resistor not digital
the value not that important as long as it can ghange perhaps 10 to 1.
As for the voltage controlled low pass filter the frequency's Im interested in are between 20K to 100K

I have heard that you can use a Cmos 4007 as a voltage controlled resistor is this true and if so how?.

Many thanks
D101

 

[melone]

Any FET transitor can be used as a voltage controlled resistor. Most of the time, these transitors are used in either "Off" or "On" mode meaning that the gate voltage is either 0V or high enough to cause the FET to fully "turn on". However, if you apply a gate voltage between those two values, the channel size is reduced and the voltage drop across the drain-to-source will increase. By modifing the gate voltage value, the drain-to-source resistance changes (increase voltage -> increase resistance, etc.).

Hope this helps!

 

[melone]

Let me modify my previous post. The lower the gate voltage, the larger the drain to source resistance!!!!

Sorry for the confusion!

 

[MMAshwin]

Hi,

Just a thought. How about a microcontroller with A/D built in, connected to one of those resistors as mentioned in my previous post. You could also use it to swicth different capacitances and/or resistances in a filter design to affect its Low Pass characteristics.
The controller digitizes the analog voltage and controlls the addressable pots or the capacitor bank
If the above is a bit OTT (Over The Top) then some examples of Melones post are as follows

http://www.national.com/an/AN/AN-32.pdf

(see page 5 on the above link)

http://www.semiconductors.philips.com/acrobat/various/SC07_98_IDEAS_1.pdf

(see page 4 on the above link)

http://physics.bu.edu/py681/lb7_201.pdf

(see page 8 on the above link)


As for the filters then you could try the MAX297 capacitor switched filter however this only goes to 50Khz.
See:-

http://pdfserv.maxim-ic.com/arpdf/MAX293-MAX297.pdf

Any help ?, yes no let me know.

Regards

 

I appreciate your ideas Laffalot.

I've been searching for a light or voltage controlled potentiometer for an art project I'm working on. I contemplated hooking up a CDS cell (Light dependant resistor) to a PIC and then control a digital potentiometer. This seems excessively complicated when I consider the programming of the microcontroller.

Hmm... there must be a more simple solution out there.

Thanks for considering this problem

I found this DC controlled potentiometer circuit. I imagine it would be very difficult to source and there is not a very good data sheet to go along with it.

http://www.nteinc.com/specs/7000to7099/pdf/nte7072.pdf

Rickam.

 

Oh, and if you doo need a voltage controlled resitor, I think you can place an LED facing a light dependant resistor in a piece of black shrink tubing and that may work. Or you may want to look intoa device called the CLM600 wich serves the same purpose.

 

[jbartos]

Suggestion: Reference:
E. James Angelo, Jr., Electronics: BJTs, FETs, and Microcircuits, McGraw-Hill Book Co., 1969, Sect. 5-4 A Voltage Controlled Diffused Resistor
gives a reasonably good background.