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Old 11th May 2013, 09:41 AM   #1
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Default Stuck on Circuit Guitar Preamp LED mirror

I am hoping someone from this forum can give me some suggestions and design tips on how to get this circuit working. What I want to have happen is this -- plug a guitar directly into the circuit and have two LEDs mirror the dynamics of the sound while the guitar is being played. One LED will be lit as the guitar is idle, when the guitar is played this LED will dim to darkness and the other LED will come to life and light itself based on how hard the guitar is played. The two LEDS will mirror each other and always be in response to the input signal.

I`ve uploaded a hand drawn circuit sketch -- it doesn`t work. I`ve been at it a few months in my spare time and am utterly frustrated. it doesn`t work. Any ideas? Do these two initial transistors provide enough gain to feed the two "Logic" transistors in the end....is my design faulty? Please assist. -swissCHEESERClick the image to open in full size.
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Old 11th May 2013, 10:28 AM   #2
Mooly is offline Mooly  United Kingdom
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I haven't the time to give you a fully worked example but the main errors are (working right to left...

1) The final transistor will only conduct on "peaks" of over around 0.7 volts. There's no static bias.

2) Same problem applies for the stage before.

3) Connecting a 10K pot to ground in this way as alevel control is a no no and even more so seeing the ratio of 10K to 220 ohm.

4) The base bias networks for the first stages look wrong value wise... I'd have to calculate it out but the values "look wrong" to give Vcc/2 on the collectors.

A better approach might be to use an opamp.
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Old 11th May 2013, 10:39 AM   #3
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Thanks for the reply Mooly. I will apply the bias transistors to the last 2 transistors & remove the 10k (although i was under the impression I would have some gain control with it). Initially I tried an opamp and it looked too darned digital....either ON or OFF...wasn`t too much dynamic representation in the LEDs. In your experience would you say it is possible to replace the first two transistors from left to right with an opamp and get a rather....analog....representation of the dynamics of the guitar signal?
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Old 11th May 2013, 10:50 AM   #4
Mooly is offline Mooly  United Kingdom
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I would think an opamp or dual opamp would do the trick. Just thinking aloud... two series LED's across the supply (with suitable limiting resistors. Opamp output connects to the centre point so that the voltage swing of the opamp modulates the LED's... workable but would need the operating point (DC) shifting so one LED was off with no signal.

Use an opamp to drive one LED... that would work... and feed the other LED from the same opamp output but to the other rail. So one driven from opamp to V+ and the other to ground. One goes on, other goes off.

Problems or refinements... use a simple rectifier to process the input signal (opamp again configured as an active rectifier to overcome volt drop of diode/s on there own). Would a time constant or sample and hold be needed ? The LED's might flash and flicker and be generally dim in response to audio. Sample and hold or even just a peak detector would overcome that.

Is the input impedance of the circuit to low ? If it connects to a "pickup device" (I'm no expert on what instruments actually use for pickups) then it may need to work into a high impedance. Again an opamp is the answer.

You have to break the circuit down into blocks. Design an LED driver stage. Design the input stage that will send the correct signals to the LED driver etc. Its not so bad when you think of it like that.
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Old 11th May 2013, 01:27 PM   #5
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Mooly, do you think you could make a sketch of your opamp idea? I don`t really follow what you mean. Tried the bias resistors on the base of the last two transistors and have absolutely ZERO positive to report. It doesn`t react at all to the incoming signal. Not enough gain, not well enough designed. Would love to get a look at your sketch for the opamp LED on/off analog dynamic signal mirror.
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Old 11th May 2013, 03:11 PM   #6
Mooly is offline Mooly  United Kingdom
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This will drive two LED's. As the input voltage (a DC voltage) goes from 0 to 12 volts the LED's light progressively one to the other. At 6 volts both are equally lit.

Thats a starting point for one of many possible LED drive options. So you need an amplifier stage in front of this that is scaled so that no signal to max level audio gives 0 to 12 volts approx. It doesn't matter if it clips at high level. So you might get away with one more opamp (have a preset for gain control) and by AC coupling the output, feed that into a rectifier/cap ino the LED driver.

I've sketched an idea for the rectifier and "peak hold" that would go between the amp stage and the LED driver. You would have to experiment with time constants for best visual effect.

The 100K feedback resistor gives the option of adding "AC" gain to the LED driver stage later if needed (R and C to ground) or it can be linked out if not needed.

I show a 741 on 12 volts. Using lower voltages (if it has to be 5 volt) means choosing low voltage opamps.
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Old 11th May 2013, 09:44 PM   #7
Zero D is offline Zero D  United Kingdom
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@ swissCHEESER

Hi, i'm not sure why you need 2 LED's ? Just one would "appear" to achieve more or less what you want.

Expanding on Mooly's circuit i've simmed this. I made it to run off 9 Volts, as i "expect" this is likely.

Without a signal the LED will be lit, & as the signal increases it will flash more & more.

Adjust R1 for Input Level Sensitivity. Adjust R2 to set LED on Threshold.
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Old 14th May 2013, 08:38 PM   #8
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Default Mooly Circuit

Hey Mooly, I found some time to build your circuit. I put a quad op amp on the front side of it and am amping my signalup to 50 times with a 50k pot tied to a 1k to ground through a cap on the feedback. then into your circuit which i have the another 50k pot tied to a 1k to ground. In short, it doesn`t work.
If I plug the input into a guitar, both lights are dim. If I plug it into a preamp and my guitar into the preamp - it works to a certain extent, but doesn`t seem to get enough signal even with the preamp and 2500 times gain. I`m amplifying a signal that is roughly .01 to .100 milivolts from the guitar....should be well amplified to give some sort of result in the opamp. Not really sure what to do next other than pull some more hair out. Any ideas?
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Old 14th May 2013, 09:01 PM   #9
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Default Mooly Circuit

Mooly, here`s a drawing of what I built. From what I understand of what you drew, I think this should work. Before I spend a few hours "debugging" this can you take a moment and look at it and see if perhaps I mis-understood or perhaps I have something completely wrong? About the only thing I`m sure of at this point is that what I built sure isn`t working. Thank you very much for your assistance and designs!
-swissCHEESER
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Old 15th May 2013, 07:54 AM   #10
Mooly is offline Mooly  United Kingdom
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What is missing is the rectifier I mentioned. You have to change the AC (audio) into a varying "DC" quantity. Try adding the rectifier I drew after the first opamp. The resistor I showed with a question mark can be the lower 1Meg you already have (you might need to play around with values) The upper 1meg can also be altered. You have far to much gain too.

Try the rectifier first.

Remember I said it was just to get you started Its not a fully worked and tried and tested design although the LED drive part definitely works as I described earlier.
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