OPAMP troubles

[casio3476]

Hello all,

For any of those who are familiar with the science olympiad program (www.soinc.org) , I am preparing for the Mission Possible event for my state competition (TX) that will be held next weekend. I need to build a sound activated switch, so I just went out to radio shack the other day and bought a 741CN opamp (8pin)

Anyway, I will be using this piece as an amplifier for a condensor mic element and sending the output to a simple transistor swith that will flick a relay on and off.

For this competition I have had to teach myself all about the functioning of transistors, diods, etc... I thought i had it all figured out until this opamp circuit reared its ugly head.

Ive narrowed down my problems to how i am attaching the mic to the opamp, but besides that i am completely and utterly lost.

Here is my current setup:
8.5v dc current

Non-inverter input is grounded through a 10k ohm resistor
Inverter input is through a 1k ohm resistor
inverter input and output are crossed with a 10k ohm resistor

Thats about all i have actually, Im not exactly sure how to attach this to the microphone. I was thinking it would be something like this (+voltage, mic element, .47u capacitor, inverter input) but again i cannot get an output current. How exactly should I prepare the signal from the microphone for it to be transfered into the opamp circuit?

In this case the output current should be negative correct?

Any tips/tricks/hints you guys can provide on building circuits such as this will be GREATLY appreciated. Im a newB but eager to learn! If anyone has an idea for a simler sound activated switch, i would be much obliged to hear it!

Thanks again for any help

Zac

 

[BullsOnParade]

Zac, tell me more about your circuit. Check these lectures out:

http://users.ece.gatech.edu/~alan/page4.html
lectures 28 and 29 should help with opamp theory.
As for wiring the 741 lemme know more about what you're
setting up and maybe i can help.

dM

 

[mAdD INDIAN]

Originally posted by: BullsOnParade
Zac, tell me more about your circuit. Check these lectures out:

http://users.ece.gatech.edu/~alan/page4.html
lectures 28 and 29 should help with opamp theory.
As for wiring the 741 lemme know more about what you're
setting up and maybe i can help.

dM

where was that site when I had my microelectronics exam two days ago!! :|

 

[blahblah99]

The signal picked up by the condensor mic is going to be very low, so you will need some gain stages (built out of opamps, of course) to amplify that signal. Once that signal is amplified sufficiently enough (say to 5Vpp or so, 10Vpp max), you will need to rectify that signal, filter it, and finally feed it into a comparator circuit (again, more opamps) that'll trip a transistor to light an led or buzzer.

This can be done with a single supply with the opamps biased at 1/2 the supply voltage (instead of reference to ground), or it can be done with a dual supply with ground as the reference.

 

[casio3476]

Ill try to make a little diagram here

(+12v)---------------MIC---------.47u------------1kohm(r1)---l---l(inverter input)
l
---------10kohm(r2)----------------(output to trans stage)
(-12v)------------------10kohm(r3)----------------------------------l(noninverer input)


The idea is that the opamp pics up the signal from the mic and boosts it, as im sure all that have replied already know. I will send the boosted signal to a simple transistor switch that I already have working. If the output of the mic is so low could I simply increase the ratio on r1 and r2 to something like 100:1 by popping in a 100k ohm resistor for r2? Or is there a max resistance that that is possible with this IC.

Im not entirely sure im hooking the MiC up either, its a condenser unit i bought at radio shack, but it doesn't have the little diagram on the back that is o-so-helpful. A similar unit had a diagram like this.



-------------
l ------->---l----l---(capacitor)------Terminal 1
l l l l l------(resistor)------------
l (cap) l l
l l-------l----l--------------------------Terminal 2
---------------

Its the best i can do w/ ascii text :/ Does this mean that the way i have the microphone set up in my circuit above is incorrect because it does not have a second input branching off of terminal 1. I dont even really understand why that branch is there. I though a condenser mic was merely a capacitor. Then why do you need three inputs?

O yes, BullsOnParade, the links for the entire column with lectures 28 and 29 seem to be dead. Thanks anyway.


zac

 

[casio3476]

Originally posted by: blahblah99
This can be done with a single supply with the opamps biased at 1/2 the supply voltage (instead of reference to ground), or it can be done with a dual supply with ground as the reference.

I dont understand, when you say dual supply you mean to completely seperate power supplies, like to different bateries? What exactly does "biased" mean and how would i bias the ground to 1/2 of my supply voltage? Unfortunately the rules of my game allow me to only use one 12v power source.

