Can anyone help me improve my design?

Hi,

Just breadboarded a VERY simple Theremin using a 4093. Pretty happy with it but jus wondering the best way to increase the effective operating pitch distance beyond a couple of inches.........thanks in advance!

Schematic?

Stick one or more linearizing inductors in series with the antenna (check their self resonance point to make sure they will function in this mode).  Use 3/8" tubing to make real antennas. 

I'd suggest you avoid Schmitt trigger devices.  Use buffered 4000B series inverters (NOR or NAND with the other input tied hi/lo as necessary, for some reason a decent buffered hex inverter does not exist).  Make the oscillators LC.  Do something for static protection like a neon bulb and/or littelfuse SP721.  Regulate the supply voltage(s) with inexpensive three terminal regulator(s). 

That may sound like a lot, but it really doesn't take much to move a design out of the squeaker toy category.

As long as there are (most probably) RC-oscillators around the 4093, additional coils will not help at all. Only a longer and/or thicker antenna.

Thanks for the replys guys! I'll plough through all of your tips!!!

It is a RC-oscillator by the way

Several things to note: ( I am presuming you are using the 4093 hysteresis - resistor [R1] from output to input, input to antenna / capacitor to ground)

1.) The hysteresis 'window' on the 4093 is quite small, which gives a low level triangle waveform on the antenna.. put a resistive divider on the input to increase the antenna voltage.

2.) Put a resistor and capacitor in series and connect this across the resistor R1

Make the resistive divider have 47k to antenna and 100k to ground - if circuit fails to operate make the 100k larger until it does oscillate. Make the TC of the series R + C about 1/4 of the period of the oscillator, and make R = 1/2 the value of R1.

The above mods will improve performance (range and linearity) a bit - but not much. Increasing the length of the antenna (as Thierry suggested) is, IMO, better than increasing its thickness - A long antenna will "see" your arm (at an angle) and the geometry helps to improve linearity.. (This is one of my quirky ideas - I believe that antenna geometry greatly affects performance of non-compensated "antennas") and think I have proved this in practice.. RC oscillators usually make unplayable theremins, but with a really long thin antenna (1.2m - or twice the arm length) one can actually play a couple of octaves quite easily) - Even better - (but more difficult, and it looks silly) is to angle the antenna towards the player.. 45 degrees gives great linearity!

Fred.