A while ago, I modified my daughter’s little kid kitchen. In that project, I found my design self teetering on that edge between two goals: get-it-done on the one side, do-it-right on the other. I had chosen the former and even my 2 year old (at the time) could see that this was a critical design mistake.
As a quick refresher, I had thought Those Thoughts 1. “Wouldn’t it be cool if her cute little IKEA oven had a light that turned on when the cute little door was opened?” Well, yes, but perhaps it would have also been cool if I’d considered the very likely fault condition wherein my toddler would leave the door open. Toddlers do that.
And you guessed what happens next: batteries died.
A very wise engineer once told me, “Rob, there’s never enough time to do it right the first time. But there’s plenty of time to fix it later.”
Well, here I am, fixing it finally.
I knew that a timer was in order. So I blew the dust of my Forrest M. Mims III Radio Shack booklets, particularly the indispensible 555 timer book.
When I was done, this is what I cooked up:
So to explain the “truth table“, the operation works like this:
- user (in this case, the kiddo) opens the oven door
- added lights inside oven turn on and remain on for about 10 seconds
- if user does not close door within this time, the lights go off to conserve battery life
- closing the door within the time frame resets the timer and turns the lights off
One other critical circuit requirement that the above Mims diagram does not account for was with the trigger input. His classical circuit only triggers on level changes, active low. In other words, for as long as the input is held low, the timer would be in standby.
That wouldn’t work for me, since I wanted the timer to start when the door opened. That’s an edge-sensitive requirement. I found a nice, easy capacitive switch solution:
The results, finally, were as they should have been: nice well-lit oven that doesn’t wear out the toy batteries. My engineer buddy would agree.