Introduction: $2 Sun Tracker Circuit - Healthy Discussion!

Picture of $2 Sun Tracker Circuit - Healthy Discussion!

My apologies to any of the 1400 or so people who've read this Instructable in the day since it was published; I described an idea I was sure would work, and I was so excited to get it out there where people could try it, that I didn't wait until I could get the parts I needed to fully test it myself, and I posted based on a partial test and an assumption about the performance of small motors that turned out not to be accurate.

Now normally I would take a faulty Instructable down (and run away and hide), but in this one instance I'm going to leave this online after editing it to correct my mistakes, because I thought that there was still a good idea here that could be developed even if it had to be by someone with more electronics experience than me, and I wanted to put this germ of an idea out there so that maybe it could inspire someone to come up with a solution that works.  I offered the 1Yr pro membership upgrade I received when this went front-page as a prize, and as you can see from the comments a lively discussion ensued.  (I gave the prize to jtlowe for his suggestion of using a clockwork rotating platform, but although there's no more prizes to give, I would still welcome any more suggestions you all can offer).

The problem is sun tracking: pointing a solar panel directly at the sun so that it can harvest significantly more light - and the difficult aspect of the problem is that the cost of adding a sun tracker to a solar panel in order to gain X% extra output has to be less than X% extra cost, otherwise it's more cost effective to simply add more solar panels.

The solution that I had was to take two small PV cells from a couple of solar garden lights, and connect them not in series or even in parallel, but head to head, connecting the ground lead from one to the ground lead from the other, and determining which of the two panels was receiving more light by looking at which one was able to generate more voltage than the other.  For instance if one generated 2V and the other generated 3V, then the voltage between the two positive outputs would be 1 volt, and that volt would be used to drive a DC motor to turn the platform in the direction of the cell that was reading the stronger light signal.  (Although in practice the voltage was actually less because driving current through a PV cell against its natural direction (since it acts a little like a diode) causes a voltage drop in excess of just the voltage that would be cancelled out, though that's not relevant to the problem)

Although that is indeed what happens which you can confirm by looking at the voltage on a voltmeter, what I didn't realize was that these panels don't generate enough amperage to drive even the smallest motor, as far as I can tell.  I'm talking about 3V motors that need so little power that they'll spin from a single half-dead AA battery!

So what I'm going to show you here is half a solution, and I'm asking the smart readers of Instructables to help come up with the other half.

Step 1: What You'll Need

Grab a couple of those cheap garden path solar lights, the kind they sell at the Dollar Store for... well, a dollar.  (If you miss the occasional Dollar Store deal on those, damn, you'll probably be out of pocket by two dollars from a more expensive store! :-) )

That's it - to test the circuit you won't need any more components.  Just your trusty volt meter, and a soldering iron...  (I finally treated myself to a programmable temperature controlled iron from Radio Shack and I have to say I'm really enjoying using it compared to what I had before.  The volt meter is that cheap one that's on sale for a couple of dollars every weekend at Harbor Freight)

When I was trying to drive the motor and realised I didn't have enough power I added a second pair of PV cells in parallel, so some of the images below have 4 cells and some have 2.   As long as we're just using these as light sensors, 2 will do fine - I didn't feel it was necessary to reshoot the photos...

Step 2: Here's the Clever Part...

Picture of Here's the Clever Part...

Unscrew the top of the light and pull the battery terminals from the plastic case.  With the cheapest lights, you need to pull the lugs on the battery terminals back a little before you can pull the AAA battery out; more expensive lights have a more accessible AA battery holder but it may not be as easy to pull the battery terminals out of the holder.

One way of building this circuit is to leave the circuit board in place and use the wires connected to the battery pads - in this version, it's easiest just to pull both negative battery pads from each light and solder them together.  On my lights, these wires are black (ground) and yellow (positive).

(The circuit board is a "Joule Thief" circuit.  It scavenges otherwise unusable low voltages and then sends a burst of higher voltage to charge the battery.  So the output from that circuit is either zero or 1.5V or higher.)

However it's probably better that you clip off the red and black wires coming directly from the PV cells, maybe a centimeter short of the circuit board (in case you want a little wire left to make subsequent experiments with that board easier?) and use the output of the PV cells directly, which depending on the strength of the incident light will range from 0 to about 3V,

In either case, connect the black wires from both cells.  (<--- yeah, that was the clever part.  Not a lot to it!)

