LED, Solar Charging, Light Sensing Assembly – Help needed

Hi all,

I am new to this site and hope i have posted in the correct area.

I’m in the process of attempting to put a prototype together. I need to have some LED’s to turn on at night (using a light sensor), powered by 2 AA rechargeable batteries and charged via a small solar panel. I have selected a panel which suits the physical requirements of the project (but i don’t know if it would be adequate) and i have also selected some LED’s. In terms of performance i am hoping that the system will charge for 12 hours during daylight and 12 hours running from the batteries (at night). I need help with the circuit diagram, light sensor selection, power clauclations life cycle estimates. I need to keep the costs down as i may go into production if this part of the project works.

By day i’m a mechanical engineer, my electrical engineering training was a long time ago and it isn’t my strong field, so please help and keep it simple for me.

Thank you.

The solar panel is here:- https://www.selectsolar.co.uk/prod/538/powerfilm-mp337-50ma-3v-mini-solar-panel

The LED type is here i am looking to use 7 of them:- **broken link removed**

I think the solar panel you have mentioned is inadequate. These solar panels are used for 3v button cells. But the LED you mentioned requires atleast 5v source (3.6v forward) with current capacity of 30ma x 7 = 210ma (7 LED's). The panel's current rating is very less-- 50ma. So it will require too much time to charge a battery of 3Ah so that it can run for 12 hrs. Even if you connect 2 panels in series, only voltage will be added up. Current will be same.

For your project, I think either you connect 4 AA batteries in series.
Battery's capacity should be 3Ah | 30ma x 7led x 12hr = 2520mAh => 3000mAh considering efficiency approximately 75%.
Solar panel could be => (2 in series) x (6 sets in parallel) i.e. (6v 50ma) x 6 = 6v 300ma. Or a single 6v 300mA or equivalent.

try to connect more panels in parallel to get the current ratings up that can solve the issue

and also it is better to use a little higher rating battery in the ckt though your battery is sufficent

it is better t increase the rating as the period of usage can be prolonged

if this battery is used there is a need for frequent recharging which becomes a concern with solar panels

Genovator said:

I think the solar panel you have mentioned is inadequate. These solar panels are used for 3v button cells. But the LED you mentioned requires atleast 5v source (3.6v forward) with current capacity of 30ma x 7 = 210ma (7 LED's). The panel's current rating is very less-- 50ma. So it will require too much time to charge a battery of 3Ah so that it can run for 12 hrs. Even if you connect 2 panels in series, only voltage will be added up. Current will be same.

For your project, I think either you connect 4 AA batteries in series.
Battery's capacity should be 3Ah | 30ma x 7led x 12hr = 2520mAh => 3000mAh considering efficiency approximately 75%.
Solar panel could be => (2 in series) x (6 sets in parallel) i.e. (6v 50ma) x 6 = 6v 300ma. Or a single 6v 300mA or equivalent.

Click to expand...

Thank you for your help.

I have to compromise somewhere as to maintain a design that doesn’t look silly.
I can easily accommodate 4x AA batteries in series. And propose Duracell Supreme Rechargeable 2450mAh AA (or equivalent manufacturer).
I have re-evaluated the LED specification. I am proposing of using four LED an example below:-

https://www.watercoolinguk.co.uk/p/3mm-hyper-bright-LED-white-15000mcd-20-31V_19660.html

Apparently these consume a similar amount of power and give out 10 times the light of the previously specified LEDs:-

Technical details:
Color: White
Material: InGaN
Lens: Water clear
Brightness (in mcd): 15000 max (ultra bright)
Block voltage: typical: 3.1 V, max: 3.4 V
Current draw: 20 ~ 30 mA (130 mA peak)
Beam angle: 20 °
Solder temperature: 260 ° C for 5 seconds
Wavelength: 520nm

The Solar panel is the sticking point for me. I don’t not have a vast amount of room and have found two which hopefully will work out:-
The first one is the smaller of the two and the one I would prefer to use:-

Power Output: ~6V @ 1W (~167 mA) * Output Type: DC Voltage
Dimensions: 4.9 x 2.5 x 0.13 in (125 x 63 x 3.4 mm)
Operating temp range: +32 to +158°F (0 to +70°C)

The second one is slightly larger:-

Output DC voltage : 6V, 1.5W
Output Current : 250mA ( depend on the power of sunlight )
Weight of Solar Panel : ~54g (0.12lbs, 272ct )
Size of Solar Panel : 140 mm X 110 mm X 3 mm / 5.5" X 4"3/16' X 1/8"

Following your calculation:-

30mA * 4 (number of LEDs) * 12 = 1440mAh
Therefore 4 batteries would give me:- 4 x 2450 = 9800mAh
I am then assuming that the batteries (when fully charged) would have enough to drive the LEDs for 12 hours.

