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5v battery backup? Answered


I have a 555 timer circuit running in monostable mode which I need a battery backup for.

Anyone have a simple, reliable circuit that can achieve the following results :

1. Switch power between the 5v cell phone charger and the battery in the event of a power outage. The "switch" needs to be seemless otherwise it will trigger the monostable as it is set to trigger or re-trigger whenever the power goes off and on again.

2. Charge the battery when the power is on.

3. Stop charging the battery when it is fully charged.

Any other suggestions will be much appreciated. I am not rigid on the design and use of parts, although....the cheaper the better without compromising on reliability (of course).

This is for a security system that drives two lasers, a camera and a cell phone. Not sure if the cell phone battery can be used to power the "backup system" as well as run the cell phone.

Thanks in advance.

GK ;)


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2 years ago

Get a CMOS 555 add a diode, a battery and solar cell..

Then run on daily lighting to charge the battery whilst the 555 runs as my 8 year old never wind watch which is in fact a solar cell running stepper motors while charging a uP !

WatchClock.jpgSolar .jPG

Answer 2 years ago

thanks. what about overcharge and how would one use the cell phone battery in such a setup. could you please provide a diagram.


Answer 2 years ago

Well I would go for simple..

How much does your load need during the dark hours call it (neg_Ah) = current x hours..

Then find out what is your battery efficiency.

Lets say your Batteries need 1.4 more amp hours to recharge for every hour used..

Nothing is 100% efficient ! (need_Ah) = (neg_Ah) x 1.4

Then select a solar cell array that delivers (need_Ah) during the charging hours of your average day.

The diode should be a low forward drop schottky and put your battery in place of the 9v.

There you are..


If you want to be engineer, then you need a higher voltage and current solar array that can run electronics that powers a current and battery voltage regulator to charge a small cell battery and does so when clouds interrupt the light.. That takes a smart inverter and may need a battery of its own for memory.. See life is getting complex when you want a diagram..

The advantage of a small solar cell array is that usually it cannot overpower the battery

That is why I suggested a low power CMOS 555..