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Which Circuit Is Best To Use? Answered

I am trying to configure a LED circuit that allows me to charge 1.2v batteries using a solar panel. But i also want the solar panel to be used as a light sensor(detector) so during the day it charges..then at night the battery kicks in and lights up my LED strips. This is my first upgraded version using a schematic software.. Please help.

Should i add 2 indicator LED's to let me know when the battery is being charged and when the battery is in use?

I attached two images..one containing a LM317T and the other without. Just need some advice! Thanks



Best Answer 3 years ago

First one assuming the red led fwd voltage even lets current flow. You may have to reduce the led count.


Ok what if im using a LED Strip of lights...the strip contains about 60-80 LEDS?

The strip uses much more current so you need a higher current transistor as the N channel MOSFET.

BUT there is no way you can expect a small solar array to charge an 80 AH battery needed to run the LED strip :-(


My battery will be 1800 mAh 10 1.2v AA batteries. Sorry i didn't Mention this, and the solar array will be 1.25 - 1.5 watts

78 led / 3 => 26 groups x 19ma => 500ma @ 12V => 6 watts

1800 mah/ 500ma => 3.6 hours

Your solar cell needs to be bigger and stronger.

Your battery will run out in 3 hours.

I sent you a message in your inbox. What i wanted to know was your schematic does this have the two most important features i need which is:

1. Using Solar Cell AS Light Detector (Photo Resistor)

2. Battery charge protection (so battery doesn't over charge or drains?)

As it gets dark base current begins to flow through the solar cell turned into a resistor turning on Q1 which raise the gate voltage about 10 volts which turns Q2 on to deliver heavy current to the LED strip...


Designed it in my software and here is your design on left...question about the battery being over drained.

Screen Shot 2015-03-21 at 5.57.32 PM.png

Your circuit does not have a resistor to guarantee the mosfet gate is truly at zero and non-conducting discharged when going from dark to sunlight.

ok understood ...if i wanted to add a feature of red and green led to indicate when battery is being charged (red & green) via solar cell and the other led (green) when the battery is now being used after the sun goes down?

Also for the mosfet should i add a heat sink also because this circuit will be running 24/7.

ok awesome thanks...this is the final design with pcb..top and bottom layer. Let me know what you think.

Screen Shot 2015-03-22 at 1.36.15 AM.png

Could you mean over charged ??

over drained means over dis-charged..

For over charging add resistance in series with charge path.

I worry more about not turning the mosfet fully on.

Your LEDS are probably ~12W. you need more than 12 hours of light for each hour of operation.


:-( Looks like i need to redesign...What i am trying to create is a back light sign that uses solar/battery which gets charge during the day and after dark the led strips turn on. But i have a small space to work with for the solar array.

There are some changes you could make, by not using off the shelf LED strip, you can eliminate the dropper resistors and make a bunch of LEDs more efficient.

Ok i will use my original solar panel instead...its a 5 Watts Solar Panel

For less than 5 bucks you can get a solar garden light that has everything you need inside.

Your images show 12V batteries but your text says 1.2V - which one is it??

For 12V you want to use a proper solar charger with automatic shut off and overcharging protection.

Only a few bucks on Fleabuy.