But will the Solar panels specified have the ability to recharge the system it 12 hours?

Im assuming to calculate this I multiply the panel output (mA) by the charging time (12 hours) and compare it to the power consumption (less 25% for losses)
If this is the case; the first panel would give 167mA x 12 = 2004mAh * 0.75 = 1503mAh
The second panel would give 250mA x 12 = 3000mAh * 0.75 = 2250mAh
So both potentially could work? Or am I missing something blindingly obvious?


Thank you for your help and please excuse my ignorance, but im quite enjoying learning about all this..

There are some slight mistake in battery capacity calculation. Your batteries are in series, therefore its mAh will be as same as of a single battery, i.e. 2450mAh. Only voltage adds up.
For 3000mAh battery to charge, you would require atleast (in minimum) 300mA of curent with more than 7.5v (for 6v battery pack). Charging time considered 12Hrs.

Your battery pack of 4 cells, with 2450mAh capacity is enough to drive these 4 LED's for 12Hrs. But your solar panel should have the potential of more than 7.5v (9v recomended). Charging current should be 300mA or more. Charging requires higher potential, and to protect your battery from overcharging you need a smart charging circuit.

Genovator said:

There are some slight mistake in battery capacity calculation. Your batteries are in series, therefore its mAh will be as same as of a single battery, i.e. 2450mAh. Only voltage adds up.
For 3000mAh battery to charge, you would require atleast (in minimum) 300mA of curent with more than 7.5v (for 6v battery pack). Charging time considered 12Hrs.

Your battery pack of 4 cells, with 2450mAh capacity is enough to drive these 4 LED's for 12Hrs. But your solar panel should have the potential of more than 7.5v (9v recomended). Charging current should be 300mA or more. Charging requires higher potential, and to protect your battery from overcharging you need a smart charging circuit.

Click to expand...

Thank you!

I have redesigned the unit to house the following;

Solar Panel :- https://sunboy2011.en.made-in-china...00ma-Small-Photovoltaic-Module-125-162mm.html

Batteries:- 4x AA batteries in series. And propose Duracell Supreme Rechargeable 2450mAh AA (or equivalent manufacturer).

LEDs 4x :- https://www.watercoolinguk.co.uk/p/3m...31V_19660.html mounted on a PCD in parallel (i assume, i don’t need a controller of some sort?)

I’m also assuming that all this would satisfy the consumption and charging requirements?

The challenge I have now, is installing a light sensor and control circuit and understanding the required circuit detail. If i were designing an industrial control panel, i would run sensors through relevant relays to give the required result through a control circuit. However (at this level) I’m assuming that i require transistors instead of relays and resistors in various places mounted on a pcd to make it all work. Is there any possibility you could help me with this?

Thank you

- - - Updated - - -

Genovator said:

There are some slight mistake in battery capacity calculation. Your batteries are in series, therefore its mAh will be as same as of a single battery, i.e. 2450mAh. Only voltage adds up.
For 3000mAh battery to charge, you would require atleast (in minimum) 300mA of curent with more than 7.5v (for 6v battery pack). Charging time considered 12Hrs.

Your battery pack of 4 cells, with 2450mAh capacity is enough to drive these 4 LED's for 12Hrs. But your solar panel should have the potential of more than 7.5v (9v recomended). Charging current should be 300mA or more. Charging requires higher potential, and to protect your battery from overcharging you need a smart charging circuit.

Click to expand...

Thank you!

I have redesigned the unit to house the following;

Solar Panel :- http://sunboy2011.en.made-in-china.com/product/joyEcghbywkW/China-9V-300ma-Small-Photovoltaic-Module-125-162mm.html

Batteries:- 4x AA batteries in series. And propose Duracell Supreme Rechargeable 2450mAh AA (or equivalent manufacturer).

LEDs 4x :- http://www.watercoolinguk.co.uk/p/3m...31V_19660.html mounted on a PCD in parallel (i assume, i don’t need a controller of some sort?)

I’m also assuming that all this would satisfy the consumption and charging requirements?

The challenge I have now, is installing a light sensor and control circuit and understanding the required circuit detail. If i were designing an industrial control panel, i would run sensors through relevant relays to give the required result through a control circuit. However (at this level) I’m assuming that i require transistors instead of relays and resistors in various places mounted on a pcd to make it all work. Is there any possibility you could help me with this?

Thank you