[sdiy] LM358 datasheet VCO on spilt rails

[James Patchell]

As I recall (this was 40 years ago...well, 39), I just used a voltage 
divider and a bypass cap.

The control range was 0 volts to about +18 (just a hair less as I 
recall).  If you look at the frequency control part of the circuit, one 
leg is basically a voltage divider that goes to the + input of the 
op-amp.  The common mode input range of the LM358 is 0 to VCC-1.5 
volts.  So, you can run the control voltage up to the point where the + 
input is at that voltage.

And yes, it is a very cheap VCO.  At the time I used this circuit 
myself, I had no way to check to see how accurate it was.

http://www.noniandjim.com/Jim/FirstSynth/FirstSynth.html

On this web page, I have a picture of this thing as I built it.

-Jim

On 2/6/2012 2:19 PM, Matthew Smith wrote:
> Just out of curiosity, is the V+/2 shown on the schematic simply the 
> supply voltage split with a potential divider, or does it require 
> buffering?
>
> Jim - what was your actual control range - 0V to +18V, or -9V to +18V? 
> (Assuming the first, wanted to be clear.)
>
> Given me an idea. Excluding PCB cost, a VCO for about $0.25. 4 for a 
> dollar. Hmm.
>