input signal LED on volume control - total beginner!

[jonschorah]

can anyone help here please?

i am using several of the below to reduce the level of some audio signals i'm recording through my soundcard.

i would like to add an LED to the circuit to register an input signal, but i don't want it to affect the audio signal level by drawing off current.

at the moment i have 8 volume controls like the diagram and would like to add an LED to each and run the whole lot off a 9v battery.

(sorry if the diagram is wrong, it's supposed to represent a couple of mono jack plugs wired up to a potentiometer)

 

[mstechca]

Even though that circuit represents a simple volume control, It would be better if the output faces right instead of left.

i would like to add an LED to the circuit to register an input signal, but i don't want it to affect the audio signal level by drawing off current.

I assume that it means you want the LED to represent the speaker (or the output).

It depends on your application. If you are just taking sound from your PC's soundcard, then you can connect an LED directly to the output of your circuit. In most LED's, there are two different sizes of pins (inside the LED's bottom). The bigger and fatter pin connects to ground, and the other (small thin pin) connects to your "output".

 

[audioguru]

Hi Jon,
You can't simply connect LEDs across the outputs of your volume controls. Their output voltage level is probably much too low for the 2V requirement of LEDs. Therefore each LED needs an amplifier circuit to amplify the voltage, a diode to prevent reverse-voltage breakdown of the LED and a current-limiting resistor. An couple of LM324 quad opamps will drive 8 LEDs.
You say the volume controls are used to reduce the audio signals. Maybe they are from suitable amplifiers already. If they are speaker amplifiers then they might be able to drive LEDs with a diode and resistor without too much distortion.

 

[jonschorah]

thanks for the pointers - audioguru, i understand some of what you are saying!

the input into the system is from the inserts/direct outs of a PA. this just a hot line level signal (but a bit too hot for my soundcard). i don't want to interfere with this signal, other than to turn it down a bit with the pot.

i want the LED to light when there is an input signal, but as you say, it will need it's own power supply, after that you are loosing me.

so far i am thinking that i need a kind of switch, to detect an input signal and then allow my LED circuit to light.

the diagram below should get across my idea, even if it's a load of rubbish from an electonic point of view. i've not tried it, but i assume it's not complete/won't work, but can the idea be used to make a working circuit?!

looking forward to some more lessons

 

[audioguru]

Hi Jon,
A simple opamp circuit is very much better than your transistor circuit:
1) The input of an opamp is a very high resistance and won't affect your input signal whatsoever. The input of a transistor is basically a low-resistance rectifier diode. It would load-down your input signal and maybe rectify it.
2) An opamp isn't affected by temperature while a transistor is a thermometer. You could bias the input of a transistor so that it has the LED just barely turned off, but a small input would turn it on. When it does turn on it would load down half the signal of your input like a rectifier diode. If the transistor heated a small amount, it would turn on continuously, and the LED would change its brightness with the volume of the input. If the transistor cooled a small amount, it would be off most of the time and not give the indication you need.

The gain of a simple opamp circuit can be set so either the LED lights with any input volume, or lights when the input volume exceeds a certain level then change the brightness of the LED with the volume of the input, your choice.

 

[jonschorah]

thanks audioguru, i am convinced by your argument, sounds like exactly what i need.

is this what you mean? (i'd like the LED to get brighter with a stronger signal)

to be honest i'm now at the limit of my understanding. i think the opamp is acting as a comparator over the input signal and is then amplifying this signal to light the LED using power from the battery -stop me if i'm wrong!

how do i set a 'threshold' level as i don't want the LED lighting at backgound levels?

 

[audioguru]

Hi Jon,
You were close and you made the opamp a comparator. I turned the LED the correct way around and added a threshold adjustment.
Nya, nya. I can draw lines straighter than you can!

It still has a couple of problems:
1) Since an opamp has a gain of a few hundred-thousand without negative feedback, the LED will flicker brightly with any signal level that is slightly above your threshold setting, no dimming.
2) Since the circuit doesn't have the signal rectified then filtered, the LED will flicker with each cycle of every sound above the threshold setting, intantaneously.
3) The absolute maximum negative input voltage of the opamp is only 0.3V and its input signal is AC which swings both positive and negative. Therefore if the peaks of the input signal exceed 0.3V, it must be attenuated before the opamp's input.

 

[jonschorah]

ok, i'm starting to get lost, but i think i can get there (i've found my old school physics book now!)

i can follow the threshold vari-resistor ok.

1) this i can understand, i was wondering what all that gain would do. i can live with this if it's hard to resolve without affecting the audio signal.

is negative feedback where you put a resistor over the output and -ve input? if so will this interfere with the audio signal?

2) looking up what a rectifier does, to sort out the flickering i need to turn the AC signal into a DC signal and average the level a bit so it's not responding instantly? (i never realised audio was AC, though it stands to reason when you look at an audio waveform). this sounds compilcated...

