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

 

The short answere is no. A 530 needs 10 volts on the gate for full conduction. Even a smaller logic level FET needs 5 volts.

Add an op amp to amplify the sensor signal and it should work. The IRF530 is overkill - something more like a 2N7000 is more like it, although the 530 will work. That would require about 15 volts for the op amp. With a logic level FET like a VN10LP, 8 to 9 volts would work fine.

 

hey...thanks,

mosfets can be used as a kind of variable resistor, right? If I were to place the drain and source pins of the 2N7000 across pins 6 and 7 of the astable 555...could I modulate the oscillation frequency?

I would try it out but I don't have a 2N7000 yet.

thanks again
wes

 

You might get some effect, but the devices are intended to operate in full on or off states. You could simply add a pot across those pins and get a more predictable result.

 

yes and no, Mosfets are Voltage controlled, transistors are Current controlled. If you use a MOSFET like the one that i have set up in this small diagram, this is a really simple amplifier. The variable resistor will trim the voltage going into the MOSFET. When the signal on the input is high, ( I think for a MOSFET, it switches on when the gate pin has a volt or so higher than the source pin (-). the output will be a fullscale value from (GND - 0V) to (V+), depending on the position of the Variable resistor. You should also use a resistor in series with the input if you are planning on using a guitar amp, because guitar amps have a high input impedance.

I apologise for the small-ness lol.

I hope this helps in some way.
http://cid-53de3ab2cc0c162e.skydrive.live.com/self.aspx/Public/amp.bmp

 

You might get some effect, but the devices are intended to operate in full on or off states. You could simply add a pot across those pins and get a more predictable result.

you mean mosfet's are meant to be full on or off? The reason I chose a mosfet was for the availability of intermediate states. In other words, I'm trying to make a sensor actuated volume knob, not a switch. I hope this makes sense. Sorry...I could have stated this more clearly to begin with.

yes and no, Mosfets are Voltage controlled, transistors are Current controlled. If you use a MOSFET like the one that i have set up in this small diagram, this is a really simple amplifier. The variable resistor will trim the voltage going into the MOSFET. When the signal on the input is high, ( I think for a MOSFET, it switches on when the gate pin has a volt or so higher than the source pin (-). the output will be a fullscale value from (GND - 0V) to (V+), depending on the position of the Variable resistor. You should also use a resistor in series with the input if you are planning on using a guitar amp, because guitar amps have a high input impedance.

Thanks. I will try a resistor in series with the amplifier input... I think otherwise, though that is a very nicely illustrated circuit, I need to incorporate the sensor as a means to control the volume.

Ideally, what I would like to have is the signal from the sensor, which is DC analog control the resistance of the mosfet across pin 6 and 7 of the 555 so that when the sensor's signal goes high current flows across allowing an oscillation, which I would then output to the amp.

I'm going to be brave and risk blowing up my components and try this out. I gather this is a rather unusual use for a 555, but I'll let you know what happens. In the meantime I'd be grateful for any suggestions.

Thanks for the comments...

 

when i get out of work, ill post a entire circuit for you to try out, gimme about an hour or so lol

 

You might get some effect, but the devices are intended to operate in full on or off states. You could simply add a pot across those pins and get a more predictable result.

Im not sure if i agree with this...

If they were intended for use in on or off states, why would they be so popular in Audio applications? audio signals are AC with variable signal voltages and currents. Please explain why you think they are on/off devices? im curious, maybe you know something that i don't

Unless you guys are lookin at a specific part number, in that case, my mistake.

 

For your application, one might work. They do generally get used for switching, as their linear region is not comparable to a bipolar transistor. Most of the audio apps are in class D amps which use PWM to drive the speakers.

Get a data sheet for the device you want to use. The device conduction will start with the gate voltage around 2 volts above the source. It will be in full conduction when the gate is 10 volts above the source terminal. If you want to use that sensor, an op amp would be needed to amplify the signal somewhat for changing the 555 output pulse.

I've never used one as a linear device, so you might want to apply voltage frm the wiper of a pot to see if you get usable results before making up a full circuit. Let us know how it turns out.