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CMOS oscillator problem

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 03 February, 2011 » Computer fan as anemometer » CMOS oscillator problem « Previous Next »

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bruce
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Username: bruce

Post Number: 532
Registered: 04-2008

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Posted on Saturday, 22 January, 2011 - 07:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
This is really a follow-up to a previous thread of mine in which I was having troube clocking a counter. Thanks for all your help: it was in fact the oscillator. CMOS inverter oscillators make cheap timers, but for digital clocking you need to use Schmitt devices.
My circuit finally worked as advertised; well, almost. The time frame before the device tripped was unexpected ( too long ). The timing components were a resistor and an electrolytic capacitor. Suspecting the capacitor, I changed it to a polystyrene type ( or similar hi quality component ) It still doesnt work properly.
The resistor is 5Meg, the capacitor 1uF. The period I calculated as 10 secs ( roughly ). When I started the circuit up, I did indeed get a period of around 10 secs. However, an hour later it had changed to nearer 5 secs. This is something I've not encountered before and is making life very difficult.
Does anyone have any bright ideas ( apart from changing my hobby )

Bruce
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alec_t
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Username: alec_t

Post Number: 464
Registered: 03-2009

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Posted on Sunday, 23 January, 2011 - 12:03 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

...er, this shouldn't really be a sub-thread of the 'computer fan' thread, Bruce. I blame the software. Perhaps ARW could shift it?

If this is an actual circuit rather than a simulation, I'd suspect moisture may be shunting the 5 Meg and/or C. Could you not reduce the time constant to 0.5 sec and clock a secondary counter to get your 10 sec period?

Alec
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bruce
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Username: bruce

Post Number: 534
Registered: 04-2008

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Posted on Sunday, 23 January, 2011 - 12:12 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Alec,
Sorry about that: I've been caught out again, thinking I was in the root directory, when I wasnt.
I'll restart the thread.

B

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