Upper limit of timing resistor value in a tropical setting

I've experienced damp getting into the laminated structure of a printed circuit board and causing tracking in all directions. Under those conditions it would be better to ignore all the usual advice and use 10k or less and an electrolytic capacitor.

Remember the capacitors and ICs have exposed terminals which can track-across, so the resistor might not be the only limiting factor. In really damp circumstances, or with high voltages, I would consider using PTFE standoffs with wire-ended components - they should be good up to about a Megohm.

I made a gadget with a similar purpose back in 2008 and it's still in use in a video studio, although this one is of a completely different design.

I used an opamp as a Schmitt trigger as the timer in that earlier one with the timing set by a 100k resistor and an electrolytic cap. The current one is to be used with CMOS gates.

Yes, I suppose I could use one of the simpler ATTiny MCUs or an Arduino Nano. But the whole thing uses a common 12V supply and using an MCU will require a buck regulator and scaling of the input and output.

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.

What is the time-keeping mechanism?

bob prohaska

Maybe 1 Kohm. Others have said 10K and 100K.

But why not dip the board in hot wax, to seal the surface? One can repair the board without removing the wax, and just re-wax when done.

Back in the dead-bug era (late 1960s), we would put the vectorboard with wired and ICs in a shallow tray of molten wax, and let it cool and solidify, protecting the circuitry both physically and electrically from damp and salty dust. If repairs or upgrades are needed, melt wax, pull board out, make changes, return to wax tray.

Joe Gwinn

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

CD4060 ? (combine oscillator with divider) this is what the automatic room perfumers use SFAICT