help needed with designing low frequency oscillator

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
I am trying to build a tremelo effect for a guitar, and I need help designing a low frequency oscillator that will produce a sine wave from about 1-10 hertz. I also need it to be powerful enough to flash an LED or a small bulb on and off. does anyone have any suggestions?
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
This sounds like the circuit in the Fender Super Reverb amplifier. If you're using vacuum tubes, you can copy that one.
I wanna use transistors. Im gonna put it in a stompbox enclosure, so I really dont have room for tubes. I wanted to design a circuit that uses optocoupling to produce the tremelo effect, so I need it to have a voltage swing suitable for powering a small bulb
 

AmpAmateur

Joined Aug 18, 2012
6
How about a comparator oscillator? You could easily put a circuit together that would give you peak to peak voltage of 7 volts with your 9V supply. I like this oscillator for your application because you could put out a square wave which would flash an LED or an LED string and then you could smooth the square wave with a filter to turn it into a sinusoid. Do you know what frequency range you are looking for?

If you're interested in the comparator approach, let me know, I could get you pointed in the right direction.

On making the above circuit adjustable:
Where did you find this circuit? Is there some supporting information on the elements of this circuit? All you need to know is how the timing is set. The transistor will be turned on and off at some time constant, set by an RC value. If you can figure out what part of the circuit sets this value, all you need to do is switch a resistor to a potentiometer, a capacitor to a variable capacitor, or both. If you have trouble figuring out which part of the circuit sets the timing, let us know. Before you switch out a resistor for a potentiometer, be cautious and do a power analysis (V^2/R).
 

AmpAmateur

Joined Aug 18, 2012
6
Skip over anything you're not interested in at the moment. Getting something working is sometimes just too important to spend time understanding the details. You can make sense out of it later if you're interested.

Time constant:
Resistance in Ohms = R
Volts = V
Charge in Coulombs = q
Capacitance in Farads = C
Current in Amperes = I
[R] = V/I
= q/second
[C] = q/V

Time constant, tau = RC = (qV)/(IV) = q/I = q/(q/second) = seconds. This is your period. Frequency, f = 1/T. A frequency of 3-10 Hz corresponds to a time constant of 0.33 to 0.1.

A 1uF capacitor and a 100k resistor would give you a time constant of .1. You would need a 330k resistor to get a time constant of 0.33 with the same 1uF cap. It might be interesting to use a 100k resistor in series with a 250k potentiometer. This would set your max frequency to 10 Hz and your min to 2.85 Hz.

Take a look at the circuit attached. Let me know if you have any problem viewing it. I used open office. I can find other formats to save in if necessary.

Look at an LM239 comparator. It is a standard comparator. You can compare it to others which might give you better output current. That is probably the only spec to try to improve upon in this application. If you're not happy with the output current, you could use this to drive a transistor with a higher current capability.

Remember, this is a square wave. A low-pass filter is required to turn it into a sinusoid.

Post again with further questions.
 

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Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
Do i just use a simple passive low pass filter to turn it into a sine wave? I would make it so that it filters out frequencies above 10hZ, right. Also, could I use an LM339 quad comparator. This is what I have lying around and it seems close enough to the LM239.
 
Last edited:

Ron H

Joined Apr 14, 2005
7,053
Skip over anything you're not interested in at the moment. Getting something working is sometimes just too important to spend time understanding the details. You can make sense out of it later if you're interested.

Time constant:
Resistance in Ohms = R
Volts = V
Charge in Coulombs = q
Capacitance in Farads = C
Current in Amperes = I
[R] = V/I
= q/second
[C] = q/V

Time constant, tau = RC = (qV)/(IV) = q/I = q/(q/second) = seconds. This is your period. Frequency, f = 1/T. A frequency of 3-10 Hz corresponds to a time constant of 0.33 to 0.1.

A 1uF capacitor and a 100k resistor would give you a time constant of .1. You would need a 330k resistor to get a time constant of 0.33 with the same 1uF cap. It might be interesting to use a 100k resistor in series with a 250k potentiometer. This would set your max frequency to 10 Hz and your min to 2.85 Hz.

Take a look at the circuit attached. Let me know if you have any problem viewing it. I used open office. I can find other formats to save in if necessary.

Look at an LM239 comparator. It is a standard comparator. You can compare it to others which might give you better output current. That is probably the only spec to try to improve upon in this application. If you're not happy with the output current, you could use this to drive a transistor with a higher current capability.

Remember, this is a square wave. A low-pass filter is required to turn it into a sinusoid.

Post again with further questions.
A filter for this circuit that tracks the oscillator frequency won't be simple. And I think this is an understatement.
Generate a square wave at 50 or 100 times the desired frequency, and use a switched capacitor filter to create the sine wave. Then the filter will track the output frequency.
 
Last edited:

Wendy

Joined Mar 24, 2008
23,396
How stuck on transistors are you? I know of a circuit that could produce a fair approximation of a sign wave, one 14 pin chippie.

Do you have an oscope or any other test equipment? While not necessary, it does help.
 
I apologize, I mislead. I skipped an important step that turns the oscillator from a triangle wave into a square wave. Instead, you could use another comparator on your LM339 to make an integrator.
 

Ron H

Joined Apr 14, 2005
7,053
I apologize, I mislead. I skipped an important step that turns the oscillator from a triangle wave into a square wave. Instead, you could use another comparator on your LM339 to make an integrator.
How do you get from the integrator to a sine wave?
 
It is true that the mathematical integral of a triangle wave is not a sine wave. This is an approximation that has been used to generate a sine wave when the wave shape is not critical. For a really good, adjustable, low frequency sine wave generator, I don't know the best approach because the comparator oscillator is the only method I've used. Maybe a Wein-Bridge oscillator would be a better path than what I've been getting at.
 

absf

Joined Dec 29, 2010
1,968
How stuck on transistors are you? I know of a circuit that could produce a fair approximation of a sign wave, one 14 pin chippie.

Do you have an oscope or any other test equipment? While not necessary, it does help.
Is that a LM324? Yes I am interested and I do have an Oscope.

Allen
 
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