Concrete Cube Low Power Night Light

In this project we will make a night light out of a concrete cube with a layer of acrylic. It‘s quite discreet and spooky (the photos can’t really show the atmosphere it creates) with a power rating of less than a Watt, so you can leave it on the whole day. The main focus of this instructable is two folds. First is upcycling (but you can always go for store bought stuff). Second is learning new stuff, hence the long text. In it’s original form it uses a very energy efficient capacitive dropper, as the LEDs are insulated from the outside there is no danger of shock. If you don’t feel comfortable though, you can always keep the LED stripes as is and provide a 12Volt power supply. I will focus on the capacitive dropper circuit but will provide info for the 12V variant too.

I used a lot of annotation in the pictures so take a look there too for extra hints

We start by cutting the acrylic sheet into 10x10cm squares. The acrylic sheet came out of an old broken 19inch TFT monitor. Then a 35mm hole is drilled in the middle. This hole size allows enough space for 2x3 LEDs. I used 3528 LED stripe, because thats what I had in hand. If you have another variety measure the perimeter of the hole by multiplying the whole diameter with π and improvise accordingly ( in my case it was 2πr = 2r*π = 35mm * 3,14 = 110mm)

If you are following the 12V variant, just cut 2 stripes and solder two cables on the plus and minus and you are done. If you follow the capacitive dropper variant we need to do some more work. First we cut the 2 stripes and with the soldering iron we remove the 2 small resistors. Instead we put a blob of solder to short them. So now we have 2 stripes of 3 LEDs each connected in series. For the capacitive dropper we would like to have all 6 LEDs in series, so we solder them according to the picture. If we did everything correctly then the LEDs should start giving some light when we provide them with 17-18V (and 12V if they are RED). We test them and we attach them on the inside of the 35mm hole on the acrylic. Make everything secure with some hot glue and go on.

As the concrete casting process will happen in 2 stages, it would be nice to provide some support for the two parts, so I just cut a small (8cm) piece of a steel pipe and attach it to the middle of the hole on the acrylic. I used a piece of wire and some hotglue to make it stable, don’t worry much we need it stable for as long as the concrete sets only. This will give some structural strengh and on we go with the cement.

Our aim is to make a cube so we calculate that we will need around one liter of dry mixture. How did I do the calculation? By using a coffee mug that holds something close to 250ml/gr of water. So for our cube we needed 1 part cement for 1,5 parts sand. I measured 2 cups of cement and 3 cups of sand, mixed them thoroughly and separated half (2,5 cups) of the mixture for the second stage. With 2,5 cups of dry mixture in my bowl, I add half a cup of water and mix with an old fork. Cement drinks up the water so mix thoroughly and add a sip of water after the first half cup. When the mixture is saturated, a small sip of water will make the whole mixture very liquid very quick. Cement works like that. Everything stays really dry and crumbly up until a point and then it gets watery quickly. You don’t need any science as you don’t care about great structural stability, but a too watery mixture will make your concrete less strong.

Tap the bowl a few times to let some bubbles come to the surface and let it aside. Time to build our form. I used these cheap pieces of wood to make a cube form out of it. Instead of a ruler, I used the acrylic piece as a guide for my cube. That way, if any of my acrylic square sides is a bit crooked, it will reflect in the form and wont give me headaches later. I then apply generous amounts of vaseline on the walls of the wood in order not to let the concrete stick where we don’t want to. I also applied a bit of vaseline on the external four sides of the acrylic for the same reason. I fixed the pieces of wood to the bottom by using hotglue. We make a mark 5cm from the bottom and we pour the concrete from the bowl until it reaches the mark. Then we take our acrylic square and we let it sit on the concrete. We press it a bit evenly in order to stick and to see some of the concrete overflowing from the hole. Of course we try to keep the 2 cables out of our way. Job done, we can now have a lunch break.

After our lunch break, we are ready for the circuit. Capacitive droppers for LEDs are extremely efficient and easy to build and if you follow a few reasonable guidelines can last forever. If you want to learn more about them, the best source I found it this old one here.

Be respectful with the circuit as it involves 220Volts, and 220Volts can be lethal. So don’t touch anything when you test the circuit, be respectful and measure twice cut once but apart from that, if we can keep the exposed parts closed away (in our case inside the concrete), there is nothing much to fear from capacitive dropper circuits (many LED lamps use them anyway).