Step 3: Test It!

Picture of Test It!

Connect your voltmeter to the other two wires - the red ones from the PV cells or the yellow ones from the Joule Thief circuit boards,

Now either test this in the sun or use a flashlight .  Compare the voltage when the light shines evenly on both photo-cells versus when it shines on each of them one at a time.

In my first test I got 1.5V when illuminating the left-hand cell, and MINUS 1.2V when illuminating the right.  The voltage went close to zero when they were equally illuminated.  The discrepancy turned out to be from one poorly performing PV cell so I swapped it out with another one I swiped from my garden and then they both output almost identical voltages for the same illumination.

(The raw output directly from the good PV cells was about 3.4V and from the bad one, 2.5V.)

Step 4: Outdoor Test

Picture of Outdoor Test

Here I've connected the circuit to a voltmeter (as I mentioned earlier, I doubled up the cells to get more power, but it didn't really help) and took 5 readings, from well left of the sun, through half as far left, then straight on, then the same angles to the right of the sun,  You can see from the readings on the voltmeter that this is a good indication of which way you need to turn and when you're pointing in the right direction.

(You'll note that the absolute values of the signals are pretty low - even when one sensor was blacked out completely and the other was in full South Texas sunlight, the best differential I've seen was 1.65V.)

Step 5: Now I Need Your Help to Control a Motor With This Circuit!

Picture of Now I Need Your Help to Control a Motor With This Circuit!

Now, I had hoped that if you feed this DC signal into a motor, it would spin one way or the other or not at all depending on whether the incoming voltage was positive or negative. (DC gearmotors are reversible by reversing the input polarity).  This motor would rotate the platform holding the PV cell sensors as well as a larger solar panel such as a USB phone charger, and when it rotated enough to face the sun, the outputs from the two PV cells would balance and the motor would halt due to receiving 0V.

But unfortunately that plan didn't work because the PV cells didn't generate enough current to drive the motor.

I have some ideas about how we can salvage this part of the design which I explain in the next section, but this failure does make the system more complicated and far less cost effective.  (Unless one of our readers can come up with a solution that will save the day?)

The best way to use the two PV cells as a direction finder is to exaggerate the amount of light falling on one versus the other when the cells are side by side but not pointing directly at the sun.  The standard way to do this is to put some sort of divider between them like a piece of card, pointing forwards, which casts a shadow when the light source is to one side but which has no relevant shadow when the light is directly in front of the equipment.

A subtle improvement over this is to use a double-sided mirror.  This has the advantage that more light is caught by the cell that is facing the sun, which may be a factor in the early morning when the sun is just coming up, especially if we can also find some way for these cells to provide enough juice to drive the motor that rotates the platform as well.

When this apparatus points in the direction of the sun, the light falling on each cell is equally strong and the voltages generated by each cell cancel out, so you'll see 0V on your meter.  Turn the meter left and right and you'll see the voltage go up or go below zero as in the photos attached.

By the way, I can't take all the credit for this idea - I based it on the underlying principle behind a circuit that a friend designed back in the 70's: he used a couple of infra-red sensitive photo-transistors tied emitter-to-emitter to detect a person's body heat, in order to turn a model skull (at least I hope it was a model...) so that it would follow people as they walked through his room!  Basically a Halloween Haunt trick way ahead of its time.  Where-ever you are now, Steve McGloin, that was a brilliant idea you had!  This hack uses the same principle, of pitting two current sources head to head and letting the stronger signal win.

Step 6: Hack a TrekBot Toy for Motor Parts?

Picture of Hack a TrekBot Toy for Motor Parts?

(The picture for this section is just a mock up of a USB charger panel on a platform with the two PV sensors.  I considered making a rotating platform from a CD stack case; I also looked at using a Lazy Susan cake stand.  But until we work out a way to drive the analog DC motor cheaply, there's no point in wasting time working on the platform.)

I mentioned that initially the two cells I used were mismatched. One generated about 2.5V max voltage rather than the other's 3.4V, so the circuit was far from 0 when the illumination was balanced.  ideally you would pick two cells that matched better, but... it doesn't actually matter that much, because (assuming you have removed the Joule Thief circuit that's trying to regulate the charging voltage) your motor and platform will still turn until the two outputs are balanced.  Although your cells may not be pointing directly at the light source, you can skew the platform to a slight angle relative to the sensor cells so that what is on it points directly at the light once the platform has stopped rotating - this is the beauty of negative feedback analog control systems!

