follow sinus/triangle with led

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Dayvi

Member
Joined
Sep 18, 2017
Messages
10
Hi,

long story short, I cloned a moog modular. It’s working great so far.
I saw that synth.com and moon have leds on the oscillators that follow(fade) the sine waveform in the frequency of the oscillator (only useful in lfo mode). Also my sub37 has this leds on the lfo.

I tried some circuits but this is not working so great. I get the led to light up, but it is only 1/3 of the sinus. The whole negative current and a lot of the positive current of sinus is not responding to the led. I have an npn and a pnp transistor circuit and the sinus/triangle output to do that. Also this circuit is somehow affecting the control voltage of the oscillator so the pitch it rising and falling with the fading led and this is very bad and I don’t know from where this is coming.

Does someone have any ideas or schematics, on how to follow a sine/triangle wave with a led with a not fixed frequency?

Thank you
 
Thanks for your feedback so far...

Here are the two schematics I tried to visualize a sinus with a led.

Schematic 1:
With a diode and two transistors. This works kind of, the led is only reacting to 1/3 of the sinus. On the oscilloscope I can clearly see that the signal that is by the led is a cut of sinus. The goal is to let the led pulse to the frequency of the osc, so the negative part of the sinus is as important as the top one.
0iiOBaI.png


Schematic 2:
With a full wave rectifier the led is lighting up the whole time. And no sinus is visible on the oscilloscope.
MsnduGi.png


I also tried the circuits with a sinus generator on various volts and herz to ensure, that the circuit is not eating away the whole current.
The output of the moog oscillator is only 0.5v.
 
LED forward current vs brightness is generally linear. PRRs circuit will probably work pretty well but if you drive the transistor with an op amp and get it's feeback from the emitter, you will have a precision current sink which should work even better. But you will also probably need to add a voltage divider in front to scale things and map CV to max LED current. If you don't want to drag in an op amp, you might be able to do the same thing with a simple current mirror.
 
thank you squarewave and PRR!

I will test your circuits asap and will let you know the results.


Thanks a lot.
 
> precision current sink which should work even better.

Yeah; but how linear does this have to be? (How linear is the eye?)

He may need offset and scaling. Since we don't know the actual voltages, I was gonna wait and see how the simplest plan looks to him. Clipping at light or dark may not matter much. It may not go to DARK; if that matters we can throw-in another resistor.

K.I.S.S. until we find a need for some more "perfect" solution.
 
PRR said:
> precision current sink which should work even better.

Yeah; but how linear does this have to be? (How linear is the eye?)
True. The 1K in your circuit would make it linear enough. And yes, we would need to know more precisely what the CV is (postive and negative or just postive w/ DC offset) and what the desired behavior is (illuminate on positive and negative or only when positive and dark on neg. peak) .... etc.
 
PRR said:
> precision current sink which should work even better.

Yeah; but how linear does this have to be? (How linear is the eye?)

He may need offset and scaling. Since we don't know the actual voltages, I was gonna wait and see how the simplest plan looks to him. Clipping at light or dark may not matter much. It may not go to DARK; if that matters we can throw-in another resistor.

K.I.S.S. until we find a need for some more "perfect" solution.

I built your circuit today and this are my results:
It is kind of working but the duration where the led is off, is too long, the “sine” that drives the led is not as wide as it should be. You can clearly see it in the photo. The led pulsing should indicate how fast the LFO is oscillating, at the moment that is hard to tell.

The sinewave from the moog is not greater than 0.5V so I think I have to amplify it to around 3V to use your circuit design, because it only starts working at around 1,8V and to get decent brightness out of the led, I have to boost the sine to ~3V.
I did the test with a sinus generator with 0.5-2Hz @ 3V, but there should not be a difference except the gain of the output Sinewave.

Would be very cool if you know a way to get more width out of the sine, so it lines up better with the actual sine that is driving the circuit. Maybe I should add a potentiometer to get the best resistor value, if a resistor is all that is wrong at the moment.


Thank you very much,
I really appreciate your help.
 

