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Let's say I have an audio oscillator, which a resistor varies the tone.

I have 5 different voltage sources, each representing a tone.

How can I control the tone (vary the resistance) depending on which voltage source is on?

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  • \$\begingroup\$ It's possible, but that's not the best way to make a Voltage Controlled Oscillator. \$\endgroup\$ Mar 24, 2014 at 3:42
  • \$\begingroup\$ This sounds like someone trying to fit a method to an application so please consider what is more important, the method (flawed IMHO) or the result being a voltage controlled oscillator. \$\endgroup\$
    – Andy aka
    Mar 24, 2014 at 8:24
  • \$\begingroup\$ I'm not sure I see the join between "a resistor controls the tone" and "5 different voltage sources control the tone". Are you saying the oscillator is voltage-controlled, either by a resistor to set the voltage or by connection to one of 5 voltages? Are you talking about selecting or mixing 5 DC voltages to provide a control voltage to the oscillator, or varying a resistance by application of voltage? \$\endgroup\$
    – John U
    Mar 24, 2014 at 10:52
  • \$\begingroup\$ I have 5 logical outputs, and I want 5 different tones corresponding to the output. \$\endgroup\$
    – skyler
    Mar 24, 2014 at 17:52

5 Answers 5

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A microcontroller taking inputs based on the voltage, controlling a Digital Potentiometer.

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Definately not the best way to make a VCO. But, the question deserves an answer, because it's an interesting and possibly educational way to do a VCO, and voltage controlled resistors are used for other things.

One way, that actually was used a long time ago in audio equipment was to shine a light at a light depedant resistor

Photoresistors aren't used much on account of RoHS these days though.

The other way is to use a FET. A FET pretty much is a voltage controlled resistor, but the resistance varies a bit with the voltage across it and not just with the input voltage, so you have to add two extra resistors to correct for this otherwise it won't quite act like a resistor.

http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm

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http://en.wikipedia.org/wiki/MOSFET#Modes_of_operation

A mosfet in the triode/linear region should be what you're after. Send the different voltages to the gate of the mosfet and you'll get a varying resistance between the drain and the source depending upon the input voltage.

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To answer the question directly: Yes, it is possible for a simple MOSFET circuit to approximate a voltage-controlled-resistance within certain limits.

This page http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm has the most direct explanation I have found. See also MOSFET as a voltage controlled resistor

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If you have a logical signal 1 of 5 (represented by 3 bits), you could very simply use a set of resistors and an analog multiplexer to select one of 5 different resistances. The multiplexer will have some internal resistance (which changes with voltage and with temperature) so this is most suitable if your desired resistances are in the K ohms.

And, if you don't have it encoded, a 74HC148 would do that for you.

If the supply voltage is 5V, you could use a 74HC4051, or if it is 15V or +/-5V you could use a CD4051. The latter has an internal resistance of about 1K, the former around 100 ohms. There are better (and much more expensive) multiplexers available that will work at higher voltages and have lower internal resistance.

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