Mosfet as volume control

[westhomas]

hello all,

I'd like to use a mosfet to control the signal from say a 555 astable oscillator as it is fed out to an amplifier (like your standard guitar amp). I've never used a mosfet before but it seems like the appropriate component for the task.

The signal from the sensor (which is a Sharp IR proximity sensor) is about 1 to 3 VDC. I would like the output to be 0 when the gate voltage is 1, so one of my questions is whether I need to modify the sensor output before it reaches the gate.

My other question is whether this will work - IRF530N even though it's way above the ratings I need.

Here's a very basic sketch...

much thanks

 

[Nigel Goodwin]

Sorry, but life isn't that simple.

 

[philba]

I think what you are asking is "can I use a MOSFET as an amplifier that I can control with an external voltage". The answer is yes but it's pretty complex as you need to operate the MOSFET in it's linear region. Horowitz and Hill (Art of Electronics) have a discussion on this point but it's a little hazy to me now. I believe that MOSFET manufacturing variances make this a tricky issue.

 

[westhomas]

Sorry, but life isn't that simple.

Ouch. Yes....life is horribly complex isn't it. But in principle what I've illustrated could work, but it needs to be articulated. Maybe it won't work at all. I'm not an idiot, I'm simply new to electronics and teaching myself.

philba - thanks, I've heard of this text before and my library has it. I will let you know what I find out if you're interested.

 

[Nigel Goodwin]

Ouch. Yes....life is horribly complex isn't it. But in principle what I've illustrated could work, but it needs to be articulated. Maybe it won't work at all.

It won't work at all - it's needs to be MUCH more complicated - what exactly are you trying to do?.

 

[crutschow]

The easiest (but not necessarily the cheapest) way to control a signal level is to use an analog multiplier such as one of these Analog Devices: Analog Multipliers/Dividers :: Other. You put the signal into one input and the DC into the other. The output it the product of the two voltages. Thus, in this case you get an output that varies in amplitude with the DC input.

 

[Hero999]

I can't think of a way of doing it without adding the 555's frequency to the signal, even an analogue multiplier would do this to a certain extent.

What you're trying to do is AM modulation, a simple way of doing this is to build a common emitter amplifier and vary the supply voltage which will alter the gain. If the 555's frequency is much lower than 20Hz you could do this and high pass filter the output which could work.

 

[Nigel Goodwin]

Use a volume control IC - 8 pin chip, audio in, audio out, power rails, and a control voltage - if you can still get them.

But we still don't know what he's trying to do - I'm presuming it's amplitude modulation of a guitar?, as you can buy pedals that do this.

 

[Hero999]

That will work but I still have a nasty feeling that the squarewave from the 555 will be added to the audio as well as multiplied. The control voltage is normally adjusted slowly on a voltage controlled amplifier.

EDIT:
Just another thought

If the signal from the 555 is <20Hz a low pass filter needs be added to the output of the 555 to prevent the harmonics from interfering with the audio signal.

 

[crutschow]

I can't think of a way of doing it without adding the 555's frequency to the signal, even an analogue multiplier would do this to a certain extent.
What you're trying to do is AM modulation,...

The definition of Amplitude Modulation is to multiply one signal by the other, so an analog multiplier makes a good (near ideal) modulator. I thought the purpose of his circuit was to control the amplitude of the 555 signal going to the amp. What else would it do?

Whether it's classified as a modulator or just an amplitude (volume) control depends only upon the relative frequency of the two signals. The function is the same in both cases.

 

[westhomas]

Gentlemen (and Ladies)

I stand corrected. I blame this illustration for generating false hopes. My apologies Nigel. I've been duped by Mims. haha

I thought it was implied, but I should have specified clearly that I want to control an output volume with the sensor, not manually. The signal is negligible...I just want to send to the amplifier any signal on my breadboard, for example a squarewave, from a 555. Sorry this caused some confusion...I want to amplify the 555's signal.

An analog multiplier seems to be exactly what I'm looking for. thank you for suggesting that.

I'll add the details and a schematic to this thread if it works out.

thanks to all...

 

[chazmania]

I am doing something of the same.. controlling volume via midi.. my idea uses two fets as a sudo volume pot control. by placing the source and drains together to form a sort of center wiper.. then adjusting the gates to go opposite in voltage using another transistor or op amps etc.. i have some vactrols that i am thinking of using as well..tied together to form a center wiper and as one turns on the other turns off by how much voltage is applied... should work great in theory.. fets are cheaper so im gona try them out like this.. basically its a VCA i think circuits for VCA's are around that use transistors.. look into the Moog circuits for some simple ideas. Anyway good luck..