What I've described here would just turn your solar panel platform east or west in the direction of the sun - but not up or down as the sun arcs across the sky during the day.  However you do not need a second motor to follow the changing elevation - if you build what's called a 'polar mount', you can use a passive physical mechanism to change the angle of tilt rather than any sort of powered motor.  (Attaching the sensors to the mount along with your primary panel, so that they are also tilted to the optimal azimuth wouldn't hurt either, but isn't necessary - they're quite forgiving)

Finally - I do have an idea how to resurrect the idea of driving a motor from the PV cells (rather than stealing power from the larger 'payload' panel that the platform would be supporting): I have a $10 toy that I bought from Woot for reuse as parts - a "TrekBot" which contains a couple of motors and some  good gearing.  It charges up from a handheld controller that contains three button  cell batteries - I suspect it has an internal super-capacitor rather than a battery.  The wheels have enough torque that I was able to spin a Lazy Susan just by friction-driving the wheel against the rim of the platform.  If we could make the PV cells charge up something like this, then switch to voltage-comparator mode once it was charged up, it could adjust the platform's orientation in a more bursty manner than the original continuously-updating design.  This level of analog design is probably beyond me, and I really don't want to use a PIC in this device, even if it is possible to buy the bare chips for a couple of dollars.  It just strikes me as inelegant :-)  But I may end up going that route if it's the only way to do this on cheap.  At the moment, the rotating platform looks to be the most expensive component.

Actually that's not quite true.  The USB charger I have - made by Soldius - appears to cost around $100 nowadays.  I swear I got it for a fraction of that some years ago, though I forget now how much exactly,  Maybe $20.  But if this solar tracker ends up costing $20, it'll be a failure.  We need to come in around $5 to be worth the effort.

That's as much as I have to contribute.  I hope you can use this as part of a new tracker design.  Good luck!



mikesnyd (author)2014-01-13

Could you throw in a small capacitor bank to run the motor? The motor isn't constanty running so the small panels would be able to keep a good enough charge in the bank.

gtoal (author)mikesnyd2014-01-13

The trekbot toys use a supercapacitor rather than a battery to store their charge, so they're definitely good enough to use as you suggest.

threewingwonder (author)2012-08-18

I always thought that if you connected the black wires together that the output fron the corresponding cells would be positive. The only way I can see getting a positive and a negative output is connecting the positive to a negative,thus giving you a positive output and a negative output. Love the instructable! I have been trying to find a simple solar tracker for my homemade solar panels, and this would fit the bill nicely. Maybe feed the 2 outputs to a camparator or even a couple of OpAmps then connect that to the motors via driving transistors. I dont know, but will post a instructable when I do figure it out!

gtoal (author)2012-07-03

I found out an interesting fact today, You can use an LED as a light sensor, especially the green ones. I have an old strip of Christmas lights that I cut up last year (I was converting them from 110V AC to 12V DC) - I put a green LED on my voltmeter on the lowest voltage scale and it does indeed reat to light, You'll either need an op-amp to am[lify the signal, or hook it up to an ADC (say in an Arduino) and add up several samples to reduce the noise.

This appears to be a fairly well documented technique (even here in Instructables) but it was new to me.

scraptopower (author)2012-07-01

Have a look at my "hard drive solar tracker" it is an adaption of this method and can drive pretty much any motor you wish.

I've also got a transistor version, not sure if I posted it on my website though.

The parts will only cost around £8 if you get the dead HDD for free.

gtoal (author)scraptopower2012-07-02

I do certainly have no shortage of dead drives, that's for sure!

It's a nice simple circuit, but are you powering it from the panels that you're shining the light on or from that DC power supply I can see off to one side? :-)

I may build this for the fun of it, but the biggest thing I still want to implement is a cheap way to power the rotation (or control it if unpowered) using no more than the small PV cells. i think your design may need more power than I have spare.

Thanks for the link, your tracker works nicely.

scraptopower (author)gtoal2012-07-02

I see, I was intending on running off the solar panel((20W) it will track, but I don't have a way to run it without a power source.

gtoal (author)2012-06-18

I have a one-year free pro membership available which I'll give to the person who comes up with the most elegant and cheapest way to use this circuit in an actual tracker!

gtoal (author)gtoal2012-06-25

Well, comments have finally trickled to a halt and this seems like a good time to award the 1Yr pro upgrade that I promised.

