# ICARUS: The Analog Heliostat

Have you ever been sitting in the shade and thought "I wish the sun could come to me"? Well now it can.

I wanted to squeeze some extra sunlight into my life to provide some free heat as well as reduce the risk of the Crabby-McNasties. There is a wonderful device called a heliostat which can reflect those powerful sun beams exactly where you want them; as in  "pew pew pew".

UPDATE JULY 18 2013: I have been contacted by a real professional (see "redrok" in the link and comments below) to advise me that I have oversimplified the geometry causing errors in aiming. I didn't get the calibration and tightening up to realize that I had this problem before Icarus died of a blown motor. Please see the next step for more details of where I went wrong (Rectilinear vs. Spherical Geometry).

A heliostat is basically a mirror that rotates to reflect sunlight on a fixed point throughout the day: into my dining room in the winter or onto a solar pool heater in the summer. This seems like a very easy project until you consider that there are two axes of rotation, and that the sun's position in the sky changes with the seasons (hugs the horizon in winter). I've been wanting to build a heliostat for a few years, even though I have never actually seen one (until today!). Heliostats are commercially available, although they are not quite mainstream yet. There are many hobbyists trying new and innovative ways of working out the geometric and mechanical challenge of splitting the angle between the sun and target:

Using clockwork: http://www.instructables.com/id/HELIOSTAT/
Using Arduino to calculate where you need to point: cerebralmeltdown.com
A complete history of Heliostats: http://www.redrok.com/main.htm#mechanical
Commercially available: http://lightmanufacturingsystems.com/order/

I am quite certain that the best solution is to have some form of computer that calculates the required position of the mirror and transfers instructions to precise actuators or step motors to position it in real time. Arduino appears to be sufficiently powerful, and the necessary code is mostly available in some form. This is where the "ANALOG" comes from in my title: I was looking to bypass the computer and precise motors to A) reduce the cost, B) make it more weather-proof and C) make it trouble-shootable (for a lowly geologist like myself). So, although I am using electricity and electronics, this is more like your grandmothers radio (analog) than you might think. I don't expect that this design would be rolled out in the western world, but the relatively simple mechanics means that it could find a niche hobbyists and in developing countries. In fact, much of the body and mechanism would lend itself well to 3D printing (future update?).

I was inspired by the many Arduino Solar Tracker projects which seem to handily provide a vector to the sun. The second vector of interest is straightforward: it points directly towards the target (window, pool heater). To get the sun reflected where you want it, the mirror needs to be positioned perpendicular to the bisector (exactly half way, or normal) between these two vectors. That seems easy, right? A simple computer or child can tell you where that bisector is; neither of which I know how to provide instructions in a form that would be executed. The point is: I am trying to eliminate computer processing from the equation, so how do you precisely split an angle in three dimensions "mechanically"? See the next step to see my low-tech solution. If I have just invented (or not) the equivalent to a "square wheel" (as in, a more elegant solution exists) please let me know in the comments below.

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Akin Yildiz4 months ago

i need this so badly

Renard_Bleu (author)  Akin Yildiz4 months ago
Thanks for the comment. Apparently I didn't get the geometry right (see the comments), but if you are looking for inspiration:

