Building a noise detecting circuit

[Averagesupernova]

LOL Hey it's your deal not ours. But gimme an hour or two and I'll post some design tips for you. The decision is still yours.

 

[Averagesupernova]

Ok. Here is what I would do. Go with a single supply because going with a dual supply complicates things unnecessarily. Also, if you run the 555 on the same dual supply it is no longer a ‘dual supply’ so to speak. Running the 555 on the dual supply means that we need to ensure that the voltage to pin 2 of the 555 will not go below the 555s negative supply. Ground is really just a reference and we can move references wherever we like to most of the time.

Make an input stage using an active high pass or bandpass filter with one or two of the op-amps on the quad op-amp package discussed earlier. Of course you will have to bias up the op-amp to run at half the supply voltage. When choosing an op-amp, make sure to get one who’s output will go ‘rail to rail’. What I mean by this is that the output should be able to go all the way to the either supply voltage. Some op-amps will only go to within a volt or even more of the supply voltage.

Use a capacitor to couple into the next stage. Configure an op-amp as an inverting amplifier biased at about 8.5 volts. This is assuming you are using a 9 volt power supply. Gain of this stage can be adjustable using a pot for feedback resistor going from the output to the inverting input. What you will have out of this stage is rectified audio pulses going negative from 8.5 volts. Can you see where this is going?

Now do me a favor and draw me a schematic of what I described and post it. I guess maybe I’m lazy or something but I think it is good practice for you to make a schematic based on a text description. That’s what I was told at one time anyway. I promise I won’t leave you hanging and I will help you finish this.

 

[Omegatron]

Ok. Here is what I would do. Go with a single supply because going with a dual supply complicates things unnecessarily.

I always had trouble with op-amp stability when I did stuff like this, which is pretty difficult to diagnose when you are just learning and don't have access to a scope, etc.

Also, running the op-amp from +-9 V and running the 555 from +-9 V would allow the trigger point to be -6 V, correct? So you would only need to scale and rectify the signal (so it goes from 0 downwards) and when it got loud enough it would dip below -6 and trigger.

 

[Averagesupernova]

Remember what I said about moving references around? That's all you have done. There will be no connection to 'ground' on the 555 at all with a split supply and the op-amp will have connections to ground as bias points on input pins. Original poster please take my advice on this one. I've done this sort of thing with op-amps many times.

 

[Soilwork]

OK this is a rough schematic of what I think you've said, but I'm most certainly wrong.
You didn't really say anything about the transistor section so I don't really know if that's right and have left it as it was pretty much.
Thanks again for your help.

 

[Soilwork]

Oh and how does the capacitance coupling along with the inverting amplifier result in a rectified signal?

 

[dlgoff]

OK this is a rough schematic of what I think you've said, but I'm most certainly wrong.
You didn't really say anything about the transistor section so I don't really know if that's right and have left it as it was pretty much.
Thanks again for your help.

Can you attach a .jpg file please?

 

[Omegatron]

Oh and how does the capacitance coupling along with the inverting amplifier result in a rectified signal?

It won't. You need diodes. Something like this:

http://www.play-hookey.com/analog/full-wave_rectifier.html

Though just a regular bridge rectifier would probably be fine for your purposes...

In fact, not rectifying the signal is probably fine for your purposes.

 

[Averagesupernova]

It won't. You need diodes. Something like this:

http://www.play-hookey.com/analog/full-wave_rectifier.html

Though just a regular bridge rectifier would probably be fine for your purposes...

In fact, not rectifying the signal is probably fine for your purposes.

Nope. No diodes. Please don't confuse him. My workday is not yet done but when I have time in a couple of hours I will explain. He does not have it drawn quite right.

 

[Averagesupernova]

Ok. I notice several things about your schematic. Some are important, some are not. You did not draw an active filter on the first stage. That is fine, we can add it later. Let’s just get the main functions working first. R3 needs to be omitted and the non-inverting input (+) on the second op-amp needs to be tied to about 8.5 volts instead of ground. This can be accomplished with a voltage divider. I assume you know how to make a voltage divider out of 2 resistors tied between the 9 volt supply and ground. If not, just ask. When I say ground, I mean the negative side of the 9 volt supply. The negative side of the mic is also tied to what I call ground. Also, tie the non-inverting input of the first op-amp to a 4.5 volt reference. Use a voltage divider to get it.

Now, to explain why this circuit does what I say. When the non-inverting input on the second stage is tied to 8.5 volts the output will also be centered around 8.5 volts. Since the cap is on the input the DC gain of the stage is one. When you couple an AC signal in through the cap the output should swing positive and negative by an amount determined by the feedback resistors. However, it cannot swing farther up than the 9 volt supply so it will mostly swing down. And obviously it cannot swing lower than ground. When there is no AC signal coming in then the output will be sitting at about 8.5 volts. This is just what we want. With adjusting the gain we can set the circuit for just the amount of noise we want to trigger the timer.

