Voltage Control Resistance circuit needed

nirvanas silence · February 19, 2004, 06:38:11 PM

I am tired of using an LED and LDR so I want to use a small circuit that varies resistance as I change the voltage from 9V-0V. Any tips on where to go?

niftydog · February 19, 2004, 07:22:25 PM

I don't have any specific circuits... but googling "voltage controlled resistor" gives plenty of results!

R.G. · February 19, 2004, 07:42:21 PM

sigh...

QuoteI am tired of using an LED and LDR so I want to use a small circuit that varies resistance as I change the voltage from 9V-0V. Any tips on where to go?

I am too. I've been tired of this for about twenty years, and have been looking for an alternative for about as long. This has not been an uninformed search.

The short answer is that there isn't any such beast - there's no general voltage controlled resistor with a wide range, a tolerance of even modest signal voltages and currents, and acceptable control characteristics.

If there were, no one would be messing around with silly LED/LDR modules or tricky, inconsistent JFETs, or any one of the several things we do to take the place of a good voltage controlled resistor.

There *is* a circuit that uses one operational transconductance amplifier (OTA) to simulate one resistor to ground. It works OK, but it is current, not voltage controlled, and most of the things you want to do with a voltage controlled resistor are done more directly by simply using the OTA to implement the circuit, not using the OTA as a VCR.

We don't, as a group, muck around with substandard substitutes because we like them - we just don't have the ideal thing to work with.

niftydog · February 19, 2004, 08:02:38 PM

I wonder if anyone has investigated the use of digital potentiometers? A simple PICAXE program and a digital pot would be easy to implement, but is it useful?

anyone?

niftydog · February 19, 2004, 08:13:02 PM

well, a quick look around, and it would seem they're not suitable. You can only get about 1mA through them! There is a way of boosting it to 50-100mA using a Howland Current Pump... but is it really worth all that trouble!? I think not.

R.G. · February 19, 2004, 08:15:42 PM

QuoteI wonder if anyone has investigated the use of digital potentiometers? A simple PICAXE program and a digital pot would be easy to implement, but is it useful?

I have. See "ASMOP" at GEO.

Digital pots have their uses, and will undoubtedly be where we go with a lot of things, but they also have problems, not the least of which is "zipper noise" which happens when you change the gain of anything in little steps at regular time intervals.

Of course, a PIC and a digital pot don't quite qualify as a simple circuit to replace a resistor.

niftydog · February 19, 2004, 08:24:24 PM

..was just checking that out in fact! I had a similar idea just yesterday... not quite as spiffy as yours thought, I was just going for simple switching!

puretube · February 20, 2004, 02:21:43 AM

H11F opto-fet, like Way Huge had it in a semi-true bypass circuit, lately
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=18973

smoguzbenjamin · February 20, 2004, 04:00:15 AM

What about a standard JFET? :? A FET is used in the EA tremolo as a voltage controlled variable resistor I think.

puretube · February 20, 2004, 06:06:44 AM

lot of people tried and did and do that for "Resistor to Ground" purposes
(but are not so happy with it...)

smoguzbenjamin · February 20, 2004, 06:39:45 AM

It works for me

puretube · February 20, 2004, 07:21:40 AM

the opto-fet mentioned above can be used bi-directional and as a series VCR ("floating")...

nirvanas silence · February 20, 2004, 10:35:12 PM

Quote from: smoguzbenjaminWhat about a standard JFET? :? A FET is used in the EA tremolo as a voltage controlled variable resistor I think.

I realized this too. I will be sure to experiment and post results. Thanks guys!

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Voltage Control Resistance circuit needed