4069 theremin

One important point I failed to make about pre-mixer waveshaping is this:

Any filter inserted pre-mixer is not (or only extremely marginally) subject to the audio frequency - If one wanted to do similar filtering on the audio side, one would need to insert a tracking filter (VCF or the like).. With pre-mixer filtering, one effectively applies the filter to the HF waveforms which only vary, proportionally, by an extremely tiny amount. One can actually filter individual harmonics with bandpass filters, and mix these to taste, making it easy to do additive synthesis.

This has been the basis for a lot of my R+D, and the reason I have built a voltage controlled heterodyning oscillator - There is no easier way to implement additive synthesis!

Generate two HF signals (ref + variable) with high content of both odd and even harmonics (ramp or pulse), and feed these signals into a set of LC  circuits tuned to the harmonic intervals - one then heterodynes each (in fact, I have a simple way to DC control the level of each HF signal, so combine them and heterodyne them all together) of these, giving a set of audio sine waves which you can mix to taste!

Fred.

"but I looked around for voltage comparators and couldn't find anything that would function the same in a DIP package."

Which brings us right back to the TS555C perhaps? ;-)

If you just tie tr + th inputs together, you have a great schmidt inverter! Q will go high when input drops below 1/3 Vdd, and stays there until input goes above 2/3 Vdd (assuming CV = 2/3 Vdd)... No need to use it as a timer! ;-)

"Which brings us right back to the TS555C perhaps? ;-)" -FredM

Good try!  But no, I just want a single VCC/2 threshold, inversion, and an output with a bit of drive to it.  I guess I prefer oscillators that use the 180 degree phase shift through the LC as their primary functional mode.

Fred I just love the 555. This is how I reverse the direction on my volume control, just reverse the operation of the trigger with a switch. It eliminates any noise on the signal for clean PWM.

Just thought I would butt in,

Christopher

"It eliminates any noise on the signal for clean PWM" - Christopher.

It certainly is good for that!

"I just want a single VCC/2 threshold, inversion, and an output with a bit of drive to it." - Dewster

Yeah - it is quite surprising that no-one (that I know of) has made a 4-pin part which is a really fast and accurate VCC/2 inverting comparator with R-R I/O and beefy output.. I think such a part might be nearly as popular as the 555!

Fred

"Yeah - it is quite surprising that no-one (that I know of) has made a 4-pin part which is a really fast and accurate VCC/2 inverting comparator with R-R I/O and beefy output."  -FredM

Well, there are the LVC1GU04, AHC1GU04, and AUC1GU04 (nice paper) that are designed for crystal oscillators (and to buffer the output).

Hi all the circuit below is actually a metal detector but has potential to be a theremin with some tweaking...it seems to operate at about 250kHz and really would be the simplest theremin ever...and something to play around with as it is such a simple circuit...I think I would put two coils in it and have less capacitance and larger coils which I would tune with a magnet proximity mechanical thingy...But I might also keep it as a metal detector and play it with rings on my fingers this would be fun to see how it could work ie with larger coils I mean one that is a large ring coil as per a metal detector..

Metal detector? Where is the hoop or sensor to do the detecting, maybe L1. With the 7805 having a 220 uf capacitor on pin 3 the output gives me the creeps about the designers understanding of proper circuit applications. IMHO

L1 is 22cm diam.14 turns AWG26 wire....The circuit above would have to be pushed in the direction of a theremin as it stands it's a metal detector but it's on my list, when I get time, of circuits to play with even just to see how crappy it is.I'm intrigued with the idea of an electromagnetic style theremin..where you have a loop coil as the antenna and you have a metal on or in your hand and you approach this loop (with beringed fingers perhaps)to change the tone...The loop doesn't have to be circular either..it can be a long oval or rectangular shape...actually changing the shape of the loop changes the inductance...and so I could imagine a musical instrument where the shape of the loop coil was altered to alter the pitch of the tone...you'd be squeezing the coil in your hand...or having some mechanical device to do it like a slide on a trombone but that wouldn't be a theremin but a therebone.

"I'm intrigued with the idea of an electromagnetic style theremin..where you have a loop coil as the antenna and you have a metal on or in your hand and you approach this loop (with beringed fingers perhaps)to change the tone." - IJ

Yeah, it is fun to play with circuits and attempt to adapt them - can be a great way to learn..

But if it is a theremin one wants, the metal detector route is, IMO, a waste of time.

The thing which makes the theremin great is the capacitive sensing - there is no need for any 'actuators' (metal objects held in the hand, rings on fingers, leds, reflective strips - whatever), and one, by subtle changes to the hand (finger movements etc) gets subtle changes in pitch.

Every other method imposes limitations or reduces the options for the player. An optical sensor, for example, senses only a single 'point' or 'distance' - fingering is not recognised in the same way as it is with capacitive sensing.. and similar would be true for any metal detection scheme.. even if one had a ring on every finger, these would still be a limited set of points by which the pitch could be 'computed'.

Many, many years ago I played with metal detector schemes (long before  I discovered the theremin - probably about 30+ years ago) - I did something like this once - a "trombone" ? - cant remember how it performed though! ;-)

Oh yes - Christophers comment about the capacitor on the regulator is spot on - its the kind of thing one learns to look for.. Seeing a bodge loke this doesnt always mean the rest of the circuit is crap (people have areas of weakness in their designs, and PSU design is often one of these) - but it does blink a red warning LED!  (however, the circuit still doesnt look as nearly as crap as the Glasgow 'didital' theremin, LOL! ;-)

Fred.

Edit -> Dont trust this circuit!

C4 connected direct to ground from output of gate shows that the 'designer' doesnt even understand loading or rc filters! - as in, the design is crap!

IMO, you have found one above average circuit - this being the 4069 theremin. If you want a metal detector, I would use that circuit, and simply replace the transformer with a loop coil and seperate parrallell capacitor (no antenna).. you need the coil to have an inductance of 680uH if you use a 180pF capacitor - but as you can change the value of the capacitor (its not fixed inside the IFT) you can change the coil to taste and add whatever capacitance is required. You could even play with combining capacitive and inductive sensing perhaps..