You also drew the output of the second op-amp going directly into the 555. We are going to change this. Insert a diode in between the op-amp output and pin 2 of the 555 with the cathode towards the 555. Tie a resistor from pin 2 of the 555 to ground.

Next we are going to make the manual reset circuit. We’ll make a sort of Schmitt trigger circuit out of one of the op-amps left on the quad package as a type of latch. Tie a resistor from the non-inverting input to the output. Tie another resistor from the non-inverting input to ground. This is positive feedback that makes a Schmitt trigger always go to either power supply rail. This is just what we want. Tie the inverting input to a 4.5 volt reference made out another voltage divider. Tie the output of this op-amp to pin 2 of the 555 with a diode inserted in series. Cathode towards the 555. Insert a diode with the cathode on non-inverting input on the op-amp and the anode on the output of the 555 timer Also insert a resistor in series with this diode. Now install a momentary switch from the non-inverting input to ground. This is the reset button. Also tie a .47 microfarad capacitor in parallel with the switch.

Draw the schematic, ask any questions and I’ll be glad to help you.

 

[Soilwork]

Thanks so much for all the help.
I've got exams on in a week and I haven't been keeping on top of my work so I'm a bit worried about it all at the moment.
Anyway I will post my schematic of what you suggested in the next two weeks if that's ok.
That's when I will be on holiday and I will have plenty of time to practice making circuits.
I just want to make sure that no one minds in particular if I post quite a lot of questions on this board about how things operate and all??

Thanks again guys (especially averagesupernova)
Tom

 

[Soilwork]

Right I've finished my exams now so I'm going to start working on those suggestions you gave me. The schematic should be up within the next few days :)

 

[Soilwork]

Tie the output of this op-amp to pin 2 of the 555 with a diode inserted in series. Cathode towards the 555. Insert a diode with the cathode on non-inverting input on the op-amp and the anode on the output of the 555 timer. Also insert a resistor in series with this diode. Now install a momentary switch from the non-inverting input to ground. This is the reset button. Also tie a .47 microfarad capacitor in parallel with the switch.

Draw the schematic, ask any questions and I’ll be glad to help you.

OK I have a couple of questions here :):

1. Why isn't that first op-amp considered a high-pass active filter? I got it from those lecture notes I had from the subject I did last semester. The input is coming from the mic and going through the capacitor, resistor and into the inverting input. The non-inverting is connected to ground. But also when you look at the complex impedance of the capactor it is 1/jwC (w is omega). So obviously the impedance is really large at low frequencies, but low at high frequencies. I know that everything needs to be changed, but my question is, if you were just looking at that one op-amp would that be a high-pass active filter?

2. I've done the corrections individually for the op-amps (1 & 2) but I haven't actually connected them up because I have sort of lost what you were saying. It says connect op-amp 2 to pin 2 of the timer, but then when you are referring to the "new" op-amp (schmitt trigger) with positive feedback that I need to add, you say that I connect that to pin 2. I can't cascade op-amp 2 and the "new" op-amp, because the inverting input has its own supply and the non-inverting is effectively connected to ground. So which one is connected to pin 2?

3. Sorry I don't really get what the bold statement in the quote above is saying?

4. The capacitor in parallel with the 'manual' switch is so that the switch is bypassed when the circuit is active right?

Those are all the questions I have for now :)
Thanks again

 

[Averagesupernova]

OK Soilwork. I don't have time at the moment to address your issues but just wanted to let you I've been watching this thread and haven't forgotten about you. I hope to get all of your specific questions answered in the next 12 to 24 hours.

 

[Soilwork]

yeah man that's fine, whenever you have the time.
I'm grateful for any time spent helping me :)
Take as much time as you need/want to :)

 

[dlgoff]

Can you attach a .jpg file please?

I can't view your .doc attachment.
Thanks

 

[Averagesupernova]

Concerning the high pass filter. You have it close. The non-inverting input is tied to ground. It cannot be. You need to bias it up to about one half of the power supply voltage like I said previously. Look deeper in your notes about active filters. You should be able to find a sharper response by incorporating several capacitors on the first op-amp stage.

I think you are confused about connecting the outputs of the op-amps to pin 2 of the timer. You are supposed to be connecting them through diodes. You also mention non-inverting inputs tied to ground. Again, not a single non-inverting input is tied to ground. They are all supposed to be biased up as I explained.

 

[Soilwork]

@dlgoff: I will post the jpg of the original circuit in the very first post of the thread. So check the first post. Sorry I didn't actually notice your post before.

@Average: I meant to add that the non-inverting inputs were tied to what you were referring to as "ground". So they are actually tied to the negative of the supply voltage. I've drawn the circuit by hand now so I will post it up soon.
Hopefully I have the right idea. Thanks :)

 

[dlgoff]

@dlgoff: I will post the jpg of the original circuit in the very first post of the thread. So check the first post. Sorry I didn't actually notice your post before.

My version of word is too old to view the .doc file. And the .jpg file you attached earlier doesn't work either. I don't know how I viewed it before.

Regards

Don

 

[Averagesupernova]

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