Solar recharging power source for LED

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1

Hello,

For a project I am working on, I want to drive an RGB LED* with a power source that charges during the day. Can anyone offer advice on what I should look for when it comes to storage and charging components? Advice in general would be greatly appreciated. I have done some simple electronics projects, but nothing complex or related to solar.

Thank you!

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For how long do you want the LED to be ‘driven’ ? Just 1 color or all three ? At full intensity or less ? Constant or blinking ? What’s the light source and for how many hours is it guaranteed to be ‘on’ for charging ?

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Thank you for getting back to my post.

The LED will need to be on for at least 4 hours in order for this to be worth it. It will be indoors, but the suggestion will be to place it by a sunny window. The idea is that when the room is dark, it will rotate between colors (no blinking) and will charge otherwise. I have not played around with intensity to give you a straight answer, but can say it should be able to cast some light in an unlit space.

4

So let’s estimate the LED uses 40 mA on average as it’s cycling through it’s colors. Lets add another 10 mA for a controller of some sort. Let’s run this off a nominal 5v. So that’s 0.05 x 5 = 0.25 W for 4 hrs, a battery that has 1W-hr of storage. That’s pretty small, a single AA rechargable might have a 2000 mah rating. At 1.25 V, that’s 3 W-hr of capacity.

Can you find a solar cell that’ll recharge that 1 W-hr of energy in some reasonable period ? Sure, even this guy should do the job.

https://www.sparkfun.com/products/7845

So now it’s a job of controlling the charging process, matching/changing voltages and controlling the LED. There are ICs for the former, SFE might even have a Break-out Board (BoB). And I’ll guess Adafruit’s Trinket would do the later (assuming you don’t care to deal w/naked parts).

5

The simplest thing I can envision is a battery holder like this …

https://www.sparkfun.com/products/552

plus 4 AA NiMH batteries. Connect the above solar cell to the pack, but through a Schottky diode. Connect the pack through an on/off switch to the 5 V output of the Trinket (bypassing it’s voltage regulator). I’d advise using the 5 V Trinket but setting it’s clock to 8 MHz. I’d even remove the regulator.

http://www.adafruit.com/products/1500#Description

Connect your LED leads, through the appropriate dropping resistors, to the Trinket’s PWM pins. The common cathode goes to ground. I think you should be able to connect the solarcell output, via a resistor divider, to the Trinket’s analog pin. This way you can gauge darkness and do some timing to turn on/off the LED.

Given the solarcell output of 5 V the batteries will never be fully charged but they should last the 4 hours. And the NiMH batt’s should tolerate the trickle charge well, no need for fancy circuitry.

6

Thank you for the response. I found it quite informative and helpful.

Why go with the 5V Trinket and not the 3.3V one?

When selecting a Schottky diode, which I be looking for in ratings?

I want to stop the effect automatically (not via a manual switch) when the environment is bright. My first thought is to handle this in the Trinket’s logic, using a photoresistor for input. However, this means the Trinket has to constantly be on, thus constantly consuming power. Is there a way to toggle power to the Trinket itself? For example, when the current coming from the cell falls bellow a threshold (i.e. it’s dark out), turn the Trinket on and when the current exceeds that threshold (i.e. it’s light out), turn it off.

Edit: I’d still want an overriding off switch, but that seems simple enough.

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Also, wont 4 AA in serial not produce enough voltage (4 * 1.2V = 4.8V) for the Trinket 5V (recommended 5.5V+)?

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timsharoo:
Why go with the 5V Trinket and not the 3.3V one?

Because you'll be driving the LEDs directly from the Trinket and 3.3v isn't enough for the G and B LEDs. Also using a 3.3v Trinket means keeping it's 3.3v regulator running. What I propose is that you bypass the 5v regulator (remove it even!) and run directly off the battery pack. The "brain" of a Trinket is an ATtiny85, which will happily run off of anything btw 2.7v to 5.5v, just at lower clock speeds for the lower voltages. A 3.3v Trinket has the same ATtiny85 as a 5v one, just at 3.3v and 8Mhz (vs 16). Hence my recommendation that you run at 8 MHz, a simple software selection from what I've read.

timsharoo:
When selecting a Schottky diode, which I be looking for in ratings?

