I'm retrofitting a toy lantern for my daughter. It has 2 AA batteries
and a cheap incandescent flashlight bulb in it, which of course eats
through batteries. I want to put a white LED in it for better
efficiency, and have a timeout function so if she leaves it on (which
she will) it shuts itself off after a few minutes.
There's many ways to do this of course. I could use a PIC to perform
the timeout, but since I need a boost converter to power the LED anyway,
I want to use the boost to get maximum operating time from the batteries
and continue operating when the battery voltage falls well below 2V,
which the boost converter will do, but he PIC won't (2V minimum for
PIC). Powering the PIC off the boost would be tricky since the PIC
would have to shut down it's own power supply!
Then I started thinking maybe I should try to simplify and use a simple
RC circuit to shut down the converter. I'm going to use a MAX1674,
which has a shutdown input (/SHDN) which shuts off the converter when
/SHDN < 20% of Vout. I figure if I put a capacitor from Vout to /SHDN,
and a resistor from /SHDN to ground, that will do it.
I haven't tried this yet because I have the MAX1674 in a SMD package and
breadboarding will take some time. So for right now I'm just trying to
come up with ideas.
Do you think this will work? Any better ideas?