http://www.dailymail.co.uk/news/article-2474800/Norwegian-town-Rjukan-enjoys-winter-sunlight-time-history-using-heliostats.html
zawy1 year ago
You can place the mirror on a polar axis (points to north star) and then be able to tilt the mirror off that axis to point to the target. As the time of year changes (sun declination) you'll have to change that tilt about once per week in spring and fall, but not as often in december and june. The polar axis only needs to turn at 15 degrees per hour (360 degrees in 24 hours), so no calculations are other mechanics are needed. The option I am going to try is a series of 4 solar cells on each of the 4 sides of my target window to power two 12VDC motors in forward or reverse, depending on where the Sun is hitting. No other electronics. It would require resetting the position of the mirror each morning. I like this method over the polar axis and even computer calculations because I don't have to do any careful set up procedure in get the base angle correct and tight, nor adjustments to the calculation if the target is not due south. The math for bisecting the angle is surprisingly hard because you can't do it in polar coordinates without first converting to rectangular coordinates. See my profile picture for how I save \$500 last year in heating my 4000 sq ft house using \$200 of materials. But I had to adjust the four 4x8 insulation boards covered with aluminzed mylar (\$50 from amazon) by hand 5 times a day. This year, I want to have some motors doing it for me!!! It can't survive strong winds, but I just had to twist 2 bent nails to remove each of the 4x8s and then lay them against the house (3 minutes work). Ideally, they would be moved to cover the outside of the windows each night for insulation. One 4x8 has up to 3,000 watts hitting it. About 30% was lost from bad aiming and the boards being wider than the windows, and 20% from the double panes. But even more was lost from the oak tree that blocked all the mid-day sun....and I still saved \$500. With motors and that tree gone, \$800 savings per year seems possible. On good days that are not too cold, I can get the house up to 80 F and keep it there until about 3 pm. Then the house loses about 1 F per 45 minutes, so I can stay above 70 F until 10 pm. Snuggle in bed warmly and suffer some morning 60 F cold until 9 am, and 50 F days with zero heating bill are tolerable. A north window with an oversized heliostat can get 3 times more sun energy than a south window because there is not a cosine-angle loss at any time during the day. It's possible to do my 4 solar cell method with only 2 cells and 1 motor (1 turning axis) but it requires careful setup. My manual adjusting was needed on only 1 axis because I set the axis approximately halfway between the polar axis and the target window.
1 year ago
Oops, I forgot the 15 degrees per hour polar axis trick doesn't work unless the target is in the exact direction of the base of the polar axis. 15 degrees per hour trick to work, the target window has to be exactly due south of the heliostat. But for all reasonable conditions where the target is approximately south of the heliostat, there is a single-axis rotation (single motor) solution that turns faster in morning and afternoon. For most of the winter, that axis will not need adjusting except by a few degrees, generally less than 5 if any. But the mirror tilt off that axis will need to change about once a week. From about 36 after fall to 45 on december 22, back to 36 degrees off the axis as spring gets near. For my situation where the window is due south and the heliostat is even with it and 32 degrees latitude, the rotating axis tilt is 13 degrees up from polar axis towards the window. Looking south, the polar axis is equal to your latitude below the horizon. My latitude is 32, so 32 minus 13 = 19 degrees main axis tilt below the south horizon, so the north end of the axis is 19 degrees above horizon. At latitude 45 degrees, the tilt above north horizon is 15 degrees (and the tilt off that axis ranges from 39 to 48. So most everyone in U.S. could use an axis set about 14 degrees above north horizon and allow the mirror to have a manually adjustable tilt of 35 to 50 degrees off that axis, if the heliostat is level with window and due north, by however many feet is needed to get out of shade of the roof. You need one north facing 6'x6' bay window or sliding glass door and two 4x8's for every 1,500 sq feet and good insulation and open sky to have zero heating bill on sunny days. Material's cost: \$30 two insulation boards, \$15 door hinges, \$20 aluminized mylar, \$50 electric motor and solar cells, \$10 of 2x4's and 1x2's. But I have not worked out the gearing for the motors.
1 year ago
oops, for the US, it's your latitude minus 14 degrees is the tilt above the north horizon that should suffice for a single axis design (not "14 degrees" as I said) . For heliostats a long way away from the window, things have to be more precise. But adjusting the mirror tilt every week is a hassle and the initial angle needs to be accurate, and it changes if the heliostat is moved, so I think I am just going use 2 normal axis and 2 motors. Instead of solar cells, I am thinking about using 4 LEDs in series on each side (16 LEDs total) to turn on 4 mosfet transistors that power the 2 motors forwards or backwards. I learned from redrok that LED's can be used to detect light and they should be plenty to turn on a mosfet. He likes more complicated electronics than me because he's good at doing electronics. He wants to do a microcontroller and everything. I just want to get my house heated ASAP. BTW, the huge amount light coming into the house erases the feeling of winter. It's more than great to have all that warm light. But you got to think big. Glass mirrors are not big enough. Don't even buy the mylar and try the aluminized side of the 4x8 insulation board, just leaning it up against something, if you can place it close to a large window or large sliding glass door. I stay home all day, and I like it warm, so out heating bill on this large old house is huge.
1 year ago
Hi zawy;

