Making of a Cheap Solar Cooker

27 09 2012

This is a little side project I’ve been working on over the last few days. I’ve wanted to build a solar cooker for a while now, so when the opportunity came up to score this Ku Band satellite dish for free, I couldn’t resist myself. These dishes aren’t perfectly circular, but more elliptical, because they are designed to be the cut out of a circle projected onto a parabolic surface but slightly off to one side of it from centre. In this way, the horn that contains the receiver and transmitter is not in the way of the incoming signal. Hence, you get more surface area to collect the signal but, more importantly, you don’t get weird effects, like scattering, degrading the incoming signal.

Ku Band Satellite Dish with Stand

The first thing to do was to go into work and grab some aluminium tape and get cracking. I cleaned the surface first of course.

The aluminium tape I used

First strip on

Curvature begins to be a problem

At this point the curvature of the dish starts to cause a fair bit of ripples in the tape, it also begins to get harder to line up the tape for a straight run across the dish. So, I changed to applying the tape vertically, in line with the major axis, instead of horizontally, in line with the minor axis.

Halfway there.

My five year old taking a photo of me whilst I am working. He does love cameras.

The dish with the reflective surface finished

As you can tell from this photo, obviously mirrors would have better reflectivity. But, I don’t really feel like cutting that many square pieces of mirror and then attaching them to the dish. This was much easier and will get a similar result cooking wise. If I were building a solar powered metal foundry, as at one point I have planned to, then I would rethink my choice.

After this, I removed the horn and then flipped the mount on the back of the dish around. I did this so I could have the focal point above the dish, instead of in front of it. That way I would get the sun light concentrated onto the bottom of my cooking pot, instead of on the side, which will allow for more even cooking.

Dish mount in original direction


Dish mount in new direction

Next, I tied a rope between the main boom of the stand, that came with the dish, and one of the support legs because I had problems with it sliding out when I tried to mount the dish on it.

The stand that came with the dish

The dish is all mounted and ready to go

Once it was all together, I had a little bit of fun with a piece of paper. The reflector didn’t have enough power to burn through plain white paper. But, if you put a black dot on it, using a permanent marker, then it catches on fire pretty much straight away.

*insert evil laugh here* IT WORKS!!!

I can’t believe he’s still smiling after that.

After I had my bit of fun, I attended to the problem of being able to hold a pot at the focal point. I ended up using a section of wire fencing I had laying around for my initial tests but I may end up making a hanging arrangement later on. This is because I’m not too sure how it will go during late afternoons or early mornings when the Sun is lower in the sky. All the contents may shift to one side and spill out. That will be one test for a later date.

All I had to do to mount the cooking rack was take off the coupler that is used to hold the horn and the three support legs together. Then slide the poles through the holes in the rack, then re-attach the coupler. With this set up, the rack is held in place very well by the support poles attached to the minor axis, but moves a bit up and down on the pole attached to the major axis. This may actually help in the long run, as it will allow for a little bit of adjustment when trying to make the cooking pot level.

Cooking rack installed

I then did a test to see how long it would take for the cooker to heat up 500mL of water in the pot I bought ages ago just for this purpose. I got the pot in a set of two for AUD$5 from a local clearance store. Mainly because they were cheap and had a black enamel coating on the bottom. This greatly enhances the effectiveness of a solar cooker due to the black absorbing more of the incoming radiation and, hence, transferring it into more heat energy for cooking.

Solar cooker aligned and with cooking pot in place.

The pot only just fits in between the support poles and required a little bit of wriggling to persuade it into place. You can see the yellow thermocouple I used to measure the temperature of the water in this picture. At the end of the experiment, I realised that either the thermocouple isn’t very accurate at measuring temperature, or the multimeter I used it with isn’t. Because, when the water was happily boiling away, I was getting a reading of only 97.6 degrees centigrade not the 100 degrees that I was suppose to be getting.

500mL of water boiling vigorously.

It only took approximately 14 minutes to get the 500mL of water to fully start boiling. So, I call this a massive success. I can’t wait to see how this thing performs in the middle of an Australian summer.

After I finished testing, I took a measurement of the ambient air temperature with my multimeter and compared it to the reading from my cheap wireless weather station that I was using for timing. The weather station took a reading of 29.2 degrees C whilst the multimeter took a reading of 24.8 degrees C. Considering my weather station is one of the cheapest available, with only indoor and outdoor temperature readings as well as humidity and barometric pressure, I don’t think it is giving a very good reading either. But it does illustrate the inaccuracies that I didn’t foresee at the start.

Anyway, here is a plot of the data I acquired during testing and a copy of the spread sheet I wrote up to go along with it.
One thing I did notice is the linearity of the plot. I must admit that I was expecting it to trail off at the end but it didn’t. This would imply to me that this thing has a lot more potential for heating up more things then just water. I’m going to try and find a carbon brick from somewhere and heat that up. That should give me a real idea of its true power.

