Fullscreen HTML5Med Res (??MB)HTML4

SunSeeking Sunflower Last October we presented a solar-powered fly – and it has been a very popular project, particularly for first-timers. Here’s another project in the same vein – an electronic sunflower which senses where the Sun is and turns toward it, just like a real sunflower. Design by Craig Maynard I f you’ve ever been out in the country where a field of sunflowers is blooming, you’ve probably marvelled at the way they all turn to face the Sun as it tracks across the sky. Thousands, perhaps millions of flowers, all facing the same direction. Naturally, they do this to extract the maximum amount of energy from the Sun; energy converted by the plant’s chloroplast. 44  Silicon Chip Words by Ross Tester In our electronic version, two solar cells do a similar job, “catching” the energy from the Sun and converting it into electricity. The electricity is stored in a capacitor and used to turn a small electric motor. The motor is controlled by a comparator circuit which gets its information from a pair of infrared diodes. If the energy being received (from the Sun) by both diodes is equal, it’s a rea- sonable bet that they are both aimed towards the Sun. But if one diode receives more energy than its mate, it’s just as reasonable to assume that it has a better aim than its mate – so the comparator turns the motor on to adjust the direction. It does this in “fits and starts” – it’s certainly not a smooth motion but is quite jerky. In most control circuitry we’d call this “hunting” and steps would be taken to eliminate it. But in the Sunflower, it actually is quite natural. If you’ve ever seen stop-action photography of a sunflower, it does move in fits and starts! The control circuit is not dissimilar to the one used in the Cybug Solar Fly. The main difference between the two circuits is that this one has just one motor, made to turn forwards or backwards, while the other circuit had two motors. By the way, while this project is very much a novelty, the control circuit could be used as the basis for something much more significant – a means of tracking the Sun to charge batteries from a solar collector, for example. In fact, this very circuit can be used to charge a 1.2V NiCd cell! But we’re getting a bit ahead of ourselves! What’s in the kit The kit, available from all Dick Smith Electronics stores, contains all the components you need to build the project. It’s packaged in a see-through box so you can see at a glance what type of flower you’re going to get (yes, there are other flowers than sunflowers!). One nice touch is that all components are mounted on a piece of conductive foam, in the same positions as they will be soldered onto the PC board. (In fact, there’s a paper label glued to the foam which reproduces the silk-screened component overlay on the PC board. You can tell at a glance whether you are missing any components). Be careful when you open the kit The completed Sunflower. Ain’t it pretty? We must confess to a slight error in this photo: the infrared LEDs were bent over before shooting – they should be pointing straight out. And, as mentioned in the text, the (white) wire we used is about 50 times heavier than the wire in the kit, so you could see it. The real stuff in the kit is about as fine as human hair! Shown below is the kit as supplied – that fine line you can see over the paper is the wire! that you don’t lose the length of very fine wire which you’ll need later on! There’s also a short length of brass wire which could be mistaken for scrap! Circuit Operation Sunlight is converted to electrical energy by four devices – the two solar cells (in series) and the two infrared diodes (D1 & D2). The diodes produce very little electricity compared to the solar cells but this doesn’t matter: as long as they produce some, the comparator (U1a) can sense which one is producing the most. If IRD1 on the non-inverting input is producing the highest voltage, the output of U1a will be high. Conversely, if IRD2 is producing more MARCH 2001  45 The Sunflower circuit can be divided into three parts: the solar charger and voltage monitor (Q8); the sunlight direction sensing circuitry (IRD1 & 2, U1a & b) and the motor and driving circuitry (Q1-Q7). Parts List: Sunflower 1 PC board, 62 x 52mm, to be snapped apart (see text) 1 6-15V DC motor 1 artificial flower 1 nylon standoff or bush 1 short length brass wire 1 320mm length 38 gauge enamelled copper wire Semiconductors 2 2N3906 PNP transistors (Q1-Q2) 4 2N3904 NPN transistors (Q3-Q6) 1 MPSA12 NPN Darlington transistor (Q7) 1 34164-3 micropower undervoltage sensing IC (Q8) 1 TLC27L2 dual low power op amp (U1a, b) 2 infrared LEDs (IRD1, 2) 2 BP-37334 1.8V Solar Batteries Capacitors 1 1000µF 16VW PC mounting electrolytic Resistors (0.25W, 5%) 5 100kΩ (brown black yellow gold 1 220kΩ (red red yellow gold) The complete Sunflower kit is available from all Dick Smith Electronics stores, Cat K-3563, for $38.40 46  Silicon Chip voltage, the output of U1a will be low. A high output from U1a will forward bias Q6, which in turn forward biases both Q1 and Q4, turning them on. This allows current to flow through the motor, turning it in the forward direction. Conversely, a low output from U1a turns Q6 off. But it also forces the second comparator, U1b, to produce a high output, forward biasing Q5. In similar fashion, this turns on Q2 and Q3, allowing current to flow through the motor in the opposite direction –which obviously turns it the opposite way. Transistors Q1-Q4 form what is called an “H-bridge” controller for fairly obvious reasons! The length of time the motor turns on (in either direction) is governed by the amount of charge in the main storage capacitor, which in turn is determined by the amount of energy received from the solar cell. When the voltage across this capacitor reaches about 7V, the 34164 voltage sensor (Q8) turns on Q7, allowing current to flow from the H-bridge motor control circuitry. The drain of the motor fairly quickly discharges the capacitor, so once the voltage falls below about 5V Q8 turns off Q7, stopping the motor. The capacitor can then recharge from the solar cells. You have probably noticed that Q7 has a different symbol to the other transistors – in fact, it is two transistors inside one package. It’s called a “Darlington” transistor and has a higher gain than a normal transistor. Don’t mix this up with the other transistors – they all look the same in their TO-92 packages. Construction Before any assembly, we need to snap the PC board into two pieces – one piece holds most of the electronics while the other holds the solar cells and infrared diodes. The PC board is deeply scored where