I was hoping someone would actually build something and try out their ideas, and if they had, that would have made the selection of a winner much easier!

However many good ideas were suggested and chosing a winner was tough - for example I thought Wroger-Wroger had a good grasp of the big picture; alarrrd and jtlowe made low-tech analog suggestions in the sprit of the instructable; perfo, eecharlie, and others contributed valuable detailed information on electronics that would definitely help in a scaled up version of this project. Every commenter had something helpful to say - there really were no bad ideas here.

In the end I decided to give the prize to jtlowe for his suggestion of using a wind-up clockwork mechanism, which was at the level of complexity that I was looking for and is what I'm going to work on next to improve the design.

Congratulations JT! I'll email you the code.

msuzuki777 (author)gtoal2012-06-21

I would certainly agree with you that you don't have enough current to drive a motor. But I would say more importantly, you don't have enough power (voltage times current).
However, if you are using solar lights you have a readily available power source, the batteries.
And another thing is that all the solar lights I've seen have another sensor to turn off the lights during the day so that the batteries will charge more. I don't know if these are phototransistors or LDSs but I would think this would be a better source of your 'light' differential signal. So what I would try would be to run the differential signal into a transistor for gain. The transistor is powered by the battery or batteries and then you would have enough power to drive the motor. Since the motor isn't going to be moving that much, you should theoretically have your efficiency. Now you may have to connect two in series to get enough voltage for the motor.
Now, I'm not an analog guy or a motor guy but I think this would have a better chance of working.


gtoal (author)msuzuki7772012-06-21

The cheaper garden lights use the PV cell to determine the light level rather than using a separate sensor. I doubt that the part of the circuit that switches the LED off it draws much current.

I'm wondering if the pulse of power from the coil in the Joule Thief circuit is enough to turn the motor a little - it would be more like a stepper motor if it worked, as long as it didn't drive it too far past the optimum position.

msuzuki777 (author)gtoal2012-06-21

The Joule Thief cannot add more energy than is already there.

Here's my suggestion. Take out the output LEDs from the solar lights, Leave the rechargeable batteries in. They will charge up to maybe 1.4V. Get a low voltage comparator. I know some can be powered by less than 1V. Power the comparator with one of the batteries. Now take the PV output from the two solar lights. This voltage is going to be higher than the battery so divide it down with a voltage dividers. Tie the two voltages to the comparator. Add hysteresis so the motor will stop when you get close. Now you know that the direction of the sun goes in only one direction so the motor needs to move in only one direction. Drive the output of the comparator to the motor. You might need a transistor to give it enough current but the battery should have any capacity to turn the motor especially since the duty cycle for the motor will be so low.

Now if this isn't enough power, you could tie two solar lights in series so the battery voltages would be about 2.8V.

By the way a cheaper solar light is the $1 solar light keychains sold on ebay. They actually put out 3 volts.


gtoal (author)msuzuki7772012-06-22

"The Joule Thief cannot add more energy than is already there."

maybe what I said didn't make sense out of context so let me explain what I was thinking: the motors need a higher current to get started turning than they do to keep turning. I know that the power output from the PV cells isn't enough to start the motors turning, but I was hypothesizing that if you collected the output and sent it all at once in a short pulse, it *might* be enough to overcome that initial inertia and turn the motor a little. I was also guessing it wouldn't turn far and wouldn't have enough duration to keep it turning with the risk of overshooting the target. Again - all assumptions, but ones I plan to test on Saturday morning in the heat of the day :-)

Now ideally I would suggest using a capacitor to build up charge, but since we have the JT circuit already - came with the $1 unit that the PV cells came in - we might as well reuse that if it would work. From what I read in another instructable, it looks like JT uses the collapsing field of a coil to generate a large pulse - at a voltage sufficient to charge a 1.2v battery, so it is plausible it would be enough to kick over a gearmotor since a AAA cell alone can do it even when the AAA cell is almost depleted.

