Arduino Theremin Frequency unstable

hello all,

I am currently working on the project from this site:

http://interface.khm.de/index.php/lab/interfaces-advanced/theremin-as-a-capacitive-sensing-device/

If you scroll down you will find a video of the theremin. You will notice a wobbly effect. The hand clearly changes the frequency, but I found the same wobbled effect of the notes. They seem to be constantly changing. So even when my hand is not there, the frequency changes.

I thought that it would be better if there was a single note/frequency. I am trying to demonstrate how a theremin works. Thus, is it possible to get rid of this wobbled effect and get a single note. I have no clue why that effect is there in the first place.

My second question is: I built this circuit on a breadboard and hooked it all up. My only problem is that the sound is much quieter than what he has. Is there any way for me to increase my volume. 

Thanks.

Regarding the volume, all depends on the connected speaker. A TTL output from a microprocessor is not an optimal driver for that. Normally, you should add a 2nd order Butterworth lowpass filter behind the Arduino before feeding the filtered signal into an amplifier which is adapted to drive a speaker.

About the stability of the frequency: Please understand that we have here a lab experiment which is rather a simple proof of concept than a true music instrument. The code does check the timer value periodically, 500 times a second which can lead to aliasing (ghost frequencies) in the audible range. There is no filtering in the code to prevent this. Second, the circuit design and board layout of the oscillator (free wiring of R2/C3, small feedback capacitances C3 and C4, no shielding) is rather poor and cheap, and is also not optimized for stability and not protected against stray effects.

There is another Arduino based theremin concept, the open.theremin UNO developed by the Swiss engineer Urs Gaudenz, which is ways more elaborated and which gives a really playable music instrument. Please have a look here: http://www.gaudi.ch/OpenTheremin/index.php?option=com_content&view=article&id=103&Itemid=93