Solar LED light circuits

Discussion in 'The Projects Forum' started by wayneh, Aug 24, 2011.

  1. wayneh

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    Sep 9, 2010
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    I'm revisiting this thread because it seems like a good place to post additional solar light circuits. But I can't post there because the thread is old.

    Here's a solar light circuit that came from a "nice" one with 5 white LEDs that pulse back and forth from a set of 2 (in parallel) to a set of 3 (also in parallel), simulating fireflies. That pulsing function is provided by another PCB black glob that's not shown. I believe this posted circuit just handles the boost and the auto-shutoff.
    Picture 2.png Unfortunately I cannot read the codes on the diodes and the caps C1 and C2 have no markings. They're both small orange discs, and not the same physical size. Any advice on replacing them appreciated. One of them appears split on the edge, and the board isn't working. The transistors are marked S8050 D331, but apparently this is a rare part number that actually refers to two different transistors, one NPN and one PNP. I've guessed NPN but would appreciate comments - I don't fully grasp this circuit. One transistor turns on the other to oscillate, I get that. But I don't have a mind for oscillators.

    I tried replacing this board with a similar one pulled from a simple amber light (will post soon). It almost works with the firefly effect, but switches between the sets of white LEDs much too quickly. I suppose the voltage for a single amber LED isn't enough to drive the circuit designed for 2 or 3 white ones (in parallel)?
     
  2. SgtWookie

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    I whipped up your schematic using LTSpice, a free SPICE simulator from Linear Technology, Inc.

    It seems to work using SS8050 models from Fairchild Semiconductor, which I tend to believe are similar to the S8050 transistors.

    Have a look at the attached. In the interests of just getting something running for you, I just simulated the solar cell snapping on and off every 20mS.
     
  3. wayneh

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    That's very interesting, thanks a bunch.

    So I guess the NPNs work as drawn. Regarding the caps, how did you guestimate the values? I wonder about the 1µF for C2. The discs are 4mm in diameter. C2 is a bit fatter and that's why I thought it was larger. I'd replace it if there is hope that this might fix the PCB. How sensitive is the circuit to the value of C2?

    FWIW, the LEDs this thing drives are white, not blue, and there are 2 or 3 in parallel at all times.
     
  4. wayneh

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    Oops, I missed that, and don't follow. It's the output of this thing that should be oscillating, not the input. The fade-back-and-forth circuit is a glob on another PCB between this one and the LEDs. It uses a common "+" for all 5 LEDs and two "-" poles, one for 2 and the other for 3 white LEDs.
     
  5. SgtWookie

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    You're welcome. :)

    They seem to work OK. You could pull them out and test them. If they turn out to be open, you might try replacing them with 2N/PN2222 transistors.

    I just tried a few values for C1 that I figured would oscillate at a reasonably high enough frequency. It's around 200kHz as shown. That's kind of high for the SS8050's, but it's just a thrown-together thing.

    Well, C2 just evens out the current flow through the LEDs. The larger C2 is, the more even the current flow will be.

    OK. Well, blue and white LEDs have about the same Vf, and I had a blue one available so just threw it in there.

    It seems that LED current increases with decreasing C1; 220pF gives around 20mA current. It's oscillating in the 500kHz range then. I don't remember offhand what the transition frequency of the SS8050 is; have a look at a datasheet. If C1 is too small, the transition frequency will be exceeded, and it'll stop oscillating.

    Q2's only function is to stop the oscillator when the sun's shining.
     
  6. SgtWookie

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    OK, I don't know about that. I didn't want to wait a whole day for the simulation to complete. ;)

    I could've left Q2, the solar cell, the battery charge diode,etc out of the simulation.
     
  7. wayneh

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    Here's the other solar light circuit I dissected today. It's for a cheap amber LED, single AA light. The two transistors are SMD and I've shown an approximation of the marking that's on them. The 2A is apparently a 2N3906 PNP, but the GC is more of a mystery. I found a two diode BAS70C that is marked this way, but I really don't know.

    Anyway this will boost the battery enough to drive a single amber LED or even 5 white LEDs in parallel. But it will NOT allow my firefly effect to operate properly. It flashes and switches far too quickly, not the nice slow fade I'm looking for. I suppose the voltage output is too low, but that's just a guess.
    Picture 5.png
     
    Last edited: Aug 24, 2011
  8. wayneh

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    Oh, I got it now! You're just turning it on and off by pulsing the solar cell. I confused that with the oscillations while "on".