The main component behind the capacitive droppers is a 220nF X2 capacitor (X2 capacitors are rated for 220VAC and are designed to fail in a safe open position) and then a bridge rectifier to turn everything to DC. To make a long story short, a 220nF X2 capacitor will allow only 22mA peak to peak to our LEDs which will be closer to 13-14mA after we smooth it out with a 50V 100μF electrolytic capacitor. This will be coupled with a 1Watt 1Kohm resistor in order to filter transients and provide a fuse for our circuit in case things go wrong. Another 680KOhm or 1MOhm resistor is added across the two leads of the capacitor in order to discharge it when not in use. For a stronger light you could use a 330nF X2 capacitor that will allow only 33mA peak to peak or around 19-20mA when smoothed with an electrolytic capacitor. Don’t go higher as the LEDs are rated for 20mA normal 30mA max and we don’t want to stress them. Overstressed or overheated LED’s are the main reason behind dead LED lamps. As we are not designing for planned obsolescence, we aim to keep our night light alive for as long as possible.

Where do we find X2 capacitors? That’s easy, in nearly every dead houshold electric appliance. Especially in the ones involving motors (it is used to filter the noise from the motor), but also in broken power supplies and nearly everywhere. How to identify them? They are plastic rectangular, quite big and usually yellow and they write X2 on their markings, apart from the capacitance. I got mine from a piece of shit blender that burned, and it was a 300nF type.

For the smoothing electrolytic capacitor I used a 330μF that I had around. As I used 6LEDs in series with a total forward voltage of around 18V, it’s better to use a capacitor rated for 35V or 50V to be on the safe side. Again the value isn’t so important. Bigger values will smooth the current more than smaller ones, but upto a point. Using my 6LEDs and 330nF capacitor as an example, a 330μF will flicker the light in between 17,5 to 20,5mA, a 220μF around 17 to 21mA, while a 100μF capacitor will flicker the light in between 14-23mA, and a 470μF capacitor between 18-20mA. Even the most sensitive eyes cannot tell the difference between 14mA and 23mA on an LED, but if you want to be on the safe side it doesn’t hurt to choose a higher value capacitor (higher than 330-470μF doesn’t make much sense).

PS if you use the 12V transformer solution you can skip this part

And now it’s time for the second part of our cake. I let the first one dry in between 5-8 hours (it’s summer). The idea is for the concrete to stiffen a bit without completely hardening. You want your concrete a bit on the stable side, as pouring the top part can crook the orientation of your acrylic part because of the weight of the new mixture.

In order to put my circuit in a safe place I cut a empty tube of vitamins at around 6cm and drill a small hole in the bottom so that the 2 cables will pass through. I attach the tube on the acrylic with a bit of hot glue and make sure the two cables pass freely from the little hole. After that I take the 2,5cups of dry concrete mixture that are left and add again half a cup of water. Mix thoroughly, pour on the top, tap for the bubbles to go away (but don’t be anal about it, bubbles are a feature of concrete IMO). After pouring I add a small piece of a straw in between the wall and the vitamin tube that will later serve as a railing for the 220V cable. And then we have to wait for around 48 hours before we release the form. You can wait less or more, that has to do with how impatient and risky you are. Too wet and your nice cube can crumble, too dry and you will have a more difficult job cleaning the acrylic parts that got covered later . Pick your poison (I personally lean on the too dry category) :)

So, 2 days later we are ready to take the form away, clean the concrete a bit, take away the concrete that spilled over on our acrylic sides (if you put vaseline or oil on the sides, you will be thankful now). When you are satisfied with the exposed acrylic, cut the protruding part of the vitamin tube, sandpaper the concrete a bit, clean it with some wet towel and you are ready for the final assembly.

Solder the positive and negative of our LED cables on the positive and negative side of the electrolytic capacitor, and solder the hot and neutral cable of a salvaged power cord on the other side of the circuit, hot glue everything that has the tendency to move, put everyting inside the vitamin tube. Cut a small round piece of paper and place it on top of your circuit and then drop a blob of silicone on top of the little piece of paper and let it dry for 24 hours. After that time, I cut little pieces of furniture pads and stick them on the bottom in order to provide for a smoother surface (and to even out any uneven parts on the concrete), and voila you are done.

PS if you are into using the 12V transformer solution, then you just need a DC plug here taking notice of the polarity, or directly the positive negative cables of the transformer