opamps & can theremin

[Jegeva]

Hello Guys & Gals,

i'm fairly new to electronics, as a lot of people Dave enthusiasm took part in reactivating the old nostalgia i had lingering from my school days about breadboard and resistors. 

Saw this : http://www.instructables.com/id/Theremin-Using-a-Soda-Can-and-the-Digilent-Analog-/?ALLSTEPS on hack a day and said to myself let's give it a try !.

But i don't even understand the operation of the 1st building block, rewatched dave's post on basic op amp operation and can't figure out why the first block :



 should produce anything on the output, + & - inputs are at the same potential and hence the op amp, doing it's best too keep them at the same potential...have nothing to do ! Both are at ground potential already...

I'm surely missing something, can you please shed a bit of light for me on the matter ?

[Andy Watson]

Put, conceptually, 1mV on the positive input and consider what happens next.

[Paul Price]

There is, kinda, put in a 100k resistor from the + input to +5 and squeals and  whistles and moans will b urs 2 hear.

With the resistor you'll always hear a tone, without the resistor you will always get it to work with some type of squeeze and make crappy sounds, and for free, mains power line hum modulated noises, like in the video linked.

Keep things simple, simply is not the ideal case here..crap circuit is tooooooo simple.

If you use an input to a CD4001 connected to a 8-in antenna or wire or a metal plate approx the same length and take the output of that same 4001 gate and feed it into the input of VCO you will have a real theremin that you can change sounds from over a very wide range and distance from the antenna.


You can make a good VCO from an op-amp or two as well, see attached

 Hint CD4046 is a good VCO, but forget about the PLL, just the VCO section of this chip.

[LvW]

Here is another way to see what the circuit does (and how):
Immediately after both voltage supplies are switched  on (never exactly at the same time) no opamp based amplifier has reached it´s desired "starting position" at t=0. Hence, assume the opamp will start with Vout=+V(supply) or -V(supply). Then, calculate both of the input voltages - taking into account the charging process of the capacitor - and see what happens. 

[Jegeva]

There is, kinda, put in a 100k resistor from the + input to +5 and squeals and  whistles and moans will b urs 2 hear.

With the resistor you'll always hear a tone, without the resistor you will always get it to work with some type of squeeze and make crappy sounds, and for free, mains power line hum modulated noises, like in the video linked.

Completely made it work  (on the breadboard, the original setup output was keeping flat at 1.smthng V on the oscillo)

Here is another way to see what the circuit does (and how):
Immediately after both voltage supplies are switched  on (never exactly at the same time) no opamp based amplifier has reached it´s desired "starting position" at t=0. Hence, assume the opamp will start with Vout=+V(supply) or -V(supply). Then, calculate both of the input voltages - taking into account the charging process of the capacitor - and see what happens. 

i don't have time for this tonight but i'll definitely give a shot a this method tomorow

thanks guys