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From: Lasse Langwadt <llc@fonz.dk>
Newsgroups: sci.electronics.design
Subject: Re: Upper limit of timing resistor value in a tropical setting
Date: Sat, 21 Sep 2024 02:07:30 +0200
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On 9/20/24 17:17, john larkin wrote:
> On Fri, 20 Sep 2024 14:18:26 +0530, Pimpom <Pimpom@invalid.invalid>
> wrote:
> 
>> Take a typical domestic environment in a tropical region, rather humid
>> at times. Assembled board is not conformal coated and some dust is
>> expected to seep into the box over the years and settle on the PCB. The
>> unit is low voltage and low power with just a few watts dissipated when
>> on, and left unpowered for days on end.
>>
>> An R-C combination is used to set timing of around 1 sec. which is not
>> critical (long-term variation of up to 10% is tolerable). If it gets
>> very bad, the board can be cleaned but this should not be required as
>> regular maintenance.
>>
>> Under these circumstances, what's the highest timing resistor value
>> you'd feel comfortable with?
> 
> Without condensation, literal drops of water on the board, 1 Meg might
> be fine. Some attention to the board layout would help... extra
> clearances on the critical node, maybe two resistors in series, small
> R and giant C, maybe use an RC oscillator and divide down.
> 
> Why not use a uP? An RP2040 is 70 cents in any quantity, and has an
> oscillator on-chip. ADCs and temperature sensing too.
> 

an AVRtiny would do the job for ~30cents, and you can get mcus much 
cheaper than that

https://cpldcpu.wordpress.com/wp-content/uploads/2019/08/3-cent-mcu-1.png