What is the frequency of your astable ? Would it be near line frequency by any chance 50 or 60 Hz ?
Gaston

Anand Dhuru wrote:

> 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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