I'll give it a try and report back. Chances are it won't work, but it's definitely worth the effort to try.

msuzuki777 (author)gtoal2012-06-23

Yes, the JT circuit doesn't add energy but it will concentrate more in a pulse. I doubt it will work also.
But instead of adding a capacitor, you could probably add the battery as it's already part of the $1 cost anyway. The battery is going to act just like a capacitor.
By the way, we only have a Dollar Tree out here but I've never seen a solar light in there. Maybe I'm looking in the wrong place.

Good Luck.


gtoal (author)msuzuki7772012-06-23

The $1 lights at the Dollar Tree are only an occasional item. I bought enough to line my whole garden last time they were here :-)  Apparently you can order online and get a case shipped to your local store.

msuzuki777 (author)gtoal2012-06-23

I will keep an eye out for them. Personally, I like the $0.99 solar keychain as they have 3V batteries and include 3LEDs. I bought some on Newegg but they're cheaper on ebay.
While this is an interesting theoretical project, I question the practical application. As you said it's not very practical outdoors.
The problems I see indoors are first, you probably need an unobstructed south facing window. Even with that I don't think there would be much tracking involved.
I have a south facing window but when the sun is out, I always have my blinds down and there ain't no room for a moving panel.
I would think a slightly larger panel would cover any charging panel.
Other questions, how much is solar charging a cellphone going to save over plugging it in.
By the way, I charge my solar keychains under a lamp. Well, these solar lights charge under room lighting? Guess I've never tried it.

I guess if you're going 'Green' I would say a bigger solar panel is more efficient as you are not wasting energy moving the panels.

A question, however. Since a solar panel is taking energy from the sun, will it be cooler? In theory, I would think that since some of the energy is being converted to electricity, there would be less heat.


Steve-Abx (author)gtoal2012-06-21

There are a couple of tricks to this - I have designed commercial units for larger solar collectors.
Here are a couple of the tricks.

Your PV cells should be polycrystalline with a resistor load, so that they work as solar power sensors not solar energy sensors. A mono crystalline cells needs full light cover to generate a poly- doesn't. Measure the voltage across the PV cell to measure exposure to the sun.

This will work for adjusting alignment but you need to turn back east when it gets dark (night time).

To save power only turn east-west.

Use a polar mount to get your panels roughly parallel to the earth's axis. Dont bother adjusting the north- south alignment, there isn't enough to gain.

I hope this helps

Consider My Solution (author)2012-06-21

It's not only very clever, but if you had done some more research you would have found that it's very common. Not dissing the effort, though.

Actually, if you put your mind to the problem and get away from the solution, you will come to conclude that the traverse we make around the sun is quite regular.

All you need to "track" the sun for a fixed solar array is a timer and some small adjustment as the seasons change.

Now, if you want some sort of "robotic" style sun tracker, then you'll probably need more than $2 worth of parts.

Keep trying.

True you can just pick up the phone with credit card in hand and have an all singing all dancing solar array auto tracking in a jiffy. But then where's the fun in that. This post is in the spirit of the site and that is doing stuff or at least trying with hack able components and investigating different ways to do it.

True but there are a lot of projects put on here just to win a prize offered. Some are the equivalent of cutting out paper dolls, gluing on a resistor for a hand and taping 9v batteries on for feet! "Look what I did! An electronic robot!"

Even this author clearly states that he posted a project that he hadn't really even built ("...I described an idea I was sure would work, and I was so excited to get it out there where people could try it, that I didn't wait until I could get the parts I needed to fully test it myself, and I posted based on a partial test..."), but he still won the prize. Projects here are a lot about winning prizes. You just have to sort out the junk.

I applaud this author for admitting his mistake and posting the retraction.

Appreciate the comment, thank you. I don't mind looking like an idiot occasionally though I try not to make a habit of it :-) It is weird what gets popular and what languishes. I actually find it a little embarassing (and frustrating) that this post about a 30-minute hack has been the most popular one I've ever made, yet the post I did about CNC art which is actually something original and reflected maybe 6 months of hard work has gone virtually unnoticed :-) Still, this has sparked a lively discussion and I'm really pleased to see that. I actually was able to use this discussion at the university where I work yesterday as an example of the design process - building on your mistakes and all - to a visiting class of high school kids that we were trying to persuade to become engineering students next year.

Well, if looks were important then I'd be in trouble all the time.

I've been vilified on here at times, because I can't always find a way to "be nice" with some of these projects. Criticism is not being un-nice, but can be quite constructive. I got flamed for pointing out that one project was based on a stolen shopping cart. Many argued that the store that owned it was OK with it being converted to something else for the "author." Why present projects that teach the wrong thing?

