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Build a Sun Tracker for Solar Panels The addition of a solar tracker to control a rotating frame can increase the daily energy output of a solar panel by 30% or more. This tracker frame is built around a 60W panel supplied by Dick Smith Electronics. 14  Silicon Chip This simple design will increase the daily output of your solar panels by around 30% or more. It was designed to suit a 12V 60W panel but it can be used without circuit modifications to control any size panel. By NENAD STOJADINOVIC Imagine the scene: an alternative energy expo with all sorts of nifty gadgets to make one’s life away from the power companies just a little easier. As I wander around, I can’t help noticing the large numbers of solar panels bolted immovably to their stands. “Surely they would deliver more power if they followed the Sun around?”, I ask. “Yes they would,” they answer, “but solar trackers are expensive items”. I checked around and let me tell you, they are not kidding. Everyone complains about the weather but nobody does any­ thing about it. Thinking these sage thoughts, I sat down at my desk and came up with a circuit that eventually evolved into the design you see before you. Tracker fundamentals When a solar panel is aimed directly at the Sun its output is at a maximum but for a fixed panel this only happens for a short time each day; before and after that the output drops off markedly. Ideally, the complete tracker would follow the Sun in both altitude and azimuth but that means two motor drive circuits would be required. It is more practical to just track from east to west (ie, altitude) and have a fixed azimuth which can be changed manually from time to time to account for the changing position of the Sun from summer to winter. Most pub- lished designs use this approach and this one is no exception. Not only should a tracker follow the Sun from east to west but at the end of each day the panel should be swung back to the east so that it faces the sunrise next day. Most published designs that we have seen do not do this. Apart from that, the tracker should incorporate limit switches so that the panel is not driven against the stops if a fault occurs. Finally, the tracker should only run intermittently, swinging the panel by just a few degrees from time to time and then consume very little power at other times. The circuit The design is based on an LM324 quad op amp where IC1a and IC1b are configured as a “window” comparator. A window comparator works on the principle that when the input to the two comparators is at the desired level, the output of both will be low Fig.1: the circuit is essentially a “window” comparator involving IC1a & IC1b. Whenever a shadow falls across LDR2, the output of IC1a goes high & the motor drives the panel westward until both LDRs are once again fully sunlit. +12V R1 10k 0V RE 4.7k LDR1 EAST ORP12 VR1 5k IC1a 3  1 D3 1N914 R8 VR2 10k 9 LDR3 ORP12  10 6 4 IC1c LM324 8 2 C1 4.7 11 3 LDR2 WEST 1 ORP12  IC1b E B VIEWED FROM BELOW C R9 D4 1N914 100k G D S D Q3 G S M 7 RM 2. 7  5W C2 0.1 D1 1N914 6 R2 10k C R5 100k D2 1N914 +4.8V 5 E 2xMTP3055 S 14 IC1d 13 RW 4.7k B D 12 8 IC2 555 Q2 G R7 22k A 7 R3 1k B 100k R10 1M Q1 S1 BC548 HG C E WEST +7.1V 2 S2 HG EAST R4 1k R6 100k Q4 D G S 2xMTP3055 D Q5 G S SOLAR TRACKER January 1995  15 Fig.2: the PC board is straightforward to assemble. Note that mercury switches S1 & S2 must be angled to set the limits on panel rotation, as detailed in setting-up procedure. Take care with component orientation. SHIELD LDR2 LDR2 S1 Q1 10k 4.7k 100k 1k D3 22k G D S VR2 LDR3 D1 D2 Q3 12V 0.1 1M 1 D4 G D S 2. 