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Fun with Cheap PV (Solar) Cells by ROSS TESTER We’ve often looked at the small PV panels now being used on and in products ranging from garden lights and decorations to self-powered instruments and thought “they’d be handy if you could get them cheap!” Now you can – and the uses are, if you’ll pardon the cliché, limited only by your imagination. I have to admit that immediately after Christmas I bought a couple of sets of “solar powered” Christmas lights – not because I wanted yet more Christmas tree lights but because they were so incredibly cheap and because I thought I could do something else with them. I knew that each of these panels contained a small PV (photo-voltaic) cell along with a small rechargeable battery, in most cases a “AA” NiMH. They’re the bits I wanted and as a bonus for my couple of dollars I received a couple of hundred (OK, 500!) coloured LEDs connected in various strings and a tiny microcontroller board which drove them. I’m not sure if I can do anything with the LEDs and controller except put ‘em up next Christmas for my contribution to National Lampoon’s Christmas Vacation. Maybe over the next nine months or so some experimentation might uncover something. But that PV cell and battery, well, they’re another matter. Just in case you’ve been hiding in outer Mongolia (no 44  Silicon Chip wait, that’s probably where the Christmas lights are made...) the theory is that the PV cell provides current to charge the battery while the Sun shines. At night (which, of course, the microcontroller knows because there is no PV cell current) it turns on and starts controlling the string(s) of LEDs in a range of intricate patterns. We’re not sure exactly how this works but it’s a fair bet that each LED, or each string of LEDs, has its own chip which the micro can address – hence the pretty patterns. At sunrise or when the battery is flat, whichever comes first (and it’s more likely the latter), the LEDs turn off, ready to go through the next 24-hour cycle. And the amazing thing is that you can get all this for less than twenty dollars (or if you wait like I did, a tiny fraction of that price). The fact that they are made to a price is evident by the fact that often these lights don’t last long. Whether it’s battery failure, PV cell failure or micro failure, they often siliconchip.com.au What to do with them? Two PV cells are available. The larger of the two (above) is 90 x 63mm and produces up to 5V <at> 120mA. . . . . . while the smaller is 85 x 50mm and is capable of up to 5V <at> 80mA. The dollar coin gives you a scale reference. don’t last the full Christmas period. To prove the point, I connected the one of the failed systems to a low-voltage DC power source and . . . presto, they worked! Oatley’s cheap PV cells After going to all the trouble of buying discounted lights just for the PV cells and batteries, you can imagine my chagrin when Oatley Electronics told us of some really special offers in PV cells in early January! How special? Packs of thirty 5V, 80mA solar panels for $16.00 including a schottky diode (we’ll explain its purpose in a moment). Or perhaps even better – a pack of 20 slightly larger cells (5V <at> 120mA) for $20.00! Do the maths: 30 x 80mA or 20 x 120mA equals 2.4A <at> 5V or 12W – an extremely useful amount of power, suitable for a whole range of applications (and we’ll get to a few of these shortly). Of course, that’s in full, direct sunlight. Early morning or late afternoon sunlight will see this taper off. Incidentally, if you measure the open-circuit voltage in full Sun, you would get close to 6V but when you start to draw power, the voltage drops. 5V is the “sweet spot” for maximum efficiency. Quite a number of applications spring to mind: Charging batteries – either mobile phone batteries (believe it or not, their original use) or connected in series/ parallel for a range of other voltages. Phone supply – use the USB socket on your phone, notebook, etc with parallel-connected cells to power your gear while away from a mains power source. Sorry, iphone users – Apple has done away with the USB socket! Solar skylight – do you have a dark room in the house? Put some of these on the roof and a suitable LED in the room and you’ll be surprised at how much light you’ll get during the day. Battery-backed solar skylight – combine both of the above with a suitable battery and you’ll have a light that can be used at night as well as during the day. Remote control garage door/gate supply – if it’s inconvenient to run mains power to your garage door controller, power it from a small 12V SLA battery, kept charged by these solar panels. For example, see the articles in April & May 1998 and October 