The hard way to turn on a LED

Thread Starter

ropak

Joined Nov 15, 2018
20
I have an 31 band graphic equaliser that uses a LED to indicate if the equaliser section is 'in' or 'out' of the audio path.

To do this, it looks to me, that an audio signal is sent to the non-inverting input of C4558 dual opamp, the output signal is then rectified and sent to the non-inverting input of the other half of the op amp. It is then 'compared' to a reference voltage on the inverting input, which depending if higher or lower will turn the LED on or off.

My problem is that the LED doesn't turn on and I can't for the life of me work out how such an arrangement is supposed to work. The LED should indicate the circuit is 'in' whether or not an audio signal is present. I'm hoping someone has come across this kind of circuit arrangement before and will be able to help me understand what the problem might be. I've mapped out a circuit diagram as best I can. The equaliser functions as it should and switches in and out as it is supposed to, it's just that the LED won't light to say so!

It seems complicated for such a simple task. Any ideas would be appreciated.
 

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AlbertHall

Joined Jun 4, 2014
8,536
The rectifier circuit you've drawn will not work but I have no way to tell how accurate your interpretation is.
There are some typical circuits here. Can you make yours into one of these?
 

wayneh

Joined Sep 9, 2010
16,114
I have an 31 band graphic equaliser that uses a LED to indicate if the equaliser section is 'in' or 'out' of the audio path.

To do this, it looks to me, that an audio signal is sent to the non-inverting input of C4558 dual opamp, the output signal is then rectified and sent to the non-inverting input of the other half of the op amp. It is then 'compared' to a reference voltage on the inverting input, which depending if higher or lower will turn the LED on or off.

My problem is that the LED doesn't turn on and I can't for the life of me work out how such an arrangement is supposed to work. The LED should indicate the circuit is 'in' whether or not an audio signal is present. I'm hoping someone has come across this kind of circuit arrangement before and will be able to help me understand what the problem might be. I've mapped out a circuit diagram as best I can. The equaliser functions as it should and switches in and out as it is supposed to, it's just that the LED won't light to say so!

It seems complicated for such a simple task. Any ideas would be appreciated.
You might check the second op amp by placing a voltage of >5V onto the non-inverting input. The LED should light. If the voltage on the cap after the first op amp is never reaching >5V, then there’s a problem with the signal detector.
 

Thread Starter

ropak

Joined Nov 15, 2018
20
The rectifier circuit you've drawn will not work but I have no way to tell how accurate your interpretation is.
There are some typical circuits here. Can you make yours into one of these?
Perhaps I've made an error in my laying out of the circuit in question so I will go back and check again with reference to the typical circuits you have linked to.
 

Thread Starter

ropak

Joined Nov 15, 2018
20
You might check the second op amp by placing a voltage of >5V onto the non-inverting input. The LED should light. If the voltage on the cap after the first op amp is never reaching >5V, then there’s a problem with the signal detector.
Yes, the LED does light with >5V on the non-inverting input.
 

Thread Starter

ropak

Joined Nov 15, 2018
20
Perhaps I've made an error in my laying out of the circuit in question so I will go back and check again with reference to the typical circuits you have linked to.
Thanks for pointing this out, I have corrected the diode/resistor array in my drawing. It better reflects a typical circuit now, however the input arrangement is different to that shown in the link you provided. The revised circuit diagram is attached.
 

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Thread Starter

ropak

Joined Nov 15, 2018
20
With regards to the signal detector, I'm unclear how it is supposed to differentiate between a processed signal and one that is unprocessed.

Switching the eq 'in' or 'out' simply sends the processed or unprocessed signal to the input of the first opamp.
 

AlbertHall

Joined Jun 4, 2014
8,536
Thanks for pointing this out, I have corrected the diode/resistor array in my drawing. It better reflects a typical circuit now, however the input arrangement is different to that shown in the link you provided. The revised circuit diagram is attached.
Yes, that version would work. It has a gain of six - +1V peak input would give 6V DC output.
 

AlbertHall

Joined Jun 4, 2014
8,536
Switching the eq 'in' or 'out' simply sends the processed or unprocessed signal to the input of the first opamp.
I can't see how it is going to do that.
Are you sure the detector isn't connected to the input or output of the audio processor?
 

AlbertHall

Joined Jun 4, 2014
8,536
To understand why the LED does not light, feed a good loud signal to the detector input and measure the voltage at its output - across C3.
If it is more than 5V the problem is with U2 circuitry, less than 5V it is U1 circuit or the input to that circuit.
In the latter case, do you have an oscilloscope?
 

Thread Starter

ropak

Joined Nov 15, 2018
20
When feeding a 2 Vac peak to peak signal directly to the input of U1, I measure 14 Vdc across C3 and the LED lights.

Feeding the same signal through the input jack of the unit, I measure 200mV across C3.

The input to U1 comes through a voltage divider. On the 'upstream' side of the voltage divider, using an oscilloscope, I can see the original signal if the switch is 'out' and the processed signal if the switch is 'in'. If the switch is 'in' I can vary the amplitude of the signal using the master level slider control, which means I can vary the voltage across C3.

On the down stream side of the voltage divider, ie the input to the signal detector, the signal is attenuated and kind of unstable, ( I only know basic oscilloscope techniques).
 

Thread Starter

ropak

Joined Nov 15, 2018
20
The LED indicator, I'm guessing, is supposed to light up whenever the equaliser is in circuit. I would have thought it would still need to indicate this even during times when there is no signal present.
 

Thread Starter

ropak

Joined Nov 15, 2018
20
I think I understand it all.

The LED is a clipping indicator! It's only meant to turn on when the signal gets too big, which goes to explain all the circuitry.

I should retitle this thread, "The hard way to turn on my brain".

Thanks for all the help to prove that the circuit was working, I can rest easy now.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,114
I think I understand it all.

The LED is a clipping indicator! It's only meant to turn on when the signal gets too big, which goes to explain all the circuitry.

I should retitle this thread, "The hard way to turn on my brain".

Thanks for all the help to prove that the circuit was working, I can rest easy now.
That makes perfect sense!
 
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