I have designed some extremely simple trackers pretty much based on just Blue LEDs and MOSFETs. See:
http://www.redrok.com/electron.htm#ledblue
Or even less parts if you have a wall wort that outputs low voltage AC power. See:
http://www.redrok.com/electron.htm#ledac
redrok
1 year ago
redrok, thanks for the circuits. It looks like I'll be going with That really clever but finicky simple AC circuit. With the Irf 530 mosfets i have the only way i could get it to work was with 5 660 nm or 5 850 nm LEDs (testing with halogen lamp which is really close to Sun with little bit extra infrared). 3 660 nm is enough voltage and coulumbs for the gate, but the circuit wanted to stay in AC on without any light, hard on the motor and really hard on the mosfets. But by adding forward diodes in series with the reverse gate-leakage diodes, all seems well, but then i need 2 more LED to be more sensitve to light. Other wise, i would have been buying 2 of your \$26 circuits. the 850 nm leds supplied 0.3 mW whereas the 660 nm suplied 0.1 mW... about 3 and 1% efficency for 0.01 cm^2 dies at 100 mW/cm^2 if i closely simulated the sun, except that the lens amplifies, so less real efficiency. I think they are nearly the same angle lens, but 1/2 the angle means 4 times the light intensity when pointed directly. I tried resistors in series with the leds but had no luck. My motor is 12 v 0.1A, but it seems like a 1A motor is possible. I used 1N4454 diodes. I don't know how, but the voltage stays the same pos or neg polarity with small peaks over the full 60 hz cycle. Since i only need a few joules per hour to turn the heliostat, 6 leds, a 1 mF capacitor and a zener could be a sufficent solar cell, especially for stepper motors. I have completed a 2 axis PVC design to hold 2 4x8s and I'll edit the instructable or add a new one hopefully this week, with video of operation.
1 year ago
Hi zawy;

The IRF530N MOSFET is not suitable for this circuit:
http://www.redrok.com/electron.htm#ledac.
The IRF530N gate threshold is about 4V, see:
http://www.redrok.com/electron.htm#IRF530N
However, it needs even more voltage to turn on sufficiently.
The IRFZ3708 is much more suitable.
http://www.redrok.com/electron.htm#IRF3708
The measured threshold voltage is about 1.7V.
http://www.redrok.com/images/MOSFET_IRF3708_Vgs.gif

You mentioned that both MOSFETs are turning on at the same time.
This is caused by leakage currents getting into the gates from the drains.
This can be dissipated a 100MOhm resistor, (generally unobtainable).
I uses 1N4148 signal diodes which have suitable leakage.
Don't use low leakage current diodes!!!

Use good quality Blue LEDs and the IRF3708 MOSFET.
If you are wanting to use higher current motors a heat sink may be needed on the MOSFETs.

redrok
1 year ago
I only have Irf 530 on hand. My cheap oscope was affetcing the circuit, so the series diode was not working unless the oscope was attached. The higher gate voltage of the 530 allows 10 Mohm instead of diode to work. But that also means I need 5 reds for forward, and 6 reds for reverse. Likewise, I don't have blues. I'm just trying to get it working today without having to order anything. And for my purposes, 1 or 6 diodes doesn't make any difference.
1 year ago
Hi zawy;

