A one chip theremin

Posted: 7/6/2009 6:07:03 PM

From: Colmar, France

Joined: 12/31/2007

So, here is the schematic (http://theremin.tfrenkel.com/opto/schema.png).

Don't care about the signal source V1, it's a simulation of your TLS chip. The point called "Audio_sig" has also no practical value, it had been designed for analysis of the audio signal before the light depending attenuator.

How does it work: C1 eliminates the DC component, so that the signal will be symmetrical to the 0V line. R1 and R2 divide the signal down to a suitable level for any keyboard or hifi amplifier. Their values have been choosen high enough to not to overload the TLS chip and low enough to minimize the influence of the following circuitry and the amplifier's input impedance. C2 does a smooth filtering so that low frequencies stay with rich overtones while higher frequencies will be less sharp. If this design sounds still too sharp for you, you may try 180n or 220n instead. You may get a similar sound as the Burns B3 theremin with that... ;-)
The diodes D1 and D2 have a double function:
a) They protect the amplifier from voltage spikes and input overload
b) They keep the output amplitude nearly constant (with this type of filtering it would normally be much higher for lower frequencies and smaller for higher frequencies.

Finally there are the CDS resistor and the sensitivity pot which attenuate the audio signal depending on the brightness (the brighter the louder). The range of the pot is big enough, so that you can use different types of CDS resistors. Just give it a try.

Amplitude constancy and the filter effect can be seen in these pictures:
100Hz (http://theremin.tfrenkel.com/opto/100hz.png)
330Hz (http://theremin.tfrenkel.com/opto/300hz.png)
1000Hz (http://theremin.tfrenkel.com/opto/1khz.png)
3300Hz (http://theremin.tfrenkel.com/opto/3khz.png)

Hope this helps!

The green line is always the TLS output and the blue line is the audio signal before being attenuated by the CDS.
Posted: 7/6/2009 6:30:21 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]" I cut the black foam that the TLS230R chip came on to block the light"[/i]

Just a word of caution - that foam is HIGHLY conductive (unlike the pink ESD plastic which only becomes really conductive at high voltage, this black foam is carbon loaded - you could easily get a few k ohms or less if it touched the IC pins)

If you get strange behaviour from your circuit, keep this in mind!
Posted: 7/7/2009 10:41:01 AM

From: Monterey Bay

Joined: 6/18/2009

Thank you Thierry and Fred!

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