I wear a dental appliance at night that I have to clean every morning. It is still a little wet after cleaning.
Its case does not ventilate enough to hygienically dispatch the water remaining from the cleaning. I am designing a ventilated case that has a built in fan that will run on rechargeable 3 AAA batteries (because I have a lot of those). I would like the fan to run between 30-min. to an hour after I put the appliance in the case. The fan needs to be very low power to conserve battery life. It does not need to move air fast. I’m looking for recommendations on a timer and an efficient fan for the case. For the timer, I was thinking something like this.
Have you tried just using a can of compressed air(keyboard cleaner) to give the appliance a couple of quick blasts to removed most liquids then place in a container having small ventilation holes on bottom and top? The bottom holes would require a few mm gap above the surface placed onto.
Great idea, but not nearly as fun. I like doing these little projects as a hobby (electronics and 3d printing). This does make me consider possible printing a CO2 powered gun I could blast it with. I wonder how long the cartridge would last…
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Weird that buying that is probably cheaper than buying individual parts using a simple RC circuit to gate a MOSFET…
Even weirder that it might be cheaper to use a small embedded processor (an attiny almost stands on its own) to count down to switch a MOSFET. 
So it kind of depends which rabbit hole would be fun to go down!
I didn’t even think about something that. I haven’t done a ton of coding, but I’ve done enough that this would be within my wheelhouse. I wonder what the power draw is on something like an attiny while its dormant.
Note: Even an LED will drain the battery faster than the ATtiny85 (in sleep mode) - its capable of extremely low sleep current - less than 0.5uA
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You can see that if the processor only sleeps then current draw is extremely low 414nA (but its not doing anything!). It is obviously the best mode for saving battery power and getting the best ATtiny ultra low power operation.
Lots of information in there for reducing current requirements. 
That’s incredibly low. And the ATtiny would save a lot of room compared to the timer I was thinking about.
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To switch the fan, you’ll need a MOSFET, a diode to protect it, and possibly a pull-down resistor (though maybe the built-in pull-down in the attiny would be enough). For the MOSFET, probably a cheap small-signal MOSFET like a BS170 (0.5A) would be fine. You’ll just want a diode across it so that it doesn’t kill the MOSFET when you turn it off.
I did an attiny85 project which drove an RGB LED for 2 hours to have my daughter a “rainbow” light to go tosleep by and then I put it to sleep and was run off one 3.7V 18650 battery. But it would only last about a month and I couldn’t figure out why and then it dawned on me. The LED on the power line. Once I disconnected the red power LED it would last a good 5 months in sleep mod after running for 2 hours each night.
simplify things by using a tiny 5V fan so the whole thing is powered by a USB 5V either from mains power or from a battery.
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Any recommendations for a low power fan?
I have used these small fans before
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Or something like this which can handle 5V and has 50% less power usage( 0.10A vs 0.20A ).
You can search for the fan size you’re interested in and look at the power requirements as most all of the specify the input voltage(or range) and current draw( some you can see on the label ).