2004. Under-floor ventilation – keep the air circulating under the house to prevent mildew and damp with a suitable fan, exhausting to outside, connected to the cells. It won’t cost you anything to run, either. The same idea can be used for a room that’s always smells “musty” when closed up. Car internal cooler – use the same idea as above to exhaust hot air from your car while it’s parked during the day. Costs nothing to run – and will actually save you money because the air conditioner won’t have to work so hard. Unattended boat keeper – a few cells on the deck of the boat can help keep alarms etc, in operation while you’re not there. Camping supply – use the solar cells to keep the tent air circulating, or to provide light at night via a rechargeable torch, etc. Or perhaps to keep your phone charged the other side of Woop Woop. Prawn lure – combine some LEDs, a battery and solar cells in a waterproof glass jar and make yourself a convenient prawn lure. OK, how do we mount them? Oatley Electronics have come up with a rather unusual method of mounting: silicone sealant and gutter guard! Sure, you could make up a frame of some sort to hold them but they’re already coated with a clear epoxy resin on front, Ten cells are powering four x 3-LED arrays in parallel, merely from the modelling light in our studio flash. In sunlight, they’re much brighter! Note the rudimentary heatsink the LEDs are mounted on. siliconchip.com.au March 2017  45 and the PCB has a conformal coating to protect it. The only bits of the PCB that aren’t protected are where you solder your connecting wires, and we think it would be wise to cover these (after soldering!) with some silicone sealant or even some spray-can conformal coating such as Electrolube Flexible Silicone Coating (www.electrolube. com.au/products/conformal-coatings.html). If you don’t cover the copper and use the PV cells outside, the copper would quite quickly corrode, leading to premature cell failure. We’ve shown a couple of photos of the cells mounted on gutter guard – it has the big advantage of being very flexible and it is also cheap! Add a tube of silicone sealant from your friendly hardware store and that’s it. You can even nail or screw the gutter guard onto an appropriate (sun-lit) surface. You could even glue the panels to the gutter guard and solder your connecting wires later, as long as you’re careful with the sealant placement. Wiring This depends entirely on the use you’re putting the PV cells to. They can be wired in series for higher voltage - for example, to keep a 12V battery “float charged” you’d want three in series (~15V); to provide a higher rate of charge, you’ll need to parallel several sets of three cells. We’ll look at some examples in a moment. The schottky diode PV cells have the unfortunate “feature” of allowing current to flow through themselves when dark, so if left wired to a battery, all the good charging done during the day can be lost at night, as the battery discharges via the cells. But the simple method of wiring a diode in series with the positive line (anode to the cells, cathode towards the battery) prevents this. While the cell is producing power, the diode will be forward-biased (ie, the anode voltage is higher than the cathode), so charging current can flow to the battery. But when the cell is in the dark, the cathode has a higher voltage than the anode, so it is reverse-biased, thus preventing self-discharge. But you can’t just use any old diode because you’ll lose too much power. All diodes have a forward voltage between anode and cathode – and that is voltage that you cannot use; it is lost. A normal silicon diode has a forward voltage of about 600-700mV which, when the PV cell is only producing 5V at its maximum output, is rather too much to lose! By contrast, a schottky diode has a forward voltage of only about 200mV, so you’re not going to waste too much of that precious energy you went to all the trouble to produce from the PV cell. In fact, with a silicon diode, the panel would struggle to charge a lot of battery types at 4.3V (5V – 0.7V); at 4.8V, it has a much better chance. Let’s look at some specific ideas The ideas mentioned above are all practical and possible – even if you don’t use them exactly, they may start your creative juices flowing for that project you’ve been thinking about. (1) Phone battery charger This is the simplest application that we can think of – it all depends on what you want to charge. mobile phone charger. As we mentioned earlier, this sort of PV cell was originally intended for charging mobile phone batteries. These days, most mobile phone batteries are nominally 3.7V. In almost all cases, the USB/microUSB socket connects to the input of a charge controller