Auto Solar Light
NEVER DO THIS WITH lithium Ion batteries!!! (They will explode because they are so efficient, but may not if they have built in protection...) It is easier than you think to make a auto solar light if you are willing to learn a little about electronics. The "R" in the picture means resistor. The "D" means diode and the "Q" means transistor. Resistors limit the amount of electrons flowing through the circuit. They do not need to observe polarity meaning that you dont have to worry about which way to solder them with respect to positive and negative. Diodes are a one way valve. In this case, the banded end goes away from solar panel! The led pictured is also a diode, hence why they only work wired in one direction. D1 is any normal diode. Some drop more voltage (E) than others. If too much is dropped, not enough charging voltage will be applied to the batteries. Schotty diodes drop much less but cost like 25¢ more. Transistors are like gates that open when a very much smaller amount of current is applied next to the gate. Kinda like one man opening up a gate for a bunch of people. In the picture, the left side is the gate (or base as it is called), the top, the collector and the bottom, the emitter. R1 and R2 are 2,000 to 20,000 ohms. (I think R1 would be smaller than R2 for darker turn on, not sure yet). Experimentation can adjust what dark level to turn on led. This is a very small amount of wasted current. If Q1 is opened by the very little bit of current allowed by R1, then current (also a small amount) flowing through R2 will want to flow through Q1 instead of opening Q2. When it gets darker (and not enough juice is coming from the solar panel), the gate of Q1 closes and thus juice from the battery flowing through R2 has no choice but to open Q2. When that happens, juice from the battery flows through the light emitting diode (led) as any normal and very simple circuit. R3 is needed to set the led voltage. Q1 (I would think) would always be a simple pn2222 NPN which can handle the small amount of current. Q2 can also be that if just a few small (explained later) leds are used. If larger amounts, (I forget how much but definately over .5 amps), then a more powerful NPN transistor will be needed since Q2 allows the full amount of current in the curcuit to flow through it. Red, orange, and yellow leds use (drop) about 2 volts and all other colors including white drop about 3, even up to 4 volts. This is why we need 12 solar cells wired in series from positive to negative. You will have to determine which side is positive after soldering them together (the front side is negative on mine). If they were wired positive to positive, they would only be .5 volts but 12 times the amps. 6 volts is the best for three 1.2v rechargeable batteries (note, in this pic, they are not even wired yet). The 3.6v pack needs at least 4.5v but the intermittant nature of solar would barely produce that except in perfect conditions. After the battery bank is fully charged, it will be of higher voltage than the nominal 3.6v, (up to around 4.3?). Thus we will have to resist the voltage down to the 3 - 3.2v that the white led would need. The extra 1.5 volts (in this case) will greatly reduce their longevity (add a few extra volts, and they'll fry and stink). A reduced longevity means that they are not nearly as bright as they used to be. A little led (called super bright or ultra bright) is not supposed to be subject to any more than .02 amp. A "high power" led is rated at five times the current or .1amp, and a 1 watt led is rated at .3amp. (Times by 1,000 for miliamps, thus 20, 100 and 300 mA respectfully, but don't forget to use amps, not miliamps for Ohms law). Ohms law... Volts divided by Amps is Ohms (and volts divided by ohms is amps). Thus amps time ohms is voltage. We want to keep these things below the rated voltage. For convienience, too much amperage will not hurt IF the voltage is kept a little less than rated. I like to use the 300mA led (it's equal to 15 of the "superbrights" and seems to have a whiter light). The formula is this... figure the highest posible battery voltage and subtract the led voltage. Then divide by the wanted amps and you will get the resistor value. Example one. In the picture, we have at most 4.5 volts minus 3 = 1.5 divided by .3 = 5 ohm resister. Example two. If you wanted to use less energy and use only one .02 led (rated at say, 3.3 volts) then 4.5 minus 3.3 = 1.2 divided by .02 = 60 ohms. Power law... amps times volts = watts. Thus the 5 ohm resister in example one will have 1.5volts times .3 amps running through it, or .45 watts (the larger voltage is already dropped by led). And the 60 ohm resister in example two will have 1.2volts times .02 = (only) .024 watts. Watts = power, thus too many and the ciruit fries. If a normal ¼ watt resister was used for example one, it would probably fry since it would have almost double the current through it (at .45 watt). But in example two, only about a tenth of a quarter watt. In example one, the normal ¼ watt resistor is too small, thus simply wire more together in this manner... If you need a 5 ohm resister, wire four 20 ohms together (paralelle, Just twist them all together, not end to end). Or two 10 ohms, or ten 50's together. Or you can string two 5 ohms end to end (to make a 10 ohm) then wire two of those together to make a 5 ohm that can handle four times the power. (I might be slightly wrong here but it has worked in my little experimentations).When there is too little current, the leds will still light up, just not as bright. Since the batteries are always charged higher than nominal, and will drain to lower than nominal, these leds will seem rather dim towards the end of the night. One way around this later... Leds in series will multiply voltage (such as 3 will drop about 9 or 10 volts). Leds in paralelle will multiply needed amps (such as three 1 watt'ers will consume up to .9 amp). If you want to use many .02 leds (and have a 12 volt battery), then you could use a series paralelle circuit which would place many strings of 3 leds connected positive to negative (the positive of each "string" is connected to the positive of all other strings). The use of a ohms meter is very useful (proved I was wrong many of times). I fried mine using it to measure (small amounts of) current from a collection of little ¼ watt solar panels (the ohms and volts still works). Thus you can determine current (amps) by first measuring the voltage difference from