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### Topic: Negative Voltages (Read 12952 times)previous topic - next topic

#### baum

##### Jan 23, 2011, 04:49 pm
I want to generate about -9V for this circuit from afrotechmods:

So I am asking, how can I make that negative voltage? Or will audio quality not be affected too much if I just ground the op amp with 0V?
I really don't want to buy a lot of ICs, can I do it simply and cheaply with standard transistors and caps and such?
Thanks!

#### shelleycat

#1
##### Jan 23, 2011, 05:16 pmLast Edit: Jan 23, 2011, 05:16 pm by shelleycat Reason: 1
If you can buy one IC (and its a useful one!), try a MAX202 from Maxim.
Its a TTL to RS232 driver (so its useful if you ever want to use softserial to something), it runs off 5V and has dual voltage generators to produce +- 10V or so, which can be used as a supply, especially if you don't use the drivers.

#### baum

#2
##### Jan 23, 2011, 05:19 pm
Nevermind. After watching afrotechman's tutorial, I found this:
@1:30.

#### AltairLabs

#3
##### Jan 23, 2011, 05:41 pm

But you dont NEED a negative voltage.  A better question might be "how can my microphone drive a small speaker using a single voltage source?"

Answer: use an LM 386 with single +9v supply like this

or this

#### baum

#4
##### Jan 23, 2011, 06:30 pmLast Edit: Jan 23, 2011, 06:31 pm by baum Reason: 1
OK. Is it okay if I don't have those exact caps?
I have only multiples of 10:
1pF, 10pf, 100pf, 1n, 10n, 100n, 1u, 10u, 100u
5 of each. So can I have 2 x 100n in series to make ~220n?

Oh and I have the lm358.

#### shelleycat

#5
##### Jan 23, 2011, 06:58 pm
2 * 100nf in parallel for 200.

#### AltairLabs

#6
##### Jan 23, 2011, 07:10 pm
None of the cap values here are critical, get the right number of decimal places and your probably OK.

Put caps in parallel to add capacitance, unlike resistors that add in series.

The LM358 is a poor choice to drive a speaker, but it will sorta halfway work.  And you can still do it without a negative rail.  If you want to do that *exact* circuit just for fun, simply use two 9V batteries to supply the +9v and -9v rails.  If you fall in love with it then you can hunt later for a way to power it without batteries.

The LM386 is very inexpensive, less than the cost of the voltage converter chip shelleykat mentioned.  And its designed specifically to drive a small speaker, does the job quite well.  Get LM386 today at RadioShack for \$1.50 or order from Tayada for 17 cents each.
http://www.taydaelectronics.com/servlet/the-223/LM386-LM386L-Audio-Power/Detail

#### baum

#7
##### Jan 23, 2011, 07:29 pm
Well... I won \$70 from freeday on sparkfun, and the 358 is the only one they have. what If i attach a mosfet to the opamps output?

#### AltairLabs

#8
##### Jan 23, 2011, 07:55 pm
MOSFET could help a weak op amp to directly drive a speaker.

#### baum

#9
##### Jan 23, 2011, 11:28 pm
Yay!
But the mosfet won't allow for negative voltages, will it? So maybe I'm better off with an op amp from radioshack for a higher power.

Besides amplifying signals, is the 358 okay for comparators and oscillators?

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