Simple Theremin

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Dr.EM

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Hi. I found this rather untidy schematic for a simple pitch only theremin. I am interested in building it, because its a pretty groovy instrument :lol: , but most the ones i've seen are very much more complex.

Anyhow, on the schematic, it specifies 3x BF195 transistors, and underneath says BC182, is the first one specified in a foriegn coding system, because I can only find the BC182. Also, it says 3 but there are clearly 4 in this schematic, are they all the same?

I would order the parts I don't have (ie. the BC182s, inductors and small caps) next time I send an order. But, I'm pretty sure this being a radio frequency circuit, it cannot be tested on breadboard, is this right? And for that matter, will be difficult to do on stripboard.
 
OMG, how did I forget that :lol:
 

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The BF195 is a standard pro-electron coded European transistor, the 'B' meaning silicon, and the 'F' meaning low power RF transistor - it was very common in british radio's back in the 70's.

However, the BF195 uses a rather inconvenient case type, with very short connections that 'lock' in to the PCB.

But this application is very simple, two RF oscillators, a mixer, and an audio amplifier - you could use pretty well any small signal silicon NPN - personally I'd probably use the BC107 or BC108.
 
Hmm, I have BC109Cs, but i'll need the capacitors and inductors anyway. You reckon theres any chance of this working on breadboard? It being simpler, does that mean it must be worse in some way?
 
Dr.EM said:
Hmm, I have BC109Cs, but i'll need the capacitors and inductors anyway. You reckon theres any chance of this working on breadboard? It being simpler, does that mean it must be worse in some way?
Click to expand...

109's should be fine, there's no need for their low noise characteristic, nor their higher gain - but it won't do any harm.

I wouldn't advise building ANY RF device on breadboard!, and if using veroboard it's best to remove unused tracks and pieces of tracks.

A theremin is simply two RF oscillators beating against each other, one of which you can vary by moving your hands (or anything else!) near it. It should certainly give you an idea of using one.

Quick thought! - EPE Nov/Dec 1996 had a design for a 'Professional Theremin', it could be worth trying for back issues? - but it might be too old now?.

However, EPE Oct 2004 had a theremin as well, dual channel for volume and pitch - and included a veroboard layout!. This would certainly be an issue that was available online as a PDF, and there have been various links posted where you can download them at times!.
 
Ok, thanks! Yeah, I hear it works by the capacitance of an object near the ariel affecting its oscillating frequency. Some people can play these really well, must be incredibly hard as its non linear response and there is no indication of pitch anywhere.
 
Here's a circuit you may find interesting.
 

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Very interesting

I have some hex invertors, but I imagine you need that very particular number for that application, probably because the high frequencies involved. Is that still the same RF operating principle? I imagine the main difference is in the output waveform, more square than sine I would think.
 
I dare say it will sound a bit different to the "classical" theramin.
On the plus side it does use elements that are readily found, the inverters are drawn merely for simplicity as they could be actually made from other types of gates with an inverted output , as for the 4069 specified in the circuit there is nothing special about it. The oscillations are well inside the upper limit for off the shelf logic chips and audio amplifiers, the oscillator stages could even be replaced with a pair of 555 timer's , a nice touch as it would allow you to experiment with changing the mark-space ratio of the square wave thus altering the sound characteristics.
 
Well, that does look like a better one to try, cheers.

Still, i'm not totally sure how this works. Does it use radio frequency signals? I don't imagine it can because they tend to be 100Mhz+, and CMOS won't work at that frequency. If it isn't, how does the plate affect the frequency? Still to do with capacitance?
 

It uses two RF oscillators, exactly the same as the other one did - as far as I'm aware theremins don't use VHF?, they use much lower frequencies, probably in the low single MHz region.

Using VHF would produce much too great a frequency change, and there's no point producing notes higher than human hearing :lol:

As you suspect, the capacitance of your hand near it alters the tuning of one of the RF oscillators.
 
Ah, I can see where you are getting stuck...


Radio waves or more precisely electromagnitic waves can be of any wavelength / freaquency , submarines and cave explorers routinely use
radio transmitters that operate lower than 5khz.
 
Ah, right. So this can probably be built on stripboard with some care? I unfortunately can't make PCB.
 
this is the revised pitch with corrections. sorry for the previous double post but hav an unstable connection
 

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Thats neat, but might be a little complex for me. I see the advantage of this is that it has better sounds, and a VCA. But, I was thinking, if I got that "mimimum theremin" working, I could surely connect the output to a frequency to voltage chip and use that output voltage on the CV input of my synth, and hey presto, a synth played by a theremin! Would make an interesting control system I reckon :lol: . Can't see why that wouldn't work with a little tuning.
 
well u have two distict sections there the pitch and volume so u can mess with them as u please and maybe make some interesting combinations. i mean think of feeding a radio or other device into the volume section.
i saw another all transistor project that said to use u guitar volume pedal but i think that was a simple get away and no longer a decent thing. this project i read is more stable especially as ther is no LC circuit that can get tricky. note that the two oscilators of each section r made with seperate chips although the four gates r enough for both i suppose this is to minimise interference. this only shown on one diagram (the revised pitch one) but it is probably advisable for both.
i will build it as soon as i can lay hands on parts (and cash hehe)
 
Hi Dr. EM,
The oscillators in the Minimum Theramin operate at only about 200kHz so just about any CD4069 inverters IC will work.
Stripboard will be fine if you use short length tracks and cut unused lengths.
I haven't heard of that low-dropout 5V regulator but there are many available but needing a 100uF output filter capacitor. A 78L05 will work only when the 9V battery is new.
The lowpass filters gradually reduce audio frequencies above about 300Hz, so bass tones will be buzzy and treble subdued and flute-like. :lol:
 
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