Why Does My DIY Theremin Sound Gravelly?

[k_squared]

I protoboarded this circuit:
http://www.popsci.com/diy/article/2008-04/build-pocket-theremin-cheap

alas, it seems to make an extremely gravelly noise - the term "hoarse cat" came to mind. Anyways, what could be causing this? I think I have developed a general idea of how the circuit works, however, it does not explain this given that I simply used a proto board to hook up that exact circuit. I mean, could I have accidentally used a low-voltage 555 (if that does anything?) or, you know, whatever. I'm thinking my part tolerances are close enough (well, radioshack). Thanks!

Oh yea... are my photocells supposed to be of very similar values? I can get a slightly better sound out of it by using a bigger capacitor on chip '2'.

Speaking of which, does anyone know any good (fairly) detailed explanations of the 555 timer? That little thing is ubiquitous. Oh, and uh, thanks a lot for any help.

 

[vk6kro]

The output of a 555 is a square wave which is going to sound distorted if you are used to sinewave or musical sound.

The MP3 on that website disguises this a bit by using a high frequency. This means you can't hear the harmonics so the sound seems acceptable.
If the frequency is lower, you can hear the harmonics and they may sound unpleasant.
You can lower the frequency by making the bottom 0.01uF capacitor larger. Maybe 0.1 uF or even 0.47 uF.


There are lots of 555 sites but the one on Wikipedia would be a good start.
http://en.wikipedia.org/wiki/555_timer_IC
Here is another one:
http://www.kpsec.freeuk.com/555timer.htm
Try Google for others




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