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?
edit: Schematic from the YouTube page 
YouTube Sound Sample of Circuit
Here's a very quick video I made so you can hear the sound? noise? that is created by the circuit.
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.
Best Answer
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).