# TL082 with 12v battery

Hi. I'm really a newbie and i'd like to make this http://www.instructables.com/id/Arduino-Guitar-Tuner/ but in a smaller version, so i don't want to use two 9v batteries. I was wondering if there's a way to amplify audio signal up to 2.5v, always using a TL082, but only one 12v battery. COuld someone help me please?

Thanks

Hi.
There's a reason why 2 batteries were used:
The TL082 needs a symmetric power supply.
Nicole (the author of that article) knew about that, but she didn't bother telling about it (but that's normal over there at that crappy site).
Nicole has great tools available like oscilloscopes and laser cutters.

Take a look at the schematics.
At the right of the amplifier block, the batteries are visible.
You can also see that there is a line from center connection between the two batteries, to GND .

If you are going to omit one battery, you need to divide the voltage from the single battery you will be using.
The center voltage would then be connected to GND of the Arduino.
You absolutely need to do that.
You can't simply create a voltage divider and use that for this function, that won't work reliable.

So there's more to do than you would think, and using two batteries instead actually is the easiest way to do this.

Hi MAS,
thanks a lot for your answer. Reading Nicole instructable i've understood the role of the two batteries, but i was thinking about the possibility to make a voltage devider as you has than said. Why you say "You can't simply create a voltage divider and use that for this function, that won't work reliable"? As I said, I'm a newbie, could you explain me why the power devider choice could be so hard to use? There is a better and simpler way to obtain the same result? Thank you very very much.

The voltage divider exists of two (in this case equal) resistors.
You can't put any load (resistance) to it, as that will change the balance you created.
You need to amplify the so created level so that it will not be influenced by some load.
You could do that by using two complementary transistors (preferably thermally coupled), but that's easier said than done.

Nicole is using one of the 9 volts batteries to also power the Arduino.
If you would do that with your 12 volt battery, then the GND will be at 0 volt.
You can't tie the self created voltage divider signal to 0 volts and expect it to be 0 volt and 6 volt at the same time, if they come from the same source.
It doesn't matter if you used an amplifier or not for this.
Of course there are possibilities to do this but that requires galvanic isolation.
That's not an easy thing to do.

Ok, it's pretty clear. So my idea to make a smaller tuner it's not really feasible with my few skills? There is some other way to do what i'd like to do? Have you got some suggestion? Thanks a lot!

Ratto84:
Ok, it's pretty clear. So my idea to make a smaller tuner it's not really feasible with my few skills? There is some other way to do what i'd like to do? Have you got some suggestion? Thanks a lot!

Yes, don't use a TL082. Use a single rail OP amp or even just a single transistor. Try googling "single transistor audio amplifier".

Russell.

Thanks russelz for your answer, but i've a doubt: using a transistor i'll make "the wave bigget", but i won't change the central voltage of the input signal, that's right? I have to read the frequency of an audio signal, but arduino can't undersant a negative value, so i've to make the central voltage of the input at 2.5v, so i can read from 0 to 5v. I think the transistor can't help in that way... It's right? Thanks a lot

A single transistor can't create a positive as well as a negative signal.
Did you look for the search items russelz gave you ?

You never need symmetric power supplies with opamps, that's a complete myth. You do however
often need signal ground to be halfway between the power rails and stiff(*). Often that's done with
split supplies, but you can create a mid-rail signal ground using one opamp for the job, a voltage
follower has extremely low output impedance.

(*) Stiff means low dynamic impedance, milliohms. This is needed so that signal feedback doesn't
happen via the power rails and cause oscillation. This means lots of lovely low frequency decoupling
too so that the higher audio frequencies are also handled (opamp open-loop gain drops fearsomely
up the decades, and its the open loop gain that gives the low output impedance.)

For audio circuits you assume symmetric waveforms and high gains (more than 10) are involved, hence
the need for mid-rail signal ground and defence against oscillation.

Thanks for your answer MarkT, but for me it's not clear, i'm sorry. You say I can make a voltage devider, right? Could you provide a schematic please? Thank you very much!

The LT1215 is a single (or dual supply) op amp designed specifically for 5V systems but capable of running on up to 18V. Order direct from Linear.

The output is 1/2 of Vcc and pretty stiff.

Not to be nit-picky but shouldn't there be a series coupling cap if it's an audio signal ?

Wow. I just looked at the schematic on the Instructables website.

Horrible. The input should have a resistor to ground and a capacitor to isolate any DC on the input.

Worst of all, one section of the IC is not connected to anything. This is just begging for it to break into oscillation or go into SCR lockup, possibly even destroying the IC.

Funniest of all, the second section could have been used to make a Vcc/2 virtual ground. But it isn't really needed, anyway. R2 could be replaced with a 47k resistor, and another 47k resistor to Vcc. But not with the TL082, it requires too much voltage.

There should also be a resistor between the output of the Op Amp and the Arduino. With a total of 18V available, the input to the Arduino can easily swing well outside of 0V and 5V.

How about just using an LM386? It is meant to power headphones or a small speaker, but no reason you can't use it here. Use 5V to power it, put a 1k resistor after C1 before A0.

Polymorph's 1st. schematic shows a way to generate the desired 1/2 VCC.
You can use that as reference, and you can use the 2nd half of the TL082 for this.
But i don't think you can use it to connect Arduino GND to VCC/2 while also powering it from the same battery.

Ratto84:
Thanks russelz for your answer, but i've a doubt: using a transistor i'll make "the wave bigget", but i won't change the central voltage of the input signal, that's right? I have to read the frequency of an audio signal, but arduino can't undersant a negative value, so i've to make the central voltage of the input at 2.5v, so i can read from 0 to 5v. I think the transistor can't help in that way... It's right? Thanks a lot

Use capacitor coupling as shown in the schematic for that instructible. The diodes on the IO ports will charge the capacitor so that the voltage doesn't go negative.

Russell.

MAS3:
Polymorph's 1st. schematic shows a way to generate the desired 1/2 VCC.
You can use that as reference, and you can use the 2nd half of the TL082 for this.
But i don't think you can use it to connect Arduino GND to VCC/2 while also powering it from the same battery.

No, I'm not saying to connect the Arduino ground to the output of that Op Amp.

Besides, the Arduino input in that design is meant to receive the input centered around 2.5V, anyway.

I know that, but the instructable tells you to do that and one might be tempted to try it.
I was pointing out that you can't do that.

Clearly the forum is the place to go when you want to know what you can and cannot do...

(like throwing chum into a shark tank...)

The input of this circuit is permanently connected to a guitar pickup.
To preserve to "tone" of a guitar, you should not "load" it more than the guitar amp already does.
So a fet opamp is a good choice.
If you would use a single 12volt supply, you would need a mid-voltage circuit on the opamp's input.
That would load the pickup if resistors were lower than 10Megohm.
Analogue input still has to have it's own mid-voltage divider.
See attached schematic.
Leo..