If you are using a breadboard, I would suspect loose connection somewhere.
I suggest soldering all components onto a perfboard instead.  Pin 6 is for
the timing cap and it is sensitive to any capacitance change.  Pin 7 is the
internal bypass transistor.  If your body has any potential at all, which it
does, then you will offset the transistor's operation and change the period
of the output pulse.

Ben

> Hi guys,
>
> I am trying to use the 555 configured as an astable, with a variable Ra
> (the resistance between 7 & 8), and measure the output pulsewidth (the
> ON time) on a 16F84.
>
> The results I get pretty much match what the specs. of the 555 would
> expect, but there is a strange problem. The circuit is so unstable, if
> I just touch either pin 6, or 7, the readings go haywire. Same happens
> if I solder a long wire to place the resistance remotely as a sensor. I
> tried a bypass capacitor, a shielded wire for the remote resistance
> etc. but nothing seems to work.
>
> My power supply, incidentally, is pretty well regulated, and I dont
> think I can suspect that.
>
> Besides this problem the circuit works so well, I am reluctant to give
> it up in favour of some other scheme.
>
> Any hints and advice would be highly appreciated.
>
> Incidentally, I tried a 7555, the CMOS version as well; same results.
>
> Regards,
>
> Anand
>
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