I wanted to be able to turn my electric blanket on for a limited amount of time without having to turn it off manually, in case I either lack the willpower to turn it off or fall asleep.   The result works beautifully.  The blanket turns itself off after a selected amount of time with a quiet click, and I have no fear of wasting electricity and overheating myself while I am asleep.

Step 1:

These are the parts I used:
Polarized extension cord (meaning one slot and one prong is bigger than the other one).  I did not use a grounded extension cord (no third prong) because there is no ground on my electric blanket.  Make sure to select an extension cord that will be able to reach from the wall outlet to where you want to place the timer switch (probably bedside) with at least a few feet extra.  I simply cut a 12 foot extension cord in half.
In-wall Intermatic EI210 electronic countdown timer with 10, 20, 30, 60 minute and "hold" settings, purchased for about $20 (I would not recommend spring-wound timers because the ticking is too loud)
Plastic receptacle box
Decora style cover plate
Cable clamp that can fit both cut ends of extension cord
Wire nuts
Homemade mounting box made out of scrap solid wood to fit the receptacle box and electric blanket controller
Great idea. I honestly looked at Instructables for some way to keep my cordless tool battery chargers from staying on too long and boiling batteries, and didn't use the right key words, I reckon, because I didn't see this. <br> <br>I &quot;accidentally&quot; came up with a very similar unit, one I call a &quot;shutdown timer&quot;, and I used a spring wound timer as that was the only kind I could find locally, but like the wisdom of using the &quot;intematic&quot; unit like yours. https://www.instructables.com/id/Shutdown-Timer-Ideal-for-cordless-tool-batteries/
This is a very useful device, congratulations.

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More by sculgin:Countdown timer or automatic shutoff switch for electric blanket or other electric device 
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