Can you use the softstart circuit as the main power switch as MrAl suggested? You could use another P-MOSFET to short across C2. Grounding its gate will turn the power off (could be a 1 meghom resistor). Connecting its gate to V+ (by the pushbutton) will allow C2 to charge and slowly turn on M1. The 1 megohm at M1's gate will be a negligible load on the battery (.02AH/year when the power is off).
This is one possibility and I need to think about it. In that case the step-up converter would be always on, and I measured it taking 0.25 mA without load. That makes about 6 mAh per day, which is probably still less than the self discharge rate of NiMH cells.
Looking at your 3 page schematic:
You want to charge C3 from 0 to 5 volts with the energy stored in C2 and (C1 through Q1).
>Try making C2 smaller. Maybe make C2 larger.
I see that some voltage from C3 runs back through Q4 and charges C11. That kills you RC delay.
>You have a computer. Drive Q4 by the computer. Make the delay in software.
I moved C1 to be before Q1 so it would be already charged when switching the power, otherwise it would also take current when charging and making voltage drop worse.
Yes, that is the problem of that soft start as I mentioned earlier.
About driving Q4 by a microcontroller, the AVR is placed after soft start so it needs power first before it can do anything.
But as you said that, I got an idea to put another smaller AVR between the converter and Q4 to drive it up by fast PWM (with small capacitor). The power on button would be then routed to that smaller AVR to make it wake up from sleep and drive Q4 slowly up, and then the same button could be used as an Off button to shut the thing off. Then there may be no need to discharge caps after shutdown. Also Q1 and those other things near it could be removed so the step-up converter will be always on, but the power consumption shouldn't be too bad when powered off. No more problems with pushbutton bouncing either, as the AVR will do its job after getting one signal.
Hi again,
What is R12 doing there, or should i say, what caused the inclusion of R12?
It looks like R12 will hold the MOSFET on longer.
Pretty neat project.
Q4 is Vgs 1.8V rated mosfet, so there's no need to pull the gate all the way from 5V to GND. I added R12 so the gate will be pulled down to about 2V instead of 0V, so at shutdown it would be faster to discharge C11. I was in a hurry when thinking that and I'm not sure if it was so good idea or not. With a normal logic-level mosfet R12 should not be there.
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Thank you folks for your thoughts, I have now lots of thinking to do. When starting this project at February I surely didn't think the power supply would be the toughest area. My earlier projects are powered by wall warts, and it's just so easy to use good old 78xx regulators when you don't have to worry much about power consumption. Before this I built few digital clocks, and there's also some pictures of them at here:
**broken link removed**