it needs to be broken, so it’s simply a matter of placing the score on a sharp corner (eg, the edge of a desk) and pushing down hard on the board edge – it should break apart very easily. Put the smaller piece to one side. Start the main board by soldering in the resistors, using the colour code guide to make sure you get the right ones in the right spots. Actually, it’s fairly difficult to make a mistake because all except one are 100kΩ. The odd one out (220kΩ) has red and yellow bands on it, whereas the 100kΩ have brown, black and yellow bands. When you snip the excess leads off the resistors on the back of the PC board, don’t throw away them away: Here’s how it all goes together. The four coloured wires in the layout at left (and the white wires below) are in fact the 38 gauge enamelled copper wire (we’ve shown them coloured for clarity). The printed circuit boards are supplied in one piece and must be snapped apart prior to construction. we’re going to need a few lengths of wire later. Next, solder in the 1000µF electrolytic capacitor – it is polarised, with a row of “–” symbols marking the negative lead. The PC board component overlay has the “+” lead marked. Now we move on to the semiconductors. First of all, insert the 8-pin IC in its position, making sure the notch on one end goes to the end marked with a notch on the PC board. Occasionally, you’ll find an IC without a notch but a painted or moulded mark or dot alongside pin 1 instead. ICs are usually soldered in hard down on the PC board. Insert and solder in Q7 in the position shown, after checking and double checking that you have the right one! A close-up view of the solar cells and infrared diodes, mounted on their own PC board. Again, the diodes should not be laid over – they should be pointing straight ahead. Likewise Q8 should be checked then soldered in, followed by Q1 and Q2, then Q3, Q4, Q5 and Q6. Transistors are normally soldered a little off the board – say about 5 to 10mm. The reason for this is that their long leads help keep them cool. The motor is next to go on: it is soldered onto the board “standing up”, with the stripe on the side of the motor going closest to the capacitor. Place the white plastic bush on the motor shaft so that it is about half-way on. It is too big to grip the motor shaft so you will probably need to place a couple of drops of glue on the shaft first (hot melt glue is ideal). But don’t fill the whole of the hole in because that’s where the flower and solar cell collectors go! Now we move on to the smaller board which you snapped off before. Solder the short length of heavy brass wire onto either of the two large holes in the centre of the small board so that it pokes out the back of the board (the side with no writing on it). Now solder in both infrared LEDs on the other side of this board, with their flat sides towards the bottom of the board. They should be about 10mm above the board, not hard MARCH 2001  47 This view of the back of the solar collector assembly also shows a different method of mounting the assembly to the motor: a length of thin brass tube slid over the motor shaft with the brass wire from the solar board soldered to this tube. In some ways this is a better method but will require you to source the tube. down on it. Using some of the resistor leads you cut off before, carefully solder four lengths to the “+” and “–” connections on the two solar cells. The two solar cells mount side-byside about 3 or 4mm apart and stick to the board with the double-sided foam pads already attached to the cells. Remove the backing paper from the cells then carefully push the “–” wire of the left cell and the “+” wire of the right cell through their appropriate holes on the board. When the cells are almost down on the board, align them with each other and then push them down so the foam pads stick. Carefully bend the “+” wire of the left cell and the “–” wire of the right cell back towards the PC board and 48  Silicon Chip solder them to their appropriate pads. Cut off all excess leads. The very fine wire in the kit is used to connect the solar cell PC board to the main PC board, giving plenty of flexibility and allowing it to turn. Note that the wire we used in the prototype is significantly thicker than the wire in the kit – we used this because you wouldn’t see the thin wire in a photograph! First cut the wire into four equal lengths, each 80mm long. The wire is insulated and we need to remove 5mm of insulation from each end. However, it’s rather difficult to remove insulation on wire you can hardly see! The easiest way is to burn it off using a cigarette lighter. But!!!!!! It’s very easy to melt the wire doing this, so be careful. Hold the wire in the blue portion of the flame for a very brief period only. You should be able to “wipe” the burnt insulation away with your thumb and forefinger, leaving bright copper coloured wire. Solder one end of each wire to the positions on the small board marked Sol+, Sol–, IR1 and IR2. The solar collector board is now finished and we move on to final assembly. First, bend the thick copper wire down 90° about 10mm out from the back of the board. The angle of the wire to the board should be such that the solar cells (and of course the board) is about 45°. Both this wire, and the wire “stem” of your sunflower poke into the hole in the top of the plastic bush. The two wires between them will probably be a fairly tight fit but if not, a drop or two of hot-melt glue will hold them in place. Angle the flower so that it aims the same way as the solar cells but not so that it covers them! Finally, solder the ends of the four very fine wires to their respective positions on the main PC board – Sol+, Sol–, R1 and R2. Your solar-powered sunflower is now finished. You’ll almost certainly find it does absolutely nothing indoors (unless you have direct sunlight streaming in a window!). Take it outside, though, and you should find the flower starts moving around, looking for the Sun. What it it doesn’t? Obviously, there’s a mistake somewhere. With your multimeter, check that you have output from the solar cells – probably several volts in direct sunlight. If so, check to see if there is voltage across the electrolytic capacitor and that the output of Q8 swings up and down. If you have output from the solar cells but nothing on the capacitor, the chances are one or more of the very fine wires are either broken or not soldered properly. Check that the output of U1a (pin 1) goes high or low as you cover and uncover each of the infrared diodes. If all these checks prove correct, the odds are that you have one or more of the transistors in the wrong place. SC It won’t work if you have!