IC designed to operate over the range from 4.5-5.5V, so directly connecting one of these cells (or a few in parallel) to the USB socket is permissible. You might have to sacrifice a surplus micro USB lead to be able to plug into the phone – it’s not real easy to buy micro USB plugs. If you can find one, Fig.1 shows the two connections required on the plug. Otherwise, cut the USB A plug off the lead, identify the two wires (+ & -, normally red and black) you need with a multimeter and solder these to the PV cell positive and negative terminals. (2) Larger battery chargers To charge a 6V battery, you need two cells in series. That gives 10V; arguably a little more than needed but once again, power is limited so there’s nothing much to worry about. To charge a 12V battery, six cells in series/parallel would be the go – two lots of three cells in series, giving 15V (see Fig.2). You could trickle-charge an 18V or even a 24V battery in a similar way – two parallel strings of four or five cells for 18V; two parallel strings of six for 24V. In all cases, include the schottky diode. (3) Solar skylight The number of cells needed is directly proportional to the power of the LED(s) used. For a 20W (or 33W) LED, two parallel strings of seven cells would be used (see Fig.3). If the LEDs are the smaller 10W types two parallel strings It’s a bit different – but it’s dirt cheap and works well! A roll of “Gutter Guard” (8m roll $2.90 at Mitre 10) and a tube of silicone sealant (~$4 just about anywhere!) and you have a flexible mounting system for your PV cells. Note our comments about covering over the bare copper on the PCB with a conformal coating (or even silicone sealant). 46  Silicon Chip 3.7V BATTERY INSIDE MOBILE PHONE 2x PV CELLS VCC Fig.1: here’s how to keep your mobile phone battery charged away from power. You’ll need a micro-USB plug to connect it (pinout shown at left). siliconchip.com.au A K SCHOTTKY DIODE A K SCHOTTKY DIODE 1-3W LED/ ARRAY 12V BATTERY 2x3 PV CELLS Fig.2: want to keep a larger battery, such as in an RV or boat topped up? Simply add PV cells to suit. Six cells in two rows of three is ideal for 12V; twelve cells in two rows of six will keep a 24V battery happy. 24V BATTERY 2x6 PV CELLS of five cells are more than adequate. Note that the LEDs should be mounted on some form of heatsink – you don’t need much, as our photo of the 10W string shows. You can connect up to four of these LEDs in parallel. (4) Solar skylight with battery If you want the convenience of charging a battery (eg, an SLA) at the same time as lighting LEDs (ie, for night and day use), you will need to add at least one more series string of cells in parallel with the others. In this case, the schottky diode will be required and you’ll want some form of on/off switch in series with the LEDs. Turning them off when not required will also allow faster and/or deeper charging of the battery. (5) Garage door remote control supply One problem with garage door and gate remote controls is that they’re often mounted in a place without access to power. You can solve that problem by connecting some PV cells to a battery in the same way as above and run without AC power. This is not intended to power the door/gate motor itself, just the remote control receiver. So you only need a small capacity battery, usually 12V. But the smallest size you can buy should be fine as the remote control is very intermittently used and in most cases, only requires a relay to pull in for a short time or even a transistor to switch on briefly. We know someone who did this several years ago (when small solar panels first came out) and the controller is still running quite happily. (6) Under-floor air circulation Many people unwittingly block air vents under their homes. A garden bed or path positioned against the wall is a common mistake. The result is that the air under the house cannot vent or circulate and ends up smelling foul. If the ground under the house is at all moist, the problem is exacerbated. It’s that “musty” smell that suggests mould or similar is thriving. If you fit a small “muffin” fan into the brickwork, a PV cell can drive it during the day and extract that air to the outside, so it continuously circulates. All you need are enough panels – a 12V fan will usually operate quite happily (albeit a bit slower) at 10V but will not be upset with 15V – so two or three panels in sesiliconchip.com.au   2x3 PV CELLS Fig.3: if you’re only powering a LED for a dark area in your home (ie, a “skylight”), no schottky diode is necessary. You’ll see the LEDs slowly light up after dawn and die at dusk. 20W OR 33W LED ARRAY 2x7 PV CELLS ries would be ideal. These fans are quite efficient so you may only need one set of panels but