Battery under-voltage cut-off TTL control

Hello all,

I would like to built power supply board for my arduino, there must be include battery backup supply. Thats why I decided to use simple gate of two diodes to select which supply is active ( higher of course :slight_smile:
Second task is to protect SLA battery from under-voltage.
So my idea is to use shutdown pin of my SMPS to cut-off battery when voltage drops under 10,5V lets say.
Question is how to switch on again, when main power supply is restored.
So SINGLE 2 INPUT NOR GATE should helps?

I drawn sketch of my idea how it might be controlled??!!??
Attached sketch include just control part, there are not any caps,inductor and thats basic stuff.
Main power supply lats say 12-35V.
R1 current limiting
3.3V zener diode as voltage regulator
R2 as pull down ( maybe not needed )
R3 is pull-up for LM2596S regulator ( active low )

Battery voltage will be monitored by analog input.
When main power supply cut-off, NOR gate will be still pulled low by TTL control pin.
When voltage of battery drops, you set TTL control pin low. NOR GATE will goes HIGH, so SMPS will goes OFF.
Shutdown pin is pulled up by battery voltage to keep SMPS off.
Zener regulator is there just to do not fry NOR GATE.
When main power supply restore, NOR GATE goes low and SMPS on.

Does it give a sens to anyone?:)))
I am not really an electric engineer, so any feedback is welcome. Miro

I have realized something…
When SMPS goes off, 3.3V regulator which will supply 74AHC gate will also goes off, so NOR GATE would be without power supply…awww.
My second idea with NPN transistor could also work and seems to be much more simple. What do you think?

OK idea 3 is pretty similar to idea2, just include forgotten resistor to limit base current of transistor :slight_smile:
I guess it might work?

Last version…standard NPN transistor replaced by N channel mosfet.
Please somebody write something to me :)))


nobody can help out?