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Simple theremin 555 timer

Appie.123

Jul 7, 2015
24
Joined
Jul 7, 2015
Messages
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Hi,

Lately I wanted to built a theremin, but since I'm quite an electronics noob I wanted to keep it very basic and use a 555 timer.
On the internet I read some articles in which LDR's were used, to influence the frequency of the 555 timer and therefor the sound produced by the speaker connected to the output.
F2AIUVTHDYZ6W2N.MEDIUM.jpg



I found this somewhat cheating, and wanted stay a bit closer to the original theremin, with which the position of your hand changes it's capacity finally resulting in a difference in tone. Instead of using LDR's I chose to use a normal resistor and replace the capacitor with a metal plate and my hand, serving as variable capacitor.

I tried a huge iron nail and later a small plate made of several sheets of aluminum. My hand would serve as the other part of the capacitor, also being connected to me, and therefor the ground.
This is the circuit used:


Schermafbeelding 2016-03-20 om 4.51.27 PM.png

The capacitor is not connected to the 0V of the battery because the idea was that via your hand would also be the earth, this however did not work, even when putting out my shoes. Therefor I had to hold the 0V with my other hand.
Also strangely enough only a sound was created if I touched the sheet; applying more pressure did lower the frequentie, still I had to touch it.
I tried tuning it by using a variable resistor for R2 but no results; I still had to touch it.

The weird thing is that this circuit should only create an oscillating signal if a capacitor is present, but by touching both the ground and the sheet your body simply becomes part of the circuit as a resistor, and no other capacitors are present.

Can anyone explain why:
- I have to touch the 0V and my body does not ground itself
- Touching is needed to make this circuit work.

Thanks for your help.
 

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hevans1944

Hop - AC8NS
Jun 21, 2012
4,878
Joined
Jun 21, 2012
Messages
4,878
You are misguided on how a Theremin works.

The two "antennas" of a theremin are part of two radio-frequency tuned circuits connected to the "tone" antenna and the "volume" antenna. Moving your hands close to these antennas changes the frequency of the RF oscillations by varying the capacitance between the antenna and ground. The antennas do not intentionally radiate RF.

A third, fixed-frequency, oscillator is mixed (not added!) with the RF in the "tone" antenna circuit to create sum and difference frequencies. The sum frequency, also an RF signal, is discarded. The difference frequency is initially adjusted for "zero beat" when your hand is remote from the "tone" antenna, but as your hand is brought near the "tone" antenna it increases the capacitance between this antenna and ground, changing its oscillator frequency. The change in frequency produces an audible "beat frequency" tone, starting at a low tone and progressing to higher tones as your hand approaches the "tone" antenna. This mechanism is called heterodyning and is essential to the operation of a theremin.

A pretty good explanation can be found in this Wikipedia article.

A 555 timer operating at audio frequencies uses a timing capacitance that is much too large to be significantly changed by bringing your hand near a metal plate. That is why heterodyning is used in theremins: small changes in capacitance will significantly affect the tuned circuit resonance of a low-frequency RF oscillator, resulting in an audible beat frequency between two oscillators tuned to nearly the same radio frequency.

So-called "simple" theremins have been constructed without RF oscillators, depending on photocells to control the frequency and amplitude of audio oscillators. These don't even come close to the playing experience of a real theremin. Here is a Google page with lots of links to electronic circuits for constructing theremins.
 
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