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Old 04-09-2011, 04:43 PM
vadimqew vadimqew is offline
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Default Powering op-amp with single power supply

Hello Forum,

I have been working on a color organ project for my school but came across one issue. Ive been trying to set up LM358N op-amp to work properly. The problem is that i cant get a simple inverting circuit to work. For example, Im setting Rf at 100K and Ri at 10K and the gain should be -10. However the output reads only the same voltage i input it. Ive set up op-amps before in class with dual power supply +15V and -15V and they worked fine. This time im trying to set them up with 9V battery and so far no success of getting anything meaningful. Ive searched countless hours on setting up single power supplies, dual power-supplies with one or two 9v batteries, but still no success. Someone please point me into the right direction. I looked at data sheets of the op amp and it clearly says that the chip supports single supply.
heres the link to pdf
http://www.onsemi.com/pub_link/Collateral/LM358-D.PDF

and this is how the simple circuit is set up. i should be getting an output of around -5V. but the multimeter reads 0. I have tried setting up dual power supply where +9V goes into Vcc and -9V into Vee of power supply. still not luck on getting the right output.


Thank you
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Old 04-09-2011, 04:53 PM
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Bill_Marsden Bill_Marsden is offline
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Two things. When you have an unused gate or op amp ground the inputs to shut it down.

Second thing, you can use that second op amp as a power supply.

Creating a Virtual Power Supply Ground

Look at figure 5.


You can use two batteries to make a dual power supply, but most cases it is not needed.
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Old 04-09-2011, 05:00 PM
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SgtWookie SgtWookie is offline
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Have a look at this section in our E-book:
http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

The battery on the left has no ground reference; it's "floating". The negative terminal should be connected to pin 4.

Since you are using an inverting topology, but a single supply, the opamp cannot output a negative voltage. A 500mV input would cause the opamp to try to output -4.5v, and it won't get there.
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Old 04-09-2011, 05:15 PM
vadimqew vadimqew is offline
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Thank you Bill_Marsden and SgtWookie for such fast replies,
i think i have it figured out, I had the left battery (floating) grounded and non used input pins grounded and my output was 5.5 V. So applied .5 volts and my output was 5.5 which is about right? This was just a test if op amps are working.

Now the hard part is applying it to my project. Im taking the input from 3.5mm audio signal then sending it through the low pass, bandpass, and hi pass filter to isole the signal. From there im switching through the transistor to power up the LEDs. Im hoping this will work. The circuit im following is
http://ecelab.com/circuit-sound-to-light.htm


fell free to point me in the right direction


Thanks again
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Old 04-09-2011, 05:18 PM
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Bill_Marsden Bill_Marsden is offline
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You look like your on the path. I have a article for simple projects and a tutorial for LEDs and 555's (a very common chip).

LEDs, 555s, Flashers, and Light Chasers

Chapter 12 covers special effects similar to what your doing. What you looked up is a color organ, my articles come close but don't quite go into those.
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Old 04-09-2011, 08:43 PM
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Why do you have two 9V batteries?
Why isn't pin 3 biased at half the supply voltage with two resistors from the battery that powers it?
Why isn't the 10k resistor connectesd in series with a capacitor to ground?
Then if the input and output have coupling capacitors it will amplify AC audio.
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