just one side of the resistor(s) to the other. Write it down, turn off switch (S1 in picture), then measure ohms across same, and then divide the volts by the ohms to find out the (probably more than you thought) current going through the whole thing. If you increase the resistor value, you decrease the amps and thus the volts go down a little (instead of up according to the math). Also, to make things more confusing, heat (from all them electrons) causes more resistance which increases ohms which is indeed more resistance. Somehow, all electronic circuits don't melt down... It is this resistance that causes a hot PV panel to deliver less than (the same) one that is just brought out. There are more rules. Being that you dont want to buy a charge controller for a little led light, you will want to make sure that only 10% of the batteries Ah (amp hour) goes into the battery (probably up to 15% for solar). NEVER DO THIS WITH lithium Ion!!! (They will explode because they are so efficient). This is called the 10% rule and works with lead acid, NiCD and (the better) NiMH. Thus if they say 2,600 mAh, (which means 2.6 Ah), you can only allow .26 amps into the battery pack. If you have 1 x 3 inch solar cells, they will probably be rated .6 amps meaning that you will have to have two sets of battery packs wired in paralelle. If this was a continuous charge, you would need three sets, just to be safe, but solar... you can get away with the .06 "too much". They will barely ever put out as much as they say and not for that long. Voltage regulation... If using 12 volt and three leds in series (and strings thereof), that sealed lead acid might go up to 14 volts, thus you would want to resist the leds to 14 volts... But what happens when the battery is back down to 12? Example three, 5 strings of three .3 amp (1 watt) leds are used as a driveway lighting project. Each led drops 2.85 volt (and uses only .26 amps just to be safe because they get hot and like to burn out). 3 times 2.85 = 8.55 volt. 14v minus 8.55 = 5.45 volts. We will put a resistor on each string, thus 5.45 divided by .26 = 21 ohms (dont know if they make one so use next higher value). And for power, 5.45 times .26 = about a 1.5 watt resister (a lot of power is being wastedinto heat thus ten 210 ohm normal little resisters all tied together should work). The power from the 8.55 volts is being converted to light and heat in the leds also. The 5 strings make the project consume 1.3 amps. 8.55v times that = a 11.11 watt led lighting system (but most would call it 15 watts). All fine until battery is at 12 volts... = 3.45 divided by 21 ohms = only .16 amps and a set of leds that are not even half as bright (even though the math says they should be over half as bright). If you put a LM2940-12 regulator chip in the circuit (hav'nt done so yet with auto, and transistors circuit), then you can resist to 12 volts and never worry about frying the leds as in the next example. This chip only drops .3 volts! (Probably more with higher current). Example four... resisted to 12 volts but is at 14v without regulator chip... 3.45 divided by .26 = 13 or 14 ohms, power is... 3.45 times .26 = less than a watt. Now, at 14 volts, 5.45 divided by say, 14 ohms = .39 amps. These leds can only handle .3 with a heat sink. They probably burned out last night...LOL But please make it such that the chip is also turned off with switch. The wiring diagram says you can leave them on but will slowly drain your battery if not charged daily! I lost a SLA that way... The reason why is with the chip directly wired, see their diagram (by searching LM2940 ), you can use your on-off-on switch to have high and low power modes. But if the chip is wired inside the switch circuit, you can only have one switching curcuit...(or something like that). There are all kinds of solar cells on ebay, just make sure you don't pay too much per watt. Again volts times amps = watts (since they don't always list all the info). Fred480v is the place to buy'em (but He's not always selling). The ¼ watt "Shell cells" are only about $1 each if bought from enginecom. They put out about 4 volts so unfortunately (to get a good charge), you need two to charge the 3 batteries needed for white led light. Each is meant to charge 2 batteries (so you might as well use 4 batteries and resist accordinly) Shell made millions but people want to inflate the price, I've seen as high as $5 for one! Please, try to keep it below $5 per watt. People will try to sell 1 watt battery chargers for $29! And others will sell 15 watt panels for only $60... go figure... Sizing... If you series solder (oh yes, they're easier to deal with too) 4 shell mini panels (as they are officially called), you will be able to charge 8 batteries with about .06 amp. Not much, thus wire 4 strings of 4 to get (the same voltage and) ¼ amp. That is well in line with the 10% rule. (The amp meter said .08 for each but my solar lights don't seem to last that long). If you get 5 good hours of sunlight, then you will have stored 1.25 Ah. Since you have 8 batteries at 9.6v, you can wire 3 leds in series. If they are the .3amp ones, they should stay bright for about a few hours at most (but remain dim for many more hours). So, to have an all night light, you need to go with the .1 amp leds (well, summer anyways). You can go with the .02 leds (but they seem to need a slightly higher voltage and appear greener). There are, however, the .02 "superflux" that seem to give off a slightly whiter light but requires less voltage. Ok, .02 times about 15 hours = only .3 amp drawn from the 1.25 amp supply. We need 3 or 4 strings to match daily input with nightly output. So thats 9 or 12 leds that are over ½ as bright as a 1 watt led. You can make your own panels by using silicone and glass (but I havn't done the time test past a few months). Add aluminum frames, and it might even look good! (Oh, and make sure you can power some of the 1 watt'ers all night). It's quite impressive to not have to rely upon an extension cord for such a light. If you want yellow leds, you could string five of 'em per 10 volt line (instead of 3 whites). Just use that handy $20 ohm meter. It'll tell you everything you need to know once you understand Ohms law...
Obviously, I am not an electrical engineer, but can make a pretty good solar light, (but it still looks like an explosive devise). Actually, I learned from this site...
 Just trying to figure the best container.
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