Nothing special. The peak forward current might be 200 mA, the average < 100. A reverse breakdown voltage enough to stop the charged 6v battery pack, so a spec of 9v or larger.

timsharoo:
Is there a way to toggle power to the Trinket itself?

I know the ATtiny85 has a number of power saving and sleep modes. I'm sure it could be put to sleep for 10 mins at a time, waking up for 1 sec to measure stuff and decide what to do. That's a common thing for microcontrollers to do. What I'm very dubious of is that it can drive the LEDs when sleeping. Moreover I will guess the LEDs are 70% of the power consumption. Reducing the 30% to even 0 won't make a critical difference. But I'll see what might be done.
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Thanks again for the helpful response! I feel like I am narrowing in on a solution.

Mee_n_Mac:
Also using a 3.3v Trinket means keeping it’s 3.3v regulator running.

I get not being able to drive green and blue, but what's wrong with having a regulator?

Mee_n_Mac:
I’d even remove the regulator.

Same question as above.

Mee_n_Mac:
What I’m very dubious of is that it can drive the LEDs when sleeping.

The point is to have it off when I don't need to drive the LED and on when I do, essentially. Sorry if I was unclear.
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timsharoo:
Thanks again for the helpful response! I feel like I am narrowing in on a solution.

Mee_n_Mac:
Also using a 3.3v Trinket means keeping it’s 3.3v regulator running.

I get not being able to drive green and blue, but what's wrong with having a regulator?

Mee_n_Mac:
I’d even remove the regulator.

Same question as above.
Power saving. A regulator uses some power to run itself as well as providing regulated voltage. If your battery is well regulated, why waste the power ?

timsharoo:

Mee_n_Mac:
What I’m very dubious of is that it can drive the LEDs when sleeping.

The point is to have it off when I don't need to drive the LED and on when I do, essentially. Sorry if I was unclear.
Having the Trinket sleep while it's sunny and charging, waking up for a sec every some mins to see if it's "LED time" should be doable.
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Also, is there any reason to use a Trinket instead of the attiny85 directly? There is documentation on programming them with an Arduino Uno (which I have) and it seems trivial. Asides for a voltage regulator (which I am bypassing) and a USB programmer (which I can replace with an Uno), does the Trinket bring anything else to the table?

Sparkfun has one here https://www.sparkfun.com/products/9378

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timsharoo:
Also, is there any reason to use a Trinket instead of the attiny85 directly?

Nope, other than my assumption up front ..."(assuming you don't care to deal w/naked parts)". I don't know how breadboarding or PCB making fits into your skill set.
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Mee_n_Mac:

timsharoo:
Also, is there any reason to use a Trinket instead of the attiny85 directly?

Nope, other than my assumption up front ..."(assuming you don't care to deal w/naked parts)". I don't know how breadboarding or PCB making fits into your skill set.
Breadboarding is fine and dandy. PCB making... If the PCB is already made, then sure! I have no trouble putting together kits, but when it comes to universal PCBs, I'm a mess. Mounds of solder everywhere and wires leading off to nowhere convenient. I'll get better if I practice.

Does that information change any of your suggestions? I have already learned a lot.

As you know, with the Trinket there’s one with an operating logic voltage of 5v and another with 3.3v. How does operating logic voltage factor in with the ATTiny85? Do I just put out roughly what I get in?

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timsharoo:
As you know, with the Trinket there’s one with an operating logic voltage of 5v and another with 3.3v. How does operating logic voltage factor in with the ATTiny85? Do I just put out roughly what I get in?

Sorry, I don't understand the question. If you mean that when you command a digital pin to a logic high output state, you get either 3.3 or 5 volts out, depending on the supply voltage, then yes ... you get out roughly what you put in. So to drive 20 mA through an LED that drops ~3.4v @ 20mA, you output ~5v through a (5-3.4)/0.02 = 80 ohm resistor to the LED. A higher resistance for the R channel as it drops less voltage.