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,
>=20
> I have a 555 based oscillator, AC couple and the probes are in salt=20
> water of a given concentration.
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> The probes are in water for about 1 hour from 8 PM to 9 PM and the=20
> reading is 6500 Hz. I switch off the unit.
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> The next day at 9 AM in the morning I see that the value has changed to=20
> 7000 Hz.
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> What could be causing the drift?
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> Cheers
>=20
> Ravi


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