Thanks again for all your help.

zac

 

[f95toli]

Normaly you would use a postive and a negative voltage to drive a opamp, the common voltages being +-15 V but +-12V is also fine. This means that you need two sources or you need to use a "trick" so that you can use a single source , this is what blahblah99 is refering to.
In your case you need to use two 12V sources(+ and -), or a single 12V source and reference the opamp to 6V.

 

[AbsolutDealage]

I'm a little rusty, but I'll give it a shot...

Try something like this: Microphone interface

The opamp has a gain of 100, you may need to change that depending upon your specific mic.

Images pilfered from here. You may want to poke around that site to give you a little more info on opamp design.

 

[TTM77]

It has been 4 years since I think if opamps. LOL Opamp is a nicely make IC to make things easy and bost the gain a lot. There is also a feedback if you like. Is there a specific sound that you want to trigger your device? or any noise will do?

If any noise will do, then use a cap to level out Ur signal so it doesn't flicker a lot. If specific sound, then you need filter to select that frequency.

Anyway, this is my 2cents knowleage. haha

Mic (creat very small signal) ==> Opamp (amplify to strong signal) ==> filter (if you want specific freq) ==> level out (so Ur LED won't turn on and off so fast).

LED need a little time to light up, so if less then that time and U already turn it off, U will not see it light up at all.

 

[casio3476]

Thanks AbsolutDealage,

Ill try this circuit out later tonight when i get home from school, its starting to look more and more like I have been interfacing the microphone incorrectly with the omp amp.


zac

 

[casio3476]

I was looking at that circuit diagram for a microphone interface that was posted earlier and came up with two questions.

Can I use non-polarized electrolytic capacitors, or do I need to pick up polarized ones. In general, why would I use polarized vs. non-polarized?

After the first 1uF capacitor, the read takes a 90degree bend downward into a 10kohm resistor. This resistor has a arrow pointing into it from the rest of the circuit. Would this resistor be be between the 1uF capacitor and the t-joint, or between the t-joint and ground? This is some nomenclature Ive never seen before.


Thanks for any who can help, I feel I am close!

zac

 

[zetter]

I don't know if it helps, but there's a good schematics web site here and they have a link to a Sound Activated Switch which may be of assistance.

 

[AbsolutDealage]

Originally posted by: casio3476

After the first 1uF capacitor, the read takes a 90degree bend downward into a 10kohm resistor. This resistor has a arrow pointing into it from the rest of the circuit. Would this resistor be be between the 1uF capacitor and the t-joint, or between the t-joint and ground? This is some nomenclature Ive never seen before.

Sorry about that, that's what I get for copy & paste...

The connection where the arrow is should be above the 10k resistor.

 

[casio3476]

I would like to thank all of you for all of your quick replies. The other night I finally got a working circuit to function.... When my project is finally finished (hopefully by thursday night). Wish me luck at my science olympiad competition this weekend, without y'alls help it could not have been possible.



zac thompson

 

[zetter]

Good luck with the competition, hope all goes well for you.

I, and probably some of the rest of us, would like to see the results of what you've done, perhaps you can make a post in one of the other forums showing us your project.

Cheers

Zetter

 

[AbsolutDealage]

Did that circuit work, or did you have to make some modifications?

I just want a little sanity check for myself here... I want to make sure that I havn't forgotten everything yet.

 

[KalTorak]

Originally posted by: AbsolutDealage

Originally posted by: casio3476



After the first 1uF capacitor, the read takes a 90degree bend downward into a 10kohm resistor. This resistor has a arrow pointing into it from the rest of the circuit. Would this resistor be be between the 1uF capacitor and the t-joint, or between the t-joint and ground? This is some nomenclature Ive never seen before.

Sorry about that, that's what I get for copy & paste...



The connection where the arrow is should be above the 10k resistor.

Uhm, someone did figure out that's a 10k potentiometer, and not a fixed resistor, RIGHT?

 

[AbsolutDealage]

Originally posted by: KalTorak

Originally posted by: AbsolutDealage

Originally posted by: casio3476



After the first 1uF capacitor, the read takes a 90degree bend downward into a 10kohm resistor. This resistor has a arrow pointing into it from the rest of the circuit. Would this resistor be be between the 1uF capacitor and the t-joint, or between the t-joint and ground? This is some nomenclature Ive never seen before.

Sorry about that, that's what I get for copy & paste...



The connection where the arrow is should be above the 10k resistor.

Uhm, someone did figure out that's a 10k potentiometer, and not a fixed resistor, RIGHT?

Yes, I know... but for his purposes they really don't need the level-adjust there. All you really need to do is design the switch on the output to adjust for the proper threshold. Leaving it at 10k should not be a problem.