Hi, i have been trying to figure out how to use an operational amplifier so i can make my own custom led organ, long story short for some reason Im getting an outrageously high gain. I have the input hooked up to a 3.5mm jack and just touching the tip of that causes the led's to hit full brightness. Any ideas? if you need i can post a picture of what i have hooked up.

 

I am using this schematic just without the 1k resistor and the cap's, with a 5.10 volt power line that goes to ground, the input is putting in around .1 volts and the output is at a blank. I am currently using a single green led 5mm just to test what kind of output i am getting but so far nothing has really happened with the led. i have tried several different resistors.

 

The 741 opamp does not work with just 5V or single supply.
Try changing your design to use LM324 quad opamp.

You must not connect the LED directly to the output of the opamp.
You need to insert a resistor in series with the LED to limit the current.

 

oop i guess i forgot to put in that Im actually using a lm339n quad that i got from radio shack.. the one part i got from that place is the one that is giving me trouble and i have tested the led and its not blown with the amount of power im using the led turns orange before it starts to get warm so lack of transistor shouldn't be to much of an issue >.<

 

The LM339 is not an opamp. It is a quad comparator with open collector outputs.
You will have to post your schematic. I suspect you have it connected incorrectly.

Also the LED will either be completely on or off, no in between.

 

oh wow.. yea it isn't an op amp, OK i will send a schematic next chance i get. Probably in around 14 hours. thanks a ton for helping me so far, i feel kinda of dumb now

 

Given your resistor values from that schematic the gain will be 100K/1K = 100.

 

The comparator in that circuit will have its output transistor turned off or floating all the time so the LED will never light.

 

Well i went back to the store and got an actual amp, i have it set up to be a low pass filter at 600 Hz and i was wondering two things.
1) how would a virtual ground effect the operational amp, it is currently running led's to the beat of music.
2) if i just adjusted one of the resister values on the active filter would it change the frequency range? or do i have to change both values? Its set up as a two pole active filter.

 

Please show your circuit schematic.

 

c1=.047uf
c2=.024uf
r1&r2=8kohms (hoping to make variable but only want to use one pot.)
power input=12.08 volts
ground=0 volts
input= .8 volt signal from op amp amplification circuit with a gain of 1 (hoping to make that variable up to a gain of 5)
output= transistor to power up to 82 LED's simultaneously
I make the schematic up myself so i cant really find one that i can paste and sadly i have no way of drawing it out or putting a picture up at the moment.. if you need a drawn out schematic i can see what i can do but drawing them on paper isn't really my forte.

 

You have an opamp with no part number and a Sallen and Key second-order lowpass Butterworth filter with the virtual ground biasing missing so it probably does not work.

The frequency of the filter is changed with a dual switch and pairs of resistors or with a dual pot replacing the two resistors.

The gain of the filter must not be changed. Add another opamp for more gain if you want.

 

It works as of yet but i feel that i can get more performance out of it. I want different gain adjustments because i will be able to make up for the difference in volume from movies to music if that makes sense. So i take it from what you said that i should get a virtual ground to increase its performance? I have no problem making one i just didn't know if i should or if it was just for special applications. The op amp is actually a lm324 quad amp. If you have any ideas for improvements that are fairly simple please let me know, this is my first ever circuit and i love to have different styles to be able to build things off of.

 

The LM324 opamp works without a dual polarity supply or virtual ground biasing. But the filter works oddly when the input goes negative by the negative swings in music or speech. it will be difficult to add virtual ground biasing unless you use one of its opamps (it has 4 opamps) as an input buffer that is biased by a voltage divider.

The output of the filter needs to have a level-shifter so is near 0V when the signal level is low. It can also have adjustable gain.

 

Yea i am currently only using 2 of its amps, so what would a schematic look like for what you are talking about if you don't mind pointing me in the direction of one, also would you recommend that i put in a virtual ground or should i just leave it the way it is? If its not broken then don't fix it i s'pose, i just was hoping to get a bit smoother waves cause it seems like the LED's flicker a bit more then other light organs ive seen but perhaps its just because of their camera.

 

Ok, i have done a bit more research on the top and from what i understand if i make a virtual ground i can then allow my op amp to go into the negative and get the rest of the signal instead of it just going to ground right? so if i were to set the v-ground how do i know what to set its value to?

 

When the input of your opamp goes negative then the opamp is messed up (its maximum recommended negative input voltage is only -0.3V) which causes its output to suddenly go positive. Maybe that is why your LEDs flicker. Also your LED driver needs a peak-detector circuit.

If you add an input opamp with a voltage divider to bias its input then the filter opamp will be able to swing normally up and down.
Then a level-shifter is needed to reduce the output DC voltage so that the transistor that drives the LEDs will be able to turn off when the signal level is low.

 

Ok i read up on the voltage divider with an op amp, its the type of virtual ground i was thinking of making so im pleased that you recommended that, and i also looked at the peak detector and i was thinking of setting that to around 200th's of a second, ill play with it as i go. the one thing i do not understand is the level shifter.. i cant find any information on it so anything you can tell me about it would be great. Thanks indefinitely for your help so far i really appreciate it.

 

Your vision cannot see short durations of light brightly. 30ms (0.03 seconds) looks bright but shorter than 25ms looks dimmed.

Is 200th's of a second the very long duration of 0.2 seconds?

If you make an inverting peak detector with an opamp that still works when its inputs are at 0V (an MC33071, LM358 or LM324) then it can be capacitor-coupled so the level shifter is not needed. The peak detector can have gain if you want.

 

Also i don't really understand the negative input voltage of -0.3v. is that something that i need to take into account when making a virtual ground?