CD4011 Oscillator Fed to Transistor Audio Amplifier; Volume Control Issue

Thread Starter

brockrwood

Joined Oct 23, 2016
47
*** Update of March 30, 2019 ***

Well, here is the newest version of he schematic. After looking at my old schematic, in Post #6, and comparing it to the breadboarded circuit, I realized there were two goofs. I was NOT using a pot at the spot where I indicated "VR1". I had a 180K resistor in that spot in the actual circuit. As for my "R1" in the Post #6 schematic, on the breadboard, THAT is where the 100K pot was. I was not using an 18K resistor in that spot on the breadboard. The schematic was drawn incorrectly. Sheesh.

I learned the hard way about the value of drawing the signal flow from left to right, without fail, so I don't become confused about what is what.

Anyway, I re-configured the circuit so that it is much closer to the website circuit in Post #13 by AnalogKid. Instead of using an 18K resistor for R1, I used a 20K pot configured as a variable resistor. Again, I am trying to make the frequency somewhat variable. The R2 in the newly re-configured circuit is a 180K resistor. Here is the schematic for the re-configured circuit:

newest_cd4011_oscillator_circuit.jpg

I tried to follow AnalogKid's advice on drawing out the 4 gates on the CD4011 as well as trying to show the signal flow from left to right. I did have to draw a wire all the way to the left side of the page for the second half of the circuit because I ran out of room on the right side of the page.

The new version of the circuit is nice and stable. I get a smaller range within which to vary the frequency, however, using the new values for R1 and R2. I can only adjust down to about 460 Hz. And I can only adjust up to about 2,000 Hz.

I should have looked to my bookshelf containing my favorite Forrest Mims books. His "Engineer's Mini-Notebook" for "Digital Logic Circuits" has some interesting ideas for CD4011 oscillators on pages 38 and 39:

mims_4011_oscillators.jpg

I notice that Mr. Mims uses a 1 megaohm resistor at the position of R2 in my circuit and a 100K resistor at the position of R1 in my circuit. That is almost 10 times my values for these resistors.
 
Last edited:

AnalogKid

Joined Aug 1, 2013
9,189
I get a smaller range within which to vary the frequency, however, using the new values for R1 and R2. I can only adjust down to about 460 Hz. And I can only adjust up to about 2,000 Hz.

I notice that Mr. Mims uses a 1 megaohm resistor at the position of R2 in my circuit and a 100K resistor at the position of R1 in my circuit. That is almost 10 times my values for these resistors.
That is part of the answer to your frequency range issue. The larger R2 is, the larger R1 can be, and the R1 resistance range sets the frequency range. The capacitor and R1's max value set the min frequency, and the range of R1 sets the max frequency. You are running outof high end because as R1 gets very small, the gate output stage cannot supply the current needed. The answer is larger resistors.

R1 should be a fixed and variable resistor in series. this prevents overloading the gate output. With the pot set to zero ohms, the fixed R and the C set the highest possible frequency. With the fixed R of 4.7K, the gate output current never is greater than 2 mA, a safe value. Safe but not perfect; the output voltage will decrease somewhat, affecting the frequency. Increasing the fixed R to 10 K will help greatly with this.

Now that you have the components chosen for the max freq, the combination of the fixed R and the max value of the variable R sets the minimum frequency. Play with the values of these three components to get the frequency range you want.

R2 in your schematic should be larger. It is there to limit the current through the input stage protection diodes when the capacitor is yanked from one voltage to the other. This current path is in parallel with the main timing resistor(s), and makes the output frequency higher than calculated.

Despite National's warnings about the unreliability of the two-gate oscillator configuration, ***everybody*** uses it, including the internal oscillator in the CD4060.

ak
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Well, AnalogKid nails it again! Thank you!

I increased the value of R2, the "stability" resistor, to 1 megaohm.

Per AnalogKid's suggestion, I added R10 to the circuit. This is a 10K resistor in series with the frequency control pot, R1.

I replaced the 10K frequency control pot with a 500K model.

Finally, to let met adjust the circuit to higher frequencies, I decreased the value of the RC timing capacitor, C1, to .01 microfarad.

The 10K resistor for R10, as AnalogKid pointed out, protects the CD4011 from outputting more gate current than it is designed to handle. The 10K resistor in position R10 also sets a maximum upper frequency based on the value of C1 chosen because R10 ensures that the RC network will have at least 10K of resistance in it even if the frequency control pot, R1, is rotated to zero ohms. I found that with the .01 uF cap, the 1 meg R2, the 10K resistor, and the frequency control pot rotated all the way to zero resistance, I get an upper frequency of about 3,800 Hz. Fine for my purposes.

The pot has a 500K value so that I can also adjust the output frequency to a fairly low number. With the pot rotated all the way to 500K (plus the 10K of the resistor, for a total resistance of 510K), I got the output frequency down to 95 Hz. That is more than low enough for my 2" speaker.

Here is the schematic of the improved circuit:

newest_cd4011_oscillator_circuit_rev_02.jpg

Thanks for all the great suggestions!
 
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