Plot of the time it took for 500mL of water to reach full boil.

Water Boil Test

I think I’m going to have a bunch of fun with this in the future.

Fisher and Paykel Smart Drive Schematic

25 09 2012

I’ve spent my free time over the last few days reverse engineering the control board out of the Fisher and Paykel washing machine I bought. I managed to find details on all of the components except one. I know it’s a MOSFET but, beyond that, I don’t have any more specifications. It is labelled H75309 G814BE and is a 4 pin SOT-223 package. If anyone has any more information on it, I eould very much appreciate it. I used this codebook to help find all the parts with SMD markings on them, which was exceptionally helpful and I would highly recommend it. I rewrote the schematic in KiCAD (Build: (2010-00-09 BZR 23xx)-stable), which I will share with you all here. Please note that this is not a complete schematic of the control board but only a partial of the power section that drives the motor. I wasn’t really interested in the rest of it and it took a long time just to do this part. So, I’m not going to bother doing the rest, except for the hall effect sensor section at a later date. I’ve also included the libraries I created for the parts that KiCAD didn’t have as standard. They must be added to your library list before the software can use them.

Next, I’m going to attempt to remove as many parts as I can off the board to reuse, then use them to develop a prototype for my controller. Hope this all helps somebody.


Electric Recumbent Motor

22 09 2012

The journey has begun for my electrically assisted recumbent bicycle. I’ve collected a lot of parts, including the donor bicycles, some steel for construction, some of the batteries and the all important motor. I’ve decided to use a BLDC motor from a Fisher & Paykel washing machine (must be a model with the Smart Drive motor). This is for a few reasons; they are cheap to get, powerful and have permanent magnets in the rotor. Cheapness is important to me as I am a low income earner, I got the entire washing machine for AUD$5.50 off Ebay. So, for an approximately 1kW motor, that is a pretty awesome deal I think. The permanent magnet side of things is also important because I am wanting to investigate regenerative braking on this build and I am led to believe that these are more efficient in that regard. Most people that use these motors as generators usually rewire them for better voltages, but I am going to go with the stock motor first and see how it goes.

Here are some pictures of the tear down of the washing machine and preparation of the motor for mounting. I got the washing machine for cheap because it had stopped working and after the previous owners spent some money replacing the water pump, they gave up on it when it still wouldn’t go. Most of the time the power drive transistors on the control board burn out on these models and a handful of components can get them working again. But, I’m not interested in riding a washing machine all day, I’m interested in bicycles and how to make them go fast.

Fisher and Paykel washing machine with Smart Drive motor. Bought off Ebay for cheap.

To remove the tub and motor assembly, the four support rods on either corner have to be removed first. The top right has already been taken off in this photo. I just grabbed hold of the motor shaft inside the tub and lifted slightly to be able to lift off the grey holders on each corner. It worked well for me.

This is a picture of the tub and motor assembly once removed. The motor is the grey part on the bottom. The problem with this design is that it is hard to remove the motor from the tub. So, I decided to just cut away the tub right back to the part that holds the bearings and shaft.

First I drilled a hole in the side of the tub big enough to fit my jigsaw blade through, then cut around the perimeter to remove the top.

At this point I removed the rotor and stator then started cutting either side of the support sections that run out from the central hub up to the next cross support. After that it was just a matter of snapping each part out along the cross support. With the wider sections near the outer rim of the tub, I had to cut them into smaller sections before trying to snap them because, if I didn’t, they would only bend and not snap.

Cut down to the last lot of support sections.

This is as far as I could cut down the tub with my jigsaw.

To remove the last bits of the supports I tried both a hacksaw and rotary tool with a cut off disk inserted. The rotary tool won out at the end of the day but I had to be careful when cutting because it melts the, what I assume to be, nylon and it can wrap itself around the shaft. As it builds up it ends up snapping the cutting disc. I got around this by cutting through in layers and removing the build up around the cut as I went.

I used the barrel sander attachment on my rotary tool to sand back the remaining stubs to leave a nice finish on the hub.


After the hub was finished I reassembled the motor. You can see on this photo that the stator has some cracks in it. Apparently this is a fairly common occurrence with these motors. I think I will end up impregnating the cracks with some epoxy using some kind of vacuum set up to prevent it splitting further.


Receiver Update

16 09 2012

I’ve had a chance to look into the destruction of the receiver module a little more. The verdict is, I had to end up buying a new receiver shield. I’m not sure why, but I replaced just the module and it wouldn’t work at all with it installed on the old shield. I bread boarded it and still couldn’t get it to work (I did forget to put the resistor in-line like the shield has though). I went and tried another module at the store I bought the first one from, before buying it, and it still wouldn’t work. At this point I gave up and just bought a new shield. This one doesn’t seem to be as sensitive as the previous one, as I am not getting as many packets as I was previously at the same distances. I’m going to redo my range tests with this new receiver and will post the results. This time I won’t point the antenna at any stray mobile phone towers 😉