    If C2 has indeed failed, it doesn't seem like that should prevent lighting the LED? It measures open, not shorted.

    I guess I could start pulling the transistors and caps to test them. Onboard testing of the diodes and resistors didn't uncover any problems and the inductors show low ohms so probably still good.
     
  9. wayneh

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    I did it! I fixed this thing. Started by removing the two 8050 transistors. The first one popped out easily, but with only 2 leads! The 3rd had corroded off where in entered the plastic case of the transistor. (Rotting leads is a big problem with cheap solar lights.)

    To my amazement I had an 8050 in my parts bag. Replaced the bad one, put everything back together, and it works! :D Nice slow pulsing of bright white LEDs.

    Now I've just got to waterproof the enclosure as best I can.

    Just to wrap up; the cheap amber LED circuit works OK and will light a white LED. And it's very simple. For whatever reason, the slightly more complicated circuit posted above works better with the higher voltage and/or current of my LED throbber circuit. It had the additional advantage for DIYers that it doesn't use SMD components.
     
  10. wayneh

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    I just noticed that the first circuit I posted - the one Sarge modeled - is nearly identical to the one described in "Circuit 1" of the link I provided.
     
  11. wayneh

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    Yet another solar light circuit, but this one is a lot different. It's from one of these, and uses a 4.0V, 1AH SLA.

    Picture 3.png

    I don't quite follow all of this circuit and need advice on one issue. R1 and Q1 appear to be used for dumping solar power when the battery voltage exceeds a voltage set by D2, assumed to be a zener. So changing the value of D2 should allow using different batteries? R2 and R3 are both tiny SMD, whereas D2 is thru-hole, so I feel better about my chances of changing it, but I could play with my SM technique if changing R2 and R3 would help. I have bunches of Li-ion batteries scavenged from spent laptop battery packs. The circuit works well with two pairs of those in series for 7.4V, but of course brightness fades as the recharging takes them back to only 5V or so. Using just one pair of the Li-ion batteries at 3.7V also works, but the LEDs are not bright and I suppose the battery won't live long with repeated overcharging.

    The rest of the circuit is a bit of a mystery although I get the overall idea. I understand that the HCF4060 is used as an oscillator, but rather than boosting voltage I think here it's used to reduce duty cycle to the LED string, to keep a 5V battery from popping them. The CdS cell sends a high to the reset pin that turns off the oscillator and somehow via Q2 and Q3 also turns off the LEDs driven by Q4. The rest of the output control is a bit of a mystery. Does it do anything but drop voltage efficiently? I mean, is it adjusting to what is happening at the output?

    As far as I can tell, the output is not tightly regulated for either voltage or current. But with a SLA battery, it's able to safely power a string of LEDs in parallel. The original device uses 50 (!) LEDs. With just one blue LED and 7.2V at the battery, it'll drive 29mA. With 3 blue LEDs, it drives 50mA, so nice and bright yet safe for the LEDs. I'm running 5 LEDs now but I haven't measured the total current. With a 3.6v supply, it drives only ~3mA through a single blue LED. I've got 4 reds hooked up now and they burn all night but not very brightly.
     
  12. wayneh

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    I made an error in the schematic above; the "195" resistor in the battery voltage tender is actually "561". I read it upside down.

    Also, I've since been able to increase the target voltage, where dumping of excess panel power begins, by soldering a resistor in parallel with R3. Specifically, I added a 220Ω resistor to raise the voltage to ~7.5. This works pretty well for 2 Li-ion cells in series.
     
  13. thatoneguy

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    I don't think LiIon would be the best for this application due to temp extremes and the possibility of overcharging. Unless these are for wintertime use blinky stuff, and need the LiIon for enough current when it's freezing out.
     
  14. wayneh

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    They're only "best" in the sense that I already had them, pulled out of spent laptop battery packs, and they're a workable replacement for the original, now dead, SLA. Lots of juice all night, and it's been working for a few months now. We'll see how it hangs in this winter.
     
  15. thatoneguy

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    Then they are the "best" in terms of candidates, v.s purchasing brand new 18650 cells to give it a go. :D
     
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