ok,
let me chime in here. I had some experience building vco circuits back when I could see to do so.

the simplest (assuming you can still find them) are the XR-2206 function generator IC (it has a vco built in) and a standard garden variety op amp. use the op-amp on the non-inverting input and use a negative feedback system (run a line through a resistance from output to inverting input). Once the gain level is set, the vco will have sufficient voltage to work with. Properly setup, this has a wide number of applications and can be run on a single 9 volt battery. check with your sighted tech friends and see if they can put this puppy together.

DE n7zzt Eric

On Nov 25, 2015, at 8:40 PM, Tom Fowle wrote:

> richard,
> How you adapt the directional system depends entirely upon what kind of
> signal is available from it.
> 
> Let's assume the unit to be modified has a regular old fashioned meter
> movement  which indicates how well the antenna is aimed at the fox.  If the
> meter simply reads "higher" when more closely pointed, it's pretty easy.
> 
>  In that case you need a VCO which can look at the voltage across
> the meter,  usually 50 to 100 Milivolts
> 
> A slightly trickier system uses a so-called "zero center" meter which shows
> whether you're aimed left or right of the fox. These meters actually measure
> current in either direction  This would need a bit of fiddling before being
> applied to a VCO.
> 
> 
> There are gobs of such vco circuits available from transistorized ones
> designed in the 1950s to specialized chips and microcontrollers. 
> 
> If all you need is a "relative" indication, not the ability to get an exact
> measurement, I'd suggest the transistorized circuit for its simplicity.
> 
> You and your friend can find many such circuits in plain text format in:
> The Smith-Kettlewell Technical File
> https://legacy.ski.org/rehab/sktf/index.html
> 
> Search the magazine for "audio gimmick" circuits or "meter readers" and
> you'll find more than you'll ever want.
> 
> the only issue for your presumably sighted friend is that the schematics are
> text discriptions not schematic drawings. Sometimes it's fun to have the
> sightlings be at a small disadvantage <GRIN>
> 
> If I can help with more specific choice of circuits or related stuff, please
> email me either on list or directly.
> Tom Fowle WA6IVG
> 
> On Wed, Nov 25, 2015 at 09:36:41AM -0800, Richard B. McDonald wrote:
>> Hi!
>> 
>> 
>> 
>> In a few months, my club is going to do some fox hunting activities.  My
>> question here relates not so much to making the "antenna" itself per say,
>> but rather to making an audible tone direction indicator.  I think this
>> thing is called a "V C O?"  Basically, the V C O causes the pitch of a tone
>> to change as the direction finding antenna is pointed towards the fox.
>> 
>> 
>> 
>> A very techy friend of mine in the club is going to build this contraption
>> for me, including the antenna itself.  He has some ideas.  However, I wonder
>> what others here on our list have done?
>> 
>> 
>> 
>> 73,
>> 
>> Richard KK6MRH