Even when you use the 10MOhm shunt resistors you still need the shunt diodes.

redrok
1 year ago
It seems to work good with just the resistors but I didn't look at the waveforms with the scope. I have it operating the heliostat. Are you sure the 1N4148, good 5 mm Blue LEDs, and IRF 3708 work good? When I update my instructable, I'll reference your page and these specs for the benefit of using 4 blues instead of the 20 reds I'm using.
1 year ago
Hi,
Why blue and green LEDs? 660 nm red LED 5mm "lamp" types are 1.9 V. I've emailed you before about my heliostat work. I believe I have the single-axis heliostat math worked out now, with less than 0.5 degree error for U.S. latitudes, winter, and a limited range of heliostat-target arrangements (window ideally due south and equal or below heliostat's south horizon. See my instructable.
1 year ago
Hi zawy;

You are thinking of the LEDs as, well, LEDs.
I think of them as little PhotoVoltaic cells.
As PV cells the don't generate nearly as much voltage as when operated as LEDs.
Rule of thumb as PV cells:
Red 1.25V
Yellow 1.4V
Green 1.6V
Blue 1.9V
redrok
1 year ago
Hi zawy;

I have designed some extremely simple trackers pretty much based on just Blue LEDs and MOSFETs. See:
http://www.redrok.com/electron.htm#ledblue
Or even less parts if you have a wall wort that outputs low voltage AC power. See:
http://www.redrok.com/electron.htm#ledac
redrok
Renard_Bleu (author)  zawy1 year ago
Wow,
I really like your concept (solar panels around your target doing your aiming and powering).

Redrok has contacted me to explain how I've underestimated the complexity of splitting the angles (as you have also noted). I will be updating this instructable once I fully understand my faulty logic. You should put together an instructable to show us your strategy!
1 year ago
I'll try to get to motorizing it this coming winter. Here's an instructable I just published for what I did last year.
1 year ago
http://www.instructables.com/id/Heliostat-for-heating/
riff raff1 year ago
Ha! Great name!
Renard_Bleu (author)  riff raff1 year ago
Pretty appropriate too, seeing that it failed likely due to overheating!
Great 'ible! Very nice ideas very nicely shown. Many thanks!
Renard_Bleu (author)  anadem1 year ago
Thanks, it was a fun build.
gpm49851 year ago
Looks awesome! I think that you are the only person I have seen who has successfully built this type of heliostat.
Gabriel @ CerebralMeltdown.com
Renard_Bleu (author)  gpm49851 year ago
Cool thanks, I am not part of the heliostat community, so it I was wondering if there were other examples out there in the wild. I have been waiting for nice weather to take a time lapse video, and one of the motors conked out today... These parts are not made for the elements. I hope to update soon.
1 year ago
I may remember someone else on Instructables tinkering with this type of heliostat, but I may be wrong. It was at least something similar I think.

I definitely know what you mean about the clouds coming out as soon as you want to test your machine. That is probably the hardest part about the whole project, waiting to see if the thing actually works. It really takes a long time to work out all of the bugs because of this.
Renard_Bleu (author)  gpm49851 year ago
Thanks for the comments.
You might be thinking this one that I provided the link as "clockwork":
http://www.instructables.com/id/HELIOSTAT/
He was using clockwork to track the sun and then use elastics to split the angles. There is quite a discussion going on in comments there.

It always works great in the garage!!! Unfortunately poor Icarus died before I could calibrate him properly, and I had more trouble than expected balancing the competing solar engines (a piece of electrical tape to handicap the strong side). They are really efficient, even in the shade.
technovative1 year ago
Really interesting, and well documented project.
Renard_Bleu (author)  technovative1 year ago
Thanks for the quick comment. I've updated with a time-lapse video, and sadly Icarus has since passed on...
Renard_Bleu (author) 1 year ago
Thanks, I've made a few changes since publishing, and am waiting for a sunny day to make a time-lapse video.