7  5W IC2 555 10k 100k IC1 LM324 VR1 Q2 1k 4.7uF G D S Q4 100k 4.7k 100k G D S Q5 S2 MOTOR The PC board has a blackened cardboard light shield placed between LDR1 & LDR2. Note that the mercury switches are fairly fragile & will break if roughly handled. Warning: do not handle mercury ­it is poisonous. (off). In this case the input voltage at point A (pins 3 & 6) is to remain between the upper and lower trigger voltages of say, 7.1V and 4.8V, re- spectively. If the voltage at pin 3 rises above 7.1V, the output of IC1a jumps to around 10.7V (on). Similarly, if the voltage on pin 6 falls to below 4.8V, the output of IC1b jumps to 10.7V (also on). Why a window comparator? Why not a simple feedback device? The answer lies in the zone of non-operation while the input voltage is in the window – the so called “dead zone”. A propor­ tional device will attempt to follow the Sun exactly and conse­quently the motor will always be either running or on the verge of running. The input voltage signal for both comparators is provided by two light dependent resistors, LDR1(E) and LDR2(W), connected in a resistive divider configuration. It can be seen that when equal sunlight falls on both LDRs they will have (roughly) equal resistances and the supply voltage will be dropped in two equal increments, leaving point A somewhere around +6V. We will pretend that RE and RW are not there for the moment. As the Sun moves west, the cardboard divider panel between the two LDRs throws a shadow onto LDR2(W), causing its resistance to rise, and so the RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 4 1 2 2 2 1 16  Silicon Chip Value 1MΩ 100kΩ 22kΩ 10kΩ 4.7kΩ 1kΩ 2.7Ω 5W 4-Band Code (1%) brown black green brown brown black yellow brown red red orange brown brown black orange brown yellow violet red brown brown black red brown not applicable 5-Band Code (1%) brown black black yellow brown brown black black orange brown red red black red brown brown black black red brown yellow violet black brown brown brown black black brown brown not applicable voltage at point A rises correspondingly. As soon as this voltage rises above +7.1V, IC1a switches on and drives the panel motor westward. LDR1E and IC1b together are there to drive the panel east­ward. Normally this function is not used since the Sun does not travel east, but is provided in case of whoopsies such as the family dog crashing into the panel. If the panel is moved too far west, a shadow falls on LDR1 and this causes point A to fall below +4.8V. This causes IC1b to switch on and drive the panel east until normal conditions are restored. The threshold voltages for IC1a and IC1b are set by R1, R2 and VR1 acting as a three-way voltage divider. Reducing the resistance of VR1 will bring the threshold voltages closer to­gether and thus reduce the size of the non-operation “window”. SC13101951 Fig.3: this is the full size etching pattern for the PC board. END OF SPINDLE THREADED TO ACCEPT MATCHING NUT ON CIRCUITRY CASE V-PULLEY H-pack output stage The drive side consists of a set of four Mosfets in an H-pack arrangement. A west signal from IC1a causes Q2 and Q5 to switch on and an east signal switches on Q3 and Q4. R5 and R6 are included to make sure that the Mosfets switch off. At the end of the day, rewind to the east is taken care of by IC1c which works as a simple comparator. As darkness falls, the resistance of LDR3 increases until the voltage on pin 9 reaches the voltage on pin 10. The reference voltage at pin 10 can be any arbitrary value above zero and so is tied to pin 5 of IC1b; ie, 4.8V. The actual darkness threshold is set by VR2. The output of IC1c is fed to a IC2, a 555 timer wired as a monostable with an ‘ON’ time of about 10 seconds. As the light faded it was found that shadows from trees and things would start to trigger the dark sensor but the west sensor would argue the point, resulting in the panel hunting back and forth. IC2 prev­ents this by fully rewinding the panel to east the moment the darkness sensor triggers. When IC2’s output goes high, it does two things. First, via diode D2, it switches on Q4 and Q3, driving the whole show