> Timing capacitor is tantalum. Pin 5 is open.
>=20
> I will change the cap from 0.47 uF to 2.2 uF as in the original circuit=20
> and try it out.

Tantalum is not a good choice for a timing capacitor. The dielectric
absorption is very high as well as leakage current.

Any kind of film capacitor is going to be an order of magnitude better.

Also, .47 or 2.2 uF are both too much for 7000Hz. It pushes the resistor
values way too low, making the current through the 555 significant
enough to shift things around.

Best regards, Bob



> > Temperature difference between morning and evening. What sort of timing
> > capacitor are you using? That's the low-hanging fruit. Lots of things
> > influence the frequency in a 555 circuit. Power supply voltage, of
> > course. Next in line is if the discharge current is high it'll be
> > somewhat temperature sensitive. Also if you have changed the divider
> > voltage(pin 5 I think) from its normal 2/3 Vcc it becomes voltage
> > sensitive. That all assumes the standard oscillator circuit.
> >
> > But the capacitor is the first place to look.
> >
> > Friendly regards, Bob
> >
> > On Tue, Sep 9, 2014, at 08:14 PM, Ravi wrote:
> >> Hi Guys,
> >>
> >> I have a 555 based oscillator, AC couple and the probes are in salt
> >> water of a given concentration.
> >>
> >> The probes are in water for about 1 hour from 8 PM to 9 PM and the
> >> reading is 6500 Hz. I switch off the unit.
> >>
> >> The next day at 9 AM in the morning I see that the value has changed t=
o
> >> 7000 Hz.
> >>
> >> What could be causing the drift?
> >>
> >> Cheers
> >>
> >> Ravi

--=20
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                          love email again

--=20
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