Audio to led amplifier using lm358

[AHacker]

Hey guys i am trying to aplify sound signal from my phone to glow LED(IR). i am using lm358 dual op amp. But i am haveing lots of problem. Like powering the ic, connemting ground and virtual ground n all. I am new to electronics. Plz sm1 hlp me make the circuit.

 

[Audioguru]

It is easy to power the LM358 because it works with a power supply that is from 3V to 32V.
You do not need a "virtual ground", instead you bias the (+) input to half the supply voltage with two or three resistors and maybe a filter capacitor. Then the input, output and feedback to ground need coupling capacitors.
An LED is a rectifying diode so if the amplifying opamp feeds it AC audio then the LED will be turned on for half-cycles and turned off for the other half cycles causing it to appear dim. Adding a peak detector circuit will fix that and make every sound activate the LED instead of only sounds with a duration of 30ms or more.

 

[AHacker]

Yhis is the aproximate circuit i want. If you could modify it plz ot will be great..

 

[Audioguru]

Your circuit does not use active rectification so it needs a fairly high input level for it to work.
Your circuit also needs special opamps that have inputs that work at ground or have a dual polarity supply.
The opamp might not produce enough output current to light the LEDs brightly. why two separate circuits for the polarities?

My circuit uses an LM386 low voltage audio amplifier that already has inputs that are already biased at 0V and does not need a negative supply. The output is already idling at half the supply voltage and the gain is already set to 20. It has plenty of output current available to light one pair or many pairs of LEDs brightly.

 

[AHacker]

Thank u very much. I will try this circuit. But what resistance should i use for infrared led. And is there any problem if i try the circuit with lm358

 

[AHacker]

What if i use this circuit after removing the capacitors in output. Will it work??

 

[Audioguru]

The opamps in that new stereo preamplifier circuit are biased with their outputs at half the supply voltage so the LEDs will be lighted all the time, even when there is no input signal.
My circuit uses a series output capacitor to block the DC voltage and uses a pair of LEDs connected oppositely so that one lights when the waveform goes more positive and the capacitor charges and the other LED lights when the waveform goes less positive and the capacitor discharges.

If the amplifier uses a series output capacitor and drives only one LED then the capacitor will charge on the first cycle of sound and stay charged because the second LED is not there to discharge it then the single LED will never light again.

A pair of IR LEDs in my LM386 circuit needs a 100 ohm series resistor to limit their current.
Why IR? You cannot see it.

The resistance for the LED current limiting resistor depends on the LED voltage and the current you want in it. An IR IED is about 1.2V and the maximum output from my lM386 amplifier is about

 

[AHacker]

Ok then i will use your circuit. And i am using ir because i am making a remote control module to control tv from mobile. And as i asked can i use lm358 instead of lm386 in your circuit asi have the ic right now. Otherwise i have to go and buy lm386..

 

[Audioguru]

Usually an IR remote control BLASTS very high current digital pulses, not low level audio that blinks an ordinary LED. The high current pulses are for short durations so the IR LED does not get too hot and burn out and the high current allows the good remote control range.
An LM358 opamp cannot produce high current and its frequency response is too poor for 38kHz remote control digital pulses.
An LM386 audio power amplifier can do it.