eastward. Second, it causes IC1d’s output to go low and thus removes the base drive from Q1. This shuts off any possible drive to transistors Q2 and Q5, ensuring that the tracker will not follow the (D) SUB-FRAME (C) TELESCOPING PROP SECTION (B) PIVOTING PROP (E) SOLAR CELL CARRIER FRAME 12.7 PILLOW BLOCK (A) BASE (D) SUB-FRAME Fig.4: this diagram shows the tracker frame in perspective view. The dimensions of its various parts are given in Figs.5-8. Moon, car headlights, or other such light sources. The mercury switches are there to set the travel limits of the panel. S1(W) disconnects the gate signal to the west drivers as the panel tilts to its westerly limit; similarly, S2(E) cuts off gate signals to the east drivers as the panel tilts to its easterly limit. Some refinements The tracker will function with January 1995  17 12.7 DIAMETER SPINDLE WELDED TO FRAME END ON CENTRE LINE 12.7 DIAMETER SPINDLE WELDED TO FRAME END ON CENTRE LINE 140 232.5 85 555 232.5 25 x 2.5 FLAT IRON LUGS WELDED TO FRAME TO SUPPORT SOLAR CELL MODULE. POSITIONS AND DIMENSIONS MAY NEED ADJUSTMENT TO SUIT YOUR MODULE 305 305 1162 (E) SOLAR CELL CARRIER FRAME MATERIAL: 25.4 SQUARE MS TUBE 1.6 WALL THICKNESS ALL CORNERS WELDED DIMENSIONS IN MILLIMETRES HOLES 7 DIAMETER Fig.5: the solar cell carrier frame on the prototype was designed to suit a standard 60 watt panel from Dick Smith Eec­tronics (Cat. MSX-64). twitching back and forth but not quite running. It took some time to figure it out but this is the result of the com­ parators being switched on by a very slowly changing input. Comparators have extremely high gains. As the voltage on the input pin approaches that of the comparison pin, peaks of noise on the input will be amplified tremendously and cause the comparator to rapidly switch on and off. Unfortunately, the driver transistors will do the same with consequent wastage of power. The solution is to 30 just these components but is not as efficient as it could be. I found a few small modifica­tions reduced current consumption dramatically – never a bad thing with alternate energy. The first mod came about when I bought some LDRs that had a very low resistance in bright sunlight. It seems that not all ORP12 equivalents are the same. The addition of RE and RW reduced the current through the LDRs and kept them a lot cooler to boot. The second mod came about when I noticed the drive motor rapidly 30 25 27 10 703 (B) PIVOTING PROP MATERIAL: 30 SQUARE MS TUBE 2 WALL THICKNESS Building the tracker 45 97 107 85 104 85 707 106 53 25.4 45ø x 6mm CHAMFER 12 (C) TELESCOPING PROP SECTION MATERIAL: 25.4 SQUARE MS TUBE 1.6 WALL THICKNESS DIMENSIONS IN MILLIMETRES ALL HOLES 7 DIAMETER Fig.6: construction details for the pivoting prop & telescoping prop sections. 18  Silicon Chip introduce some hysteresis. On IC1a, the first instant the comparator switches on causes a high signal to travel through D3 and R8 to the junction of the LDRs. The nett effect is to cause the input on pin 3 to suddenly rise about 0.4V, thus well and truly turning the comparator on. The same system is used on IC1b via D5 and R9. The hystere­sis circuit also causes IC1a to turn off a trifle later then it otherwise would, thus causing the panel to turn a bit further before the motor switches off. This is a handy thing in prevent­ing the input voltage from sticking too near the upper limit. (Note: the current drain of the tracker is around 10mA when the panel is stationary, rising to about 1.5A when the motor is running. Most of the 10mA quiescent current can be attributed to the 555 timer and this could be reduced to under 2mA by using a CMOS 555 (ie, a 7555). The circuit for the solar tracker is wired on a PC board measuring 125 x 73mm and coded 13101951. There is nothing at all tricky about the board assembly. Make sure that the diodes are all in the right way around or strange things will happen. Use sockets for the two ICs but don’t install them just yet. PARTS LIST 2.5 87 TWO PIECES OF 25 x 2.5 FLAT IRON, 87 LONG WELDED TO TOP OF UPRIGHTS TO SUPPORT PILLOW BLOCKS. HOLES DRILLED TO MATCH PILLOW BLOCKS 25.4 25.4 25.4 HINGE TANG WELDED TO UNDERSIDE 25.4 TWO PIECES OF 25 x 2.5 FLAT IRON WELDED TO BOTTOM OF HORIZONTAL FOR PIVOT 1220 (D) SUB FRAME DIMENSIONS IN MILLIMETRES HOLE DIAMETER 7 Fig.7: this diagram shows the dimensions of the frame pivot support. 