You take criticism well and must be a good instructor to those around you. Thanks for that!

Point taken I can see what you mean about alteria motive. I don't think they apply in this case but I can see how they could with others.

Actually I confess that I am deliberately doing exactly what you criticise me of - working back from a solution in search of a problem. Because as you point out there is indeed a lot of prior art and just about everything has been thought of before. My goal isn't specifically to build a solar tracker - if that's all I wanted I'd follow one of the existing Instructables. My goal is to see if the one original idea I had here is something that can be exploited somehow. The very minor intuitiion that you could use using two $1 solar garden lights to determine the direction of a lighty source is the only original thing I lay claim to here, and what I'm hoping is that there *may* be a way to exploit it that is cheap and easy, because the other trackers I've looked at (and I've read about many of them) are generally either not cheap in component terms or in terms of man hours it takes to build them.

The basic rule of thumb I'm exploring here is whether it's cheaper to add tracking or to add more solar panels. For huge panels it's a no-brainer. For medium panels it's borderline. For small ones like cell phone chargers, I'll be interested if you can show me *any* existing tracker designs that give you a bigger win than just adding another panel for the cost. Honestly the only cheap efficiency improvements I can think of that are ways of concentrating the sunlight by mirrors or lenses but those are bulky and fine for outdoors but not so practical for say behind an office window.

gaiatechnician (author)gtoal2012-06-21

If your panels are ground based, it should be cheaper to add tracking (in summer.)
But in winter the sun is low in the sky and (here it is cloudy) so it doesn't much matter where the panel points. It will perform badly no matter what!
There is a helpful site called sollumis that shows where the light comes from all day. If you track well, you must beat a stationary panel by a long ways.
I heard an interview (I thiink with the guy from red rock). He claims that the statistics for cloudy days and sunny days are just averaged over wide areas and you local climate is far more important than you would think. If you are in a sunny area, work at tracking. If you are cloudy, it might not be worth it.

gtoal (author)gaiatechnician2012-06-21

I live near McAllen Texas. We don't have a Winter here :-)

No offence taken, after all my profile says "Favorite activity: reinventing the wheel" :-)

One thing that's cool about an active follower rather than a fixed path tracker is that it works fairly well indoors for pointing at the indirect illumination through your window. And it's cool for demos where you can make it follow a flashlight :-)

Chiana_Rei (author)2012-06-24

If your only problem is generating enough voltage why not just wire in a small capacitor, or bank of capacitors that the small solar panel will keep charged and and the motor will turn slowly, as you have quite a few hours of slow very tiny movement. Then set your light following sensor right in the middle of the panel or use 4 one on each side to get 4 values and the motor turns towards the two with the highest value. and stops turning when the value drops, all you need is a way to read 4 values and come up with directional values to send to a motor driver circuit on a stepper motor. Alternately if pins are an issue then you could do the same thing with just two sensors and have the panel turn till the values across the two are the same. Then just use one panel as the master position finder and the rest have a driver circuit that gets the the turn command then you only have t create one sensor circuit. Ill have to look about but I think the easiest light seeking circuit I have ever built was the one from Mousey the Junkbot out of Make, you might even be able to use an IR mouse to build it and save some money if you can get eh values off of it, maybe using a USB to RS232 thingie. and I have now exhausted all the electronics design knowledge I might have.

jtlowe (author)2012-06-23

It sems to me that we are making htis way to complicated. Because the sun with be in a fixed position per each moment of time on a clock, why have a tracking system when you can use an old wind up clock and a gear mechanism instead? The only draw back is the return 12 hours while the sun is down and the winding the clock regularly.
Winding may be driven by a wind operated device.
If it is turning a gear against a pring force, then a reset switch can be activted onlce it reaches a set position and return the mechanism to its start postion waiting for another switch to engage the start of the days tracking activity.
Perhaps this too is too simple an idea as clock mechanisms do not move that large of a mass.

perfo (author)2012-06-23

Why not? This is an experiment that may or may not work so what is wrong with setting one of your overall design goals as cost? It isn't the best approach for a lot of good project designs I agree but that's the fun of ibles like this. The author sets his own goals no matter how relevant others think of them, and then tries to achieve it.
As for your adage "in life you get what you pay for" if only that where true. You have to work like a (insert something that is renowned for working a lot in here) just to get something anywhere near what you pay for as most manufactures sell for what they can get not what it's worth which can be the same thing but can also be very different.

