well, there is always the NE555 timer IC. it will work just as well.

DE N7zzt Eric

On Nov 25, 2015, at 9:41 PM, Tom Fowle wrote:

> Eric,
> I'm pretty sure that the XR2206, like many of the neat XR chips, is no
> longer available, but yes that is a good option if available.
> Tom Fowle WA6IVG
> 
> On Wed, Nov 25, 2015 at 09:24:20PM -0700, Eric Oyen wrote:
>> 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.
>>> =20
>>> 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.
>>> =20
>>> In that case you need a VCO which can look at the voltage across
>>> the meter,  usually 50 to 100 Milivolts
>>> =20
>>> 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.
>>> =20
>>> =20
>>> There are gobs of such vco circuits available from transistorized ones
>>> designed in the 1950s to specialized chips and microcontrollers.=20
>>> =20
>>> 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.
>>> =20
>>> 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
>>> =20
>>> Search the magazine for "audio gimmick" circuits or "meter readers" =
>> and
>>> you'll find more than you'll ever want.
>>> =20
>>> 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>
>>> =20
>>> If I can help with more specific choice of circuits or related stuff, =
>> please
>>> email me either on list or directly.
>>> Tom Fowle WA6IVG
>>> =20
>>> On Wed, Nov 25, 2015 at 09:36:41AM -0800, Richard B. McDonald wrote:
>>>> Hi!
>>>> =20
>>>> =20
>>>> =20
>>>> 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.
>>>> =20
>>>> =20
>>>> =20
>>>> 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?
>>>> =20
>>>> =20
>>>> =20
>>>> 73,
>>>> =20
>>>> Richard KK6MRH