25.4 Miscellaneous Hookup wire, solder, blackened cardboard for LDR shield. Testing Before you power up the board, remove the ICs from their sockets and make sure that your power is supplied via a 2A fuse. Alternately, use a current limited supply if you have one. Hook up the power and make sure that the correct voltage is going to pins 4 & 11 of the IC1 socket and pins 1 & 8 25.4 SQUARE MS TUBE WALL THICKNESS 1.6 HINGE TANG WELDED TO BOTTOM END OF SUB FRAME 505 Be careful with the Mosfets. These little fellows are really rugged once in the circuit but are easily damaged when being handled beforehand. Do not ever touch the pins with your fingers and if they come wrapped in foil, just peel a bit back around the pins and solder them in like that. The villain is static electricity and standard precautions include grounding yourself and your soldering iron, etc. 25.4 225 x 140 T-HINGE CUT TO CLEAR SQUARE TUBE AND WELDED TO ANGLE IRON 32 Resistors (0.25W, 1%) 1 1MΩ 2 4.7kΩ 4 100kΩ 2 1kΩ 1 22kΩ 1 2.7Ω 5W 2 10kΩ 15 Capacitors 1 4.7µF 16VW electrolytic 1 0.1µF monolithic HORIZONTAL AND TWO UPRIGHTS 25.4 SQUARE MS TUBE WITH 1.6 WALL THICKNESS UPRIGHTS WELDED TO ENDS OF HORIZONTAL 8 Semiconductors 1 LM324 quad op amp (IC1) 1 NE555 timer (IC2) 5 1N4148 diodes (D1-D5) 4 MTP3055 Mosfets (Q2-Q5) 1 BC548 NPN transistor (Q1) 3 ORP12 light dependent resistors (LDR1,2,3) 305 1 photovoltaic solar panel (see text) 1 tracker frame to suit panel 1 3V barbecue spit motor 1 weatherproof box with transparent lid 2 pillow blocks 2 V-belt pulleys 1 V-belt to match pulleys 1 4-way insulated terminal block 2 mercury switches (S1,S2) 1 8-pin IC socket 1 14-pin IC socket 1 5kΩ trimpot (VR1) 1 10kΩ trimpot (VR2) 2 OFF 25.4 x 25.4 x 3 ANGLE IRON WELDED TO SQUARE TUBE (A) BASE DIMENSIONS IN MILLIMETRES HOLE DIAMETER 7 25.4 1250 25.4 Fig.8: the hinge details for the frame pivot support. of the IC2 sock­ et. Measure around the various other pins, especially the driver transistors, for the same reason. When finished, power down and put the ICs in. The circuit falls naturally into west and east drives, and so that’s the way testing proceeds. Power up and measure the voltage on pin 3 or 6; it should be about +6V. Place your finger alternately over LDR2(W) and LDR1(E) and the voltage should swing up and down in unison. It seems logical that putting one’s January 1995  19 This view shows the tracker frame with the solar panel removed to reveal the barbecue spit motor which is driven by the tracker circuitry. finger over LDR2(W) should result in the tracker going west. Try it; pin 7 of IC1b should immediately jump to around +10.5V (high) while pin 1 stays low and vice versa. Put your finger on LDR3 and confirm that pin 14 jumps to the same high level and then low again when uncovered – you may need to adjust VR2. Note that once IC2 seizes control of the system it doesn’t let go for about 10 seconds and that S2(E) will stay high during that time. If you want to disable IC2 while you check other parts of the circuit, short out LDR3 with an alligator clip. If all is well, hook up a motor to the output pins. Covering LDR2(W) will now drive the motor one way (mark it on the motor) and LDR1(E) the other. 