3) i'm guessing the audio signal has a good chance of exceeding this.

what is a voltage reference IC? did you solve 1) 2) & 3) with this?

i will come back to linking in the other channels once i understand how to get one of them going!

 

[audioguru]

Hi Jon,
1) That's right, to add negative feedback to have say a gain of ten, you connect a very high-value resistor maybe 1M from the opamp's output to its -ve input, then a 100K resistor to the threshold adjustment pot that has a value of 20K. It won't interfere with the input.
2) Adding a rectifier with averaging to this circuit is complicated.
3) You have very hot signals that must be attenuated before the opamps.
4) A voltage reference IC has a constant small output voltage when the voltage of the battery runs down. Since its only function is to keep the threshold voltages from changing, it doesn't affect dimming or input overload.

Consider using four AA battery cells to power 8 of these circuits instead of one little 9V battery. A little 9V battery has only AAAA cells inside and won't power your project very long. Four alkaline AA cells are usually less expensive than a little alkaline 9V battery and will last 5 times as long. Rechargeable batteries have the same ratio for capacity.

 

[jonschorah]

thanks audioguru, hopefully i'm nearly there now.

1) i've added an input resistor to attenuate the input voltage to the opamp and a couple of resistors in to the feedback loop.

2) i looked up a rectifying and smoothing circuit in my physics text book and it looks like a lot of extra work to me, unless that kind of thing can be bought as a single unit. clever stuff though, i'm tempted to build one just for fun.

unless i've made a bit of an error with my feedback loop i guess i just need some resistor sizes etc. to stop the thing blowing up and the low down on opamps and voltace reference IC's as at the moment i couldn't tell an appropriate one from any other.

btw, you wil notice i have discovered the secret of straighter lines

this looks like a lot just to get an led to light at an input signal, but i've been drawing in slowly, too late to quit now!

 

[audioguru]

Hi Jon,
Yeay, straight lines! :lol:

Your input attenuator did diddly-squat so I fixed it and took a guess at your max input level.
Because the input is attenuated, I added similar gain to the opamp.
Get an LM385BZ-1.2 (ON Semi) from Newarkinone.com or maybe Digikey.com voltage reference and it needed a resistor feeding it that I added.
Here are all the details:

 

[jonschorah]

great, thanks for that ag . i can get those componets through the uk version of the site you mention (farnellinone)

last set of questions (hopefully!):

1) the LED, is 2.5V a minimum rating, as they are a lot cheaper when they are 5V?

2) the opamp chip has 4 opamps on it?

3) adding in the other inputs. taking from where you suggest, i only need the one voltage reference IC? is it possible to tap at the -ve input to the opamp for the other channels instead, as i don't need to be able to adjust the threshold or opamp gain independently.

4) there are loads of different kinds of resistors, is there anything i should be aiming for, or will the cheapest do?

5) is the capacitor you added smoothing out the fluctuations in voltage?

6) is there an equivalent voltage reference IC available here www.maplin.co.uk (i can't find the one you mention) as there is one locally, so i can buy resistors etc. singly instead of in packs of 25!

thanks for the help, as you can tell, this is my first project, so it's a bit of a steep learning curve.

 

[audioguru]

Hi Jon,
1) Ordinary green LEDs are very inexpensive and are around 2.5V. You can get ultra bright ones for about 4 times the price and they are around 3.5V. You can also get an ordinary one with a resistor inside for use with 5V but they are more expensive because hardly anyone uses them.
2) That's correct. A quad opamp IC has 4 opamps in it.
3) I designed the circuit to use a pot for each channel. You adjust the pots to cause its LED to light at whatever level you want. You cannot connect the inputs of the opamps together. Only one voltage reference IC is needed for all your 8 circuits.
4) Most people use 1/4W 5% carbon film resistors, they are inexpensive and sometimes are available in an assortment. I recently got a pack of 480 made by Velleman the kit maker.
5) Yeah, your battery voltage will fluctuate a lot without the capacitor.
6) Maplin looks like a department store to me. Farnell have them:
https://uk.farnell.com/jsp/endecaSe...Ntt=LM385BZ-1.2&Nty=1&N=401&Ntk=gensearch&y=8

I buy most of my electronic parts online and they are delivered the next day.

 

[jonschorah]

ok, thanks for your patience, i'll get my order in and let you know how the build goes

 

[fredsm]

Or simplify it by using an LM3914 and bar display.
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[audioguru]

Hi Fred,
Eight LM3914's would cost much more than two LM324's.
Since the LM3914 has a linear graph, it would show a range of levels of only 20dB with most of them crammed together at its high end and spaced 6dB at its low end. The LM3915 is more suitable for showing audio level since its indication is logarithmic like your hearing, has a 30dB range and has 3db steps.