kubulai (author)2012-06-22

Dude. I'd just feed the output of the two opposing photovoltaic cells into +/- inputs on a cheap op amp (via resisters), and run the output of the op amp to a transistor to drive the motor. It's a good idea. A LM741 op amp is cheap -- a few cents.

gtoal (author)kubulai2012-06-23

Cool. That's In1+ and In1- taken care of. Now,where does Vcc and Vee come from?

snobound2 (author)2012-06-22

If I read this right you want to use this circuit/device to point a larger array of solar cells. If that is the case, then scale the idea up a bit (or a lot!) and make the entire array the sensor. Split it in half or quarters and stick to your original concept (w/ some of the tweaks offered) . The large array will supply the adequate power to drive the motors for positioning and with blocking diodes (to prevent other segments from interfering w/ the positioning process) arranged properly the segments can be summed together for your final output.

- Ed

perfo (author)snobound22012-06-23

The problem with using your actual main array as the sensor is by design none of your panels will be in the sweet spot. The purpose of a tracker is to get the PV panels off the main array as spot on as possible to maximise the output. Using half the array tipped one way and the other half the other will work as a control solution but will always keep the array off the optimised position so sorta negates the point a little.

gtoal (author)snobound22012-06-22

Well, yes, but also no :-) This is a problem that scales up well but not down. There are pretty good solutions for larger systems. I've done something kind of similar in the past with a couple of 5 watt panels plus a 50W panel, but doing it down at this scale is more of a challenge!

I'm interested in the subset problem of small systems, around the size of a cell phone charger that could be put on a desk by a window (or maybe hanging from the ceiling as one poster suggested :-) )

The basic challenge is can it be done for less than the cost of using a larger panel (or adding a second panel)? While remembering that time costs money and even free parts salvaged from trash are not free if someone else building this can't find those parts...

I reckon seven or eight dollars and 20 minutes of effort is a fair goal to acheive, versus say and extra $20 for a larger panel.

williamj (author)2012-06-21


Some good thinking here, however…

I don’t believe that this can be accomplished for $20.00, or anywhere near there .In all probability the electronics alone would exceed that amount. ”Two” Photo Voltaic cells (PVc) could start and/or stop two motors but not reverse direction of either motor, which is necessary for proper tracking. Also you must consider what happens when the sun sets for the evening. If the PVcs are set to “go towards the light” will your tracker be hunting a sun that it won’t see until morning?

For proper tracking of a Solar Array (SA) the array should not only track east to west (for following the sun’s position during the day) but also north to south (for following the sun’s position from season to season). For this scenario four PVcs are needed. One PVc to track west, one PVc to track east, one PVc to track north and one PVc to track south. In addition to the four PVcs one addition PVc would be required to 1) automatically turn on/off the entire array and 2) to return the array to a “home” (Easterly facing) position after the sunset.

There are many, many more things that must be considered for a successful SA tracker. Perhaps you could start a group here and turn this into a true open source project.

Some really good thinking here, keep up the good thoughts,

gtoal (author)williamj2012-06-22

Depending on what we eventually use to turn the rig, we have a few choices:

1) let it die facing east and rely on the morning sun to turn it back west

2) as the sun sets, use all remaining power in the battery to return to west - this can either be by reversing the engine or by continuing to rotate through to 260deg

3) mechanical reset (eg rubber band, which makes the initial cost of rotating higher) or manual reset

if this is an indoor desktop toy, which is the way I'm inclined to go now, a manual reset and manual powering (eg a clockwork table such as a microwave turntable) tweaked to the right speed is probably my favorite bet at the moment

carmatic (author)gtoal2012-06-22

Here's a take on resetting the solar panel tracking ... Apparently they use some kind of liquid which expands from the heat it absorbs during the day, pushing the solar panel westwards... At night, it cools and contracts and pulls the solar panel back eastwards, ready for the next morning... You would need alot of tweaking for it to work just right, tho, especially in the example they showed where they also have a piston for the Y axis, implying that it is meant to work at temperate latitudes, where the heat from the sun to expand the piston would increase during summer and decrease during winter

I would assume that because you are using this indoors, the ambient temperatures that this would be exposed to is at least be kept constant? Ideally it absorbs and releases heat purely radiatively... Being thermally conductive to random temperature fluctuations in the air would render a thermal expansion piston design quite useless!

gtoal (author)carmatic2012-06-22

I can't seem to embed an image in these comments, but I was going to show you the temperature graph for yesterday - almost perfect sine curve except for a small notch when we had a short rainfall.  Usually it is a perfect sine wave for the whole day, with only a few days where there is a precipitous drop as a cold front passes over.  Few and far between.