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Hello

Seem like your LFO is bipolar, as LED show single wave rectifying.
As already stated I suppose you need buffer gain(scaling) and offset(sin always positive)

Best
Zam
 
zamproject said:
Hello

Seem like your LFO is bipolar, as LED show single wave rectifying.
As already stated I suppose you need buffer gain(scaling) and offset(sin always positive)

Best
Zam

Thanks for you input, that was the last step I missed.

I added an offset to the signal. Now I have a nice sinus pulsing LED that indicated the lfo speed.
The last step I have to make sure is, that the offset is not traveling back into the lfo, but I think a capacitor will stop that from happening.

Thank you guys!
 
PRR said:

I tinkered with some circuits and discovered a not very solvable problem for me.
Schematic 3: The schematic by PRR, it kind of works but as I described a few post above it is not very good to indicate the lfo speed because the bottom part of the sinus is cut off.

Schematic 3.1: I added an offset circuit. This is very good because now the bottom part of the sinus is also used to control the led. All seems great but the reality is that the offset I create, is influencing the input (Oscillator). Offset in the main oscillator is a very bad thing.

Schematic 3.2: I tried to put a capacitor in the signal to prevent the DC voltage to travel back into the OSC. But this capacitor renders the whole schematic useless and the led is on the whole time.

I also tried your other newer schematic with -12V but it is not working, this results in a light up led all the time, no pulsing at all.
So basically I have the whole function, but now I need a way to make sure the DC from the offset circuit is not traveling back to the input and I don’t really have an idea how to prevent this.

 

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The circuits recommended by PRR are clearly just general ideas. In practice you would need to refine the circuit for at least two other reasons:

1) If you're going to get a good range of gain, you're going to need to increase the gain of the transistor and right now that is severely limited by the size of R2. In practice that should probably be more like 220 or even 100 ohms. Otherwise, the LED is just going to vary in brightness a little. You probably want current that ranges from 1mA to 20mA.

2) Theres no way around the rectification (bottom of wave being cutoff) without either adding an offset to the base or lowering the voltage at the emitter. The former is difficult because of requirements from upstream circuitry (as you found out). So lowering the emitter voltage is going to be necessary. But -12V is too much. It should probably be more like -2V. If you only have -12V, then you can make -2V with a voltage divider from ground to -12 and then tap into the base of a PNP with the emitter connected to R2 and then a cap from the base to ground. Adjust the ratio of resistors to get -2V at the bottom of R2. Sorry no schem.

If you adjust things just so, then the LED will smoothly transition between completely off and bright with each sweep of the sine wave.
 
It might REALLY help to know what the actual voltages are out of the LFO. "MOOG modular" is not enough clue.
 
PRR said:
It might REALLY help to know what the actual voltages are out of the LFO. "MOOG modular" is not enough clue.

As I already wrote, that the sinewave is around 0.5V.
From the PS i get +12V and -6V.

@squarewave, thanks for your input, i will try that out.
 
I am LIKELY to be in error here, but VERY dusty memories say that Moog used 0 to +5 Volts DC control voltages referenced to "ground".  1 Volt per octave for the oscillators, etc.  Dusty memories from when I actually messed with Moog stuff decades ago (1970's) ....but I don't recall any negative control voltages. 

So, a LFO could modulate between 0 and +5 VDC referenced to "ground".

Bri


 
Brian Roth said:
I am LIKELY to be in error here, but VERY dusty memories say that Moog used 0 to +5 Volts DC control voltages referenced to "ground".  1 Volt per octave for the oscillators, etc.  Dusty memories from when I actually messed with Moog stuff decades ago (1970's) ....but I don't recall any negative control voltages. 

So, a LFO could modulate between 0 and +5 VDC referenced to "ground".

Bri

You mixed something up…
As you said, the trigger(gate) signal is a 6V DC that shorts to ground if you press a key. Called S-Trigger.
The Oscillator outputs 1V/oct that is right.
We speak about the LFO functionality of it, and this is just the oscillator pitched down two 0.5-5hz. So, the current that comes out is around 0.5V in that range. This is the only range that is important for visualizing the led.
The LFO does not output 0-6V(S-Trigger) because the LFO output is just the normal audio output of the oscillator.
The moog circuits need +12V and -6V to run.

 
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