20  Silicon Chip LDR3 will always drive the motor east, regardless of any other LDR you have covered. The mechanicals Being primarily a mechanical engineer, I decided to try doing a little better than some of the jury rigged trackers I had seen around. The tracker illustrated in the diagrams was designed to suit a standard 60 watt panel from Dick Smith Eec­tronics (Cat. MSX-64; price $599.) This is encased in a welded frame which, in turn, is mounted on pillow blocks so that it can be rotated. The stand is hinged at its base, allowing the panel to be raised and lowered to account for seasonal variations in the Sun’s altitude. The PC board was mounted in The tracker circuitry is mounted in a weatherproof plastic box with a transparent lid. The box is mounted on the same shaft which drives the solar panel via a V-belt. a weatherproof plastic case with a transparent lid. A blackened piece of cardboard is used as a light shield between LDR1 and LDR2 so that as the Sun moves across the sky, it repeatedly throws a shadow across LDR2(W). The weatherproof box then is mounted at the end of a belt-driven shaft and in the same plane as the solar panel, as shown in the photos. Motive power was a problem, with the commonly available windscreen wiper motors being found to have not enough torque and too much power. The best solution comes in the form of a cute little spit motor from the local barbecue place. The speed is low, torque is high and they’re cheap. The only glitch is that the actual motor is a 3V unit while the Mosfets put out about 7.5V. The answer is to use a current limiting resistor but the value has to be chosen carefully. Too much and starting torque ends up too low, while too little fries the motor. Around 2.7Ω is about right. Commissioning If everything has checked out on the bench, commissioning should be fairly simple. Point the LDR’s directly into the Sun and turn on the power. The PC board is mounted in the weatherproof box so that when the frame is rotating from east to west, LDR2 will have a shadow cast upon it as the Sun moves further west. Nudge the panel so that LDR2(W) is about half shaded and the motor should immediately scream to life. If it doesn’t, carefully turn VR1 clockwise until it does. Switch off the power and wait until sunset. Darkness level is set by VR2; start with it fully anti­clockwise. When it’s reasonably dark, adjust VR2 until the motor starts up. Watch that the panel frame does not hit the stand at the end of its travel. At the fully rewound position, check that S2(E) has switched off. If not, gently bend the glass case down until the mercury falls from the contacts. Do the same with S1(W) for the westerly limit. There are two things to be aware of before you sit back and enjoy watching your panel do its sunflower routine. First, putting a plastic lid over your LDRs can alter their light level settings and you may need to make some fine adjustments. Second­ , overcast days are bad news as there is no clear target for the LDRs. Instead, the tracker will orient on any patch of lighter sky it sees, meaning it spends all day winding itself back and forth. In that case, it is best to switch it off. (Editor’s note: as an alternative to turning the tracker off on overcast days, the output of IC1b could be disabled to prevent the unit from tracking east. This is easily accomplished by placing a toggle switch in series with SC diode D1). Is A Solar Tracker Worthwhile Having? In presenting this article on a solar tracker we should comment on whether it is worthwhile for all solar panels. In our opinion, it is probably not worthwhile for panels rated at less than about 18 watts. This is because the cost of the tracker itself, which could be $100 or more, depending on how much of it you build yourself, has to be added to the overall cost of the installation. Clearly, if you add $100 to the cost of a 10 watt panel, you could buy an 18 watt panel (using Dick Smith Electronics catalog prices as a guide) and thereby increase the output by 80%. However, for the bigger panels the use of a tracking mechan­ism is clearly worthwhile. January 1995  21