I'm trying to build a circuit that can be plugged into a headphone jack and light up some LEDs to show volume. The comparator controlling the LEDs is working fine, but the amplifier that I need to boost the voltage for the comparator is giving me trouble. I'm using a UA741CN and I used the circuit found on the website linked below, but I used 9 volts instead of 10 volts. I'm not entirely sure what the V+ and V- mean, so I followed the schematic and tried the circuit out by attaching it to a radio. It put out roughly 17.5 volts AC. I changed the volume of the radio, but it didn't change anything. I checked the audio from the headphone jack with my multimeter and it showed that the voltage was fluctuating from about 0.7 volts to about 0.1 volts. Any advice is greatly appreciated. I'm sorry if my question seems dumb, but I haven't worked with amplifier circuits or audio circuits at all. Thanks.
http://www.learningaboutelectronics.com/Articles/Non-inverting-op-amp-circuit.php

 

Hello,

Can you post an exact schematic with all the values you used?
You can upload a drawing using the "Upload a File" button.

Bertus

 

The 741 requires bipolar (positive an negative) supplies. I don't think you could have chosen a worse part for this project. Have you read the datasheet?

 

I've read the datasheet, but just to make sure that I wouldn't damage the chip. I don't know much about bipolar power supplies.

 

I've read the datasheet, but just to make sure that I wouldn't damage the chip. I don't know much about bipolar power supplies.

So a 10V bipolar supply has a +10VDC output with respect to GND, and it has a -10VDC output with respect to GND. An opamp like the 741 will not operate very well on a single supply. eg. +10VDC and GND

 

Could I take an 18 volt source and use a voltage divider to get 9 volts and use it as ground so that 0 volts would be -9 volts with respect to ground and then use 18 volts for V+ with 9 volts with respect to ground?

 

Could I take an 18 volt source and use a voltage divider to get 9 volts and use it as ground so that 0 volts would be -9 volts with respect to ground and then use 18 volts for V+ with 9 volts with respect to ground?

The problem with a voltage divider is that the voltages will change depending on the load. They would be useless in this application. Better to use two 9V batteries for ±9VDC bipolar supply.

 

so putting 2 9 volt batteries in series and making the connecting wire the ground should work?

 

so putting 2 9 volt batteries in series and making the connecting wire the ground should work?

It would be a lot simpler to scrap the uA741 and use a single supply op-amp such as LM358. Then all you need is single 9V battery.

 

The problem with a voltage divider is that the voltages will change depending on the load. They would be useless in this application. Better to use two 9V batteries for ±9VDC bipolar supply.

As long as you end up with +9V, GND, -9V you're good to go.

 

Since this isn't for a precision listening application, a single LM386 might have enough gain (200) and enough output current to drive the LEDs directly, all in one part. Really need to see the TS comparator circuit.

Do you want the LEDs to blink crisply on the beats, or vary in brightness with the volume?

ak

 

I'm currently planning on using some op amps as comparators so that more voltage from the audio jack will light up more LEDs.

 

Consider the TI (formerly National Semiconductor) LM3914/15/16. These are designed from the ground up to produce a bar graph effect (more input voltage equals more LEDs turned on) with an audio input. Each has a transient-protected, buffered input driving 10 comparators with different weightings (voltage spacings), linear, logarithmic, and VU.

ak

 

You might look at the obsolete NSM3919 here: http://www.sphere.bc.ca/test/new.html

It would save you a lot of work.

 

I'll have to check to see if I have any of those. Many of the parts I use have been salvaged from old/broken electronics.

Since the signal from the audio jack is alternating, could I use a transformer instead of an op amp?

The chip that I'm using to compare the voltage is an LM837N quad op amp. I'll try setting it up so that the leds will light up at 0.25, 0.5, 0.75, and 1 volt.

 

could I use a transformer instead of an op amp?

The transformer could step up the voltage, but at the same time it will step down the current. There may be insufficient current to light LEDs well.

 

The op amp I'm using as a comparator doesn't use the voltages it's comparing to drive the LEDs, so it should be fine.

wiring the quad op amp directly up to the radio worked. The lights mostly just flicker, but that's something that I can fix easily. Thanks for the advice.