# Using 2N2222 to generate noise - how would I make frequency lower / change frequency?

I've built the circuit shown in the snapshot below :

The parts list is:

• 2 - 1 Meg Resistors
• 1 - 2.2K Resistor
• 1 - 2N2222 transistor
• 1 - 47nF cap (Note: I actually used a 470nF cap in the one I built)
• 1 - ua741 op amp
• 1 - 3.5mm speaker output

I put the fritzing diagram together from watching the following youtube video: https://youtu.be/dAIQHkicIoo

Built Real Circuit

I have actually built that circuit on a bread board and tested it and it works great.

Question: How might I lower the frequency?

Now, I am interested in how I might change the frequency of the sound so that it might emit a lower tone.

2N2222 Generates Sound (amplified by op amp)

I am an electronics novice so I know very little about how this works but I am fairly sure the 2N2222 is generating the sound which is amplified by the op amp.

I also see the emitter seems to generate the input (signal) to pin 2 of the op amp via the 2.2K ohm 470nF cap part of the circuit. Is that the key to lowering the frequency that I hear?

If I increase resistance to that part of the circuit (lower input voltage to 741 input pin 2) would that cause the frequency to go lower? Would that mean I have to drive the circuit with more voltage since I'm adding resistance?

That's all just guesses.

Can someone tell me how I should alter the circuit to lower the frequency I hear?

Here's a very quick video I made so you can hear the sound? noise? that is created by the circuit.

https://youtu.be/cu29h9LWMTs

YouTube Video Example : Using .1uF Cap In Place of 470pF I replaced the 470pF capacitor in my circuit with a .1uF cap (thanks to user to posted that answer) and now the noise is "lower" sounding to my ear. Whatever that means. :) See what you think.

https://youtu.be/5D41GocraPE

• @NickAlexeev Will do. I thought maybe Fritzing was the way to go. I'll try to create a schematic and add it. thanks. Mar 5, 2017 at 20:32
• Per definition noise is not a periodic signal. But if there is no period, there is no frequency also. What you can change by filtering of a noise signal is the spectrum of the noise signal. A low pass would change the balance between low and high frequency parts of the spectrum. The low pass would enhance the low frequency parts and decrease the high parts.
– Uwe
Mar 5, 2017 at 20:51
• The schematic is linked in the youtube page: electro-music.com/forum/phpbb-files/whitenoise_969.jpg Mar 5, 2017 at 22:07
• OpAmp pin3 (Vin+) is floating. Mar 5, 2017 at 22:08
• This is a profoundly defective circuit. Why it seems to work in the video is anybody's guess. By rights the output should be a constant voltage with no noise. Drop the whole thing and find a different way to waste your time. The op amps is not rated for 9 volt operation, the floating + input should disable it, the lack of a DC path for the - input should disable it, and the low output power (if the op amp were actually working) should be pretty much inaudible. Total crap. Mar 5, 2017 at 23:19

EDIT: This all is unimportant. The questioner found the error by himself. See the comment. The 47nF capacitor in series with the 2200 ohm resistor kills all bass.

It's not white noise if you filter it. I bet your system oscillates and you cant stand the high pitched whistle. Why do I think so: I can't see any big capacitor at the 741's supply voltage input terminals. Connect one 50uF 10V to 25V between the same points where the wires from the battery come to the opamp.

The noise, if it is white, can sound quite high pitched and fizzy if you have a little speaker that has no bass and gives enormous boosts to some frequencies. Actually the white noise should have absolutely no pitch ( distinquishable frequency).

If you need a less treble content sound, attenuate some high frequencies (and kill the whiteness at the same time) Connect a little capacitor parallel with the 1MOhm resistor Start tests with 150 pF (that 1MOhm which is connected over the opamp).

• It doesn't seem to oscillate at all (to my ear) and there is no high pitched whistle. It is unfortunate that the original youtube I found this on does not show it in action at all, but since there is some interest in this I will fire up my circuit and do a quick YouTube video of it in action so everyone can hear it. Maybe the original YT poster was on to something because he seems to have created a circuit that shouldn't work. :) Or at least not work well. The sound? (Noise?) produced from circuit actually sounds good to me, but was interested in what creates the sound resulting in my post Mar 5, 2017 at 22:29
• Upvoting, because you gave me the proper idea for making the sound I hear seem to be lower. I know I don't understand it, but here's what I did. I changed my 470nF cap to a 104m (.1uF) cap and the sound went "lower". :) Now, I can do some more research and learn more but at least I have a proper working circuit that has changed according to one data element. I will wait and mark this as the answer in a day or two. Thanks very much. I will also make another youtube video to show the "lower" sound. Mar 5, 2017 at 23:51
• @daylight Remove the upvote. I do not earn it. I did not do the calculations and I missthought that capacitor to be large enough. Actually it was a serious bass killer. You found the right solution by yourself. Well done!
– user136077
Mar 6, 2017 at 0:04
• You still put me on the right path toward an answer and I appreciate that. This is a good learning circuit I think. :) Though it may be flawed (as others have pointed out). Thanks. And I didn't downvote you. Mar 6, 2017 at 0:12

This is a common audio white-noise generator.

Take a look at many other examples using slightly different component values: https://www.google.com/search?num=100&q=noise+generator+ocean+schematic

The noise comes from zener breakdown, applying reverse bias to the EB junction of the transistor. Hint: try using other 2N2222s, or other types of NPN and PNP transistors. Some don't work at all, but the ones that do will give a somewhat-different sound of white noise (so, actually "pink noise.) Also, try replacing the transistor's 1MEGA resistor with a 1M pot in series with 3.3K. Then, vary the pot setting to produce slightly different pink-noise sounds.

And yes, ground the pin3 input of the 741. And, for lots of bass, swap the 47nF with perhaps a 1.0uF 35v electrolytic capacitor (with its positive pin on 741 pin2.)

Here's an early version of this noise generator

• That's all great info and the kind of info I was hoping for. Thanks for reading my question and taking time to post an answer. You provided me with additional ideas to try and I will take a look at all of them. Mar 6, 2017 at 12:33

Question: How might I lower the frequency?

If it is whitenoise, it by definition has all frequency in it, high, medium and low.

You can simply put a filter to it.

Or maybe it isn't a white noise generator.

• Well, that was educational...to learn that my definition of white noise is a bit too loose and that white noise "has all frequency in it". Mar 5, 2017 at 20:37
• whitenoise by definition includes all frequencies. running it through a 741 will get you anything but. the basic operations here is an avalanche pulse generator. typically they need 12 - 18v to work. a little surprising to see it operational at 9v. Mar 5, 2017 at 20:45