I used to have a 'nodding duck' toy that bobbed up and down all day which relied on an expanding liquid as well as evaporation.  Of course it did need a cup of water to drive it but I could live with that.

carmatic (author)2012-06-21

I think that a major part of the problem is the friction in turning the solar panels, the panels themselves will likely not produce enough power to allow a motor to overcome the friction of whatever you are using to mount the panels ... And if the friction is so low that the panels can be turned with very little power, you will need to enclose the panels in a sort of dome so that the wind doesn't turn it around instead

gtoal (author)carmatic2012-06-22

Since I've almost made up my mind that this will be for an indoors installation, no wind can be an assumptions. Or go with a dome which can be done very cheaply, eg the plastic lid that covers a store-bought cake. I'm already considering a cake stand as the turntable after all!

carmatic (author)gtoal2012-06-22

I see that you have already said something about using skateboard bearings, I was just about to mention that too... The bearings would play a very important part in this, either modify your cake stand or whatever to use engineering-grade bearings , or build your own stand which incorporates these bearings from the outset

I also see that you are also contemplating mechanical means of orienting the solar panel and to return it to the same position every morning... I was thinking that a passive mechanical system was the best way too, since sunlight and mechanical items are in great abundance, while solar panels and the electricity they produce are relatively scarce

lunus (author)2012-06-22

Instead of trying to handle all of the torque using your solar power, why not counterbalance it with a physical weight and use the potential energy to move the array? The circuit would just have to maintain tension and loosen its grip to move west. Of course, the trick is that you'd have to manually switch it back to the east for morning, but you're not using any of your solar power to fight the weight of your apparatus.

Either that, or balance the weight of the panels with a counter-weight to minimize torque and use the smoothest bearings you can find to minimize friction. At that point, the weight of the whole thing will move it west until noon and then your motors move it until sunset and reset it. The circuit really would be as simple as two poly arrays, an opamp and possibly an amplifier circuit.

At the end of the day, the weight should swing the whole thing a little east of noon position, and a hinged weight could bring it all the way back east. the weight is just latched back in when daylight comes, and the whole process starts over.

gtoal (author)lunus2012-06-22

This all makes sense. Talking of bearings, I discovered today in conversation with a local maker, that the 3D printer guys are using cheap skateboard bearings in their kits, and apparently they are well enough machined to be quite good for engineering purposes!

gtoal (author)2012-06-22

A little bit of info on the motor I was using, from the RS product page:

"Comments about RadioShack 1.5-3VDC Metal Gear Motor:

I bought this to make a prototype solar-powered boat. It works great. However, it requires at least 0.25amps to spin it with no-load. It goes up to about 1.5amps full load. A single solar panel was incapable of spinning it no-load (I bought the PV from the Shack as well). Just be warned, you'll need about 5 6v/50ma PV's just to spin this thing at no load! Batteries, though, work great!"

Maybe I just need more cells or a different motor? ... :-)

gtoal (author)2012-06-22

Agree 100%

ventifact (author)2012-06-21

I remember plans for early thermo-mechanical devices that used a vertical blind between two painted black pistons to keep a collector pointed at the Sun.
Your set up is similar.
I think you need better shading between the detectors. There is too much incident light.
Maybe isolating each detector in a deepish can, like Pingils or tennis balls are packaged, and painting the insides flat black would give a bigger difference between the readings. A circuit could rotate the assembly to maintain equal readings.

gtoal (author)ventifact2012-06-22

Blinds! Damn! That gives me an idea... Has anyone ever built a solar collector by putting PV cells on the outward-facing surface of window blinds? Regular, used in the home style blinds I mean? That's a lot of available real estate that shouldn't interfere with day to day usage. At the very least the power would be enough to raise and lower the blinds themselves, which would let you install powered blinds without the hassles of concealing the external wiring. (I believe that's called "WAF" in the engineering trade...)

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