Required is a timer circuit to switch on an LED with 2 time constants, when a switch is depressed as follows:
- when switch is closed, light an LED 10 ~ 20mA
- when switch is opened again, keep the LED on for approximately 5s more
- then fade LED off for approximately 10s.
- circuit voltage = 3v
- assume a standard red LED , Vf =~ 1.8v
- prefer to have maximize brightness
- small SMD low leakage caps <~22uF
- no ICs that consume continuous quiescent current; but accept minimum idle current (< 0.25uA @ 25'C)
I'd like to use a mosfet to minimize voltage drop and maximize brightness.
I think that it would involve some chain of transistors or mosfets that are held on by capacitors, but I can't figure out how to keep it on full brightness for the first 5s, as they would start to fade immediately as the voltage drains out of the capacitor.