larger fans will require more power. So once again, a series/parallel arrangement would suffice. Since batteries are not involved, no schottky diode would be required in this application. (7) Car cooler A similar arrangement can be set up to extract hot air from your car when it’s in the Sun. You can buy some commercial units which operate from the car battery but using a solar panel and a small fan will mean you’ll never return to a cool car which won’t start! You’ll need some ingenuity in making a suitable mounting bracket for this one – commercial units sit in a slightly open window but make sure you don’t sacrifice car security for comfort! (8) Boat, caravan, mobile home battery keeper Because a lot of craft or vans are used on a very intermittent basis, you have to ensure that their house batteries are kept charged. Allowing a battery, particularly a lead-acid type, to flatten will almost certainly end up with tears! You could mount as many cells as you need on the boat deck or van roof, wired directly to the battery. The number will depend a lot on the size of your house battery – three cells producing 15V <at> 120mA are better than nothing, but not much better. You probably need a good 500mA to 1A trickle-charge to keep the battery voltage up, particularly if it also powers things like intruder alarms, anchor light etc. Don’t forget the schottky diode to prevent night-time discharge. (9) Camping supply Whether you’re after light, a mobile phone/computer charger, or even a fan on stifling hot summer nights, you can use the techniques above. Charging your phone doesn’t Fig.4: to extract musty air from under your house, a high-efficiency fan can be left connected 24/7 – naturally it will only work during daylight hours. The circuit is virtually identical to the 12V skylight circuit. 12V FAN 2x6 PV CELLS March 2017  47 A 1-3W LED DRIVER K SCHOTTKY DIODE 12V BATTERY 2x3 PV CELLS 1-3W LED/ ARRAY   If you want to charge a battery for night-time use as well, you’ll need both a schottky diode and a suitable LED driver (you cannot connect the LEDs directly to the battery or the light output will be rather brief. Bright, but brief!) take much (probably just one PV cell). Your computer may well need several, depending on the battery voltage. And charging a battery for night-time use is much the same – the larger capacity battery, the more cells you’re going to need. Just remember to keep the PV cells in direct Sun during the day (often the tent is erected in shade!). (10) Prawn/fish lure Because today’s LEDs are so bright and so efficient, it doesn’t take much battery power to run them for quite a long time. With a suitable (waterproof) clear container you could mount as many PV cells inside as required to charge a battery, with as many white LEDs as your system will allow. Make it completely waterproof by including a mercury “tilt” switch – when it’s vertical, it turns on. And don’t forget the schottky diode to stop the battery self-discharging. Leave it in the sun all day and it will be ready for your prawning/fishing trip at night. A 20W LED DRIVER K SCHOTTKY DIODE 24V BATTERY (EG, TWO 12V IN SERIES) 20W LED ARRAY         2x6 PV CELLS Oatley Electronics special offers To go with this feature, Oatley Electronics are offering some special deals at special prices! (1) A pack of 30 smaller 80mA panels (45 x 90mm) plus three 1A schottky diodes: $16.00 (Cat SP4590) (2) A pack of 20 larger 120mA panels (60 x 90mm) plus three 1A schottky diodes: $20.00 (Cat SP6090) Extras – if purchased at the same time as either of the above packs: (3) 10W LED: $1.50 each (limit of one per PV panel pack - Cat IT104) (4) 20W LED: $2.50 each (limit of one per PV panel pack – Cat IT105) (5) 60mm Brushless Fan: $2.00 each (limit of one per PV panel pack – Cat IT106) (6) USB A to B lead: (note – not a micro USB lead) $1.00 each (Cat IT107) Contact: Here are 3 x 3 cells mounted on a short length of Gutter Guard (about 10c worth!). SILICON CHIP Oatley Electronics, PO Box 139, Ettalong Beach NSW 2257 web: www.oatleyelectronics.com; email: sales<at>oatleyelectronics.com) Phone: 0490 347 297 (best to send an SMS requesting a callback) SC ONLINESHOP . . . it’s the shop that never closes! 24 hours a day, 7 days a week . . . it’s the shop that has all recent SILICON CHIP PCBs – in stock . . . it’s the shop that has those hard-to-get bits for S ILICON C HIP projects . . . it’s the shop that has all titles in the SILICON C HIP library available! . . . it’s the shop where you can place an order for a subscription (printed or on-line) from anywhere in the world! . . . it’s the shop where you can pay on line, by email, by mail or by phone Browse online now at www.siliconchip.com.au/shop 48  Silicon Chip siliconchip.com.au