Category: power supply

Lights On!

Hi there!

It sometimes happens that we build something out of necessity, to help us with little day-to-day tasks, and this is the case for today’s project.

The boiler’s room in my basement is a very cramped place that I need to access frequently because I keep in there a freezer for groceries. It turns out that I often have my hands full when I come back from taking something from the freezer, and it is difficult to reach for the light switch.

To obviate to this problem, I decided to build an automatic light switch, so I don’t have to maneuver it manually anymore. And, since I was at that, I decided to make one that not just turns off when I leave the room, but also turns on the light automatically when I enter the room.


The core of this device, technically called an Occupancy Sensor, is a PIR, or Passive Infrared sensor. I have a version of it called HC-SR501 that puts together a Pyroelectric Infrared Detector, or PID, with a bunch of other electronic components that make the sensor useable with very few external components. The PID is concealed underneath that white little dome, which is nothing more than a Fresnel lens that concentrates the light on the actual sensor, thus increasing its sensitivity.


The PID used in this device is called LHI 778, and is capable of detecting infrared emissions over a background noise of up to 85 degrees Celsius. You can see from its data sheet that it is like a small metallic cylinder with 4 pins coming out of it. This PID actually contains two infrared sensors connected in series, to increase its sensitivity.

This one is the schematic I made to use the PIR motion detector. The Detector is connected through the pin header J1 on the left. The header provides the power supply for the detector on pins 1 and 3, and captures the output signal on pin 2.

The signal from the detector goes to the base of transistor Q1 which pilots a relay that is used to control the lights of the room where the device is located.

The 5V power supply for the transistor and the relay comes from an old USB charger, so I didn’t have to build a power supply just for this application. LED1 and R1 provide a visual indication that tells us when the gadget is turned on.

The actual power supply for the whole thing comes directly from a 120V socket, goes through a 1A fuse, and through a power switch.

When the switch is set to on, both the USB charger and the common terminal of the relay receive the 120V. Power socket J3 receives the 120V only when the switch is on and, simultaneously, the PIR detects the presence of a person.

The following archive contains all the files you will need if you decided to build this device for yourself, and more.

lights_onDownload

You can also watch this video for further details on the construction of this device.

Important note: this device involves the use of potentially deadly voltages and you should not try to replicate it if you have no experience with high voltages. Build it at your own risk.

How To Make A PCB Using A Laser Printer

When working with electronic circuits, sooner or later we feel the need to make our own PCBs to get a more functional and better looking circuit board.

I already made a video in the past to show how that could be done, for simple circuits, by drawing the circuit manually on the copper clad with a special kind of pen that uses an ink impervious to the chemicals needed to etch the PCB.

This time, I am presenting you a different technique, that allows you to draw the traces, and also the silk layer, with any of the design tools of your choice available on the Internet and the market in general. All you need to have is a laser printer. You can refer to this newer video for a demonstration of the process.

The whole process works on the concept that the printouts of the laser printers are made with a toner that has the characteristic of being able to protect the copper from the etching chemicals, like the ink from the pen in the original video. This is because the toner is made with a sort of plastic material.

Unfortunately, we cannot use a laser printer to print the masks directly on the copper clad, because the PCB boards are too thick for the printer. Therefore, we need to find a way to print on paper and then tranfer the printed ink to the copper afterwords.

This is made possible by a certain quality of glossy paper that do not allow the toner to stick permanently on its surface when exposed to heat. Even paper from magazines that are printed on glossy paper works relatively well for this to happen. However, there are specialized papers, that are designed specifically for this, which are called Thermal Transfer Paper For PCBs. A quick search on-line will give you plenty of places where you can buy it at a relatively modest price.

Once you have your PCB design ready and printed on such paper, the process to create PCBs becomes really straightforward.

First step is the transfer of the traces drawing to the copper. The copper needs to be perfectly clean, so it is always better to use a piece of steel wool to scrape away copper oxide and other dirt from the copper surface. Just move the wool in a circular fashion to remove all the particles of oxide from the copper clad and make sure to use gloves, otherwise the contact with the skin of your hands will soon oxidize again the copper.

Once all the oxide is removed, you need to deep clean the copper to remove any particle of dust from it. To do so, you can use some alcohol. Once done, let the board stand for a a while to make sure it is completely dry.

Then lay the board on the printout, making sure the copper is in contact with the drawing. Wrap the paper all around the board to make sure it will not move during the transfer process.

Once the PCB is wrapped with the paper, put it on the table copper-side up and use an iron at the max temperature, with no steam, to heat uniformly the whole surface of the paper and the pcb wrapped in it. Be careful not to burn yourself in the process, of course. You do not need to press hardly, the weight of the iron is just enough. Just make sure you keep moving the iron so that the whole surface is heated uniformly. Do that for a while, until the copper clad becomes almost as hot as the iron. Don’t worry about burning the paper. it is not going to happen. Paper burns at 451 F while the iron, even at the hottest temperature, doesn’t normally go over 400F.

Once the paper and the clad are well heated, put aside the iron and unwrap the board, making sure that when you remove the paper from the copper side you do that slowly and uniformly. The ink from the printout will now have moved from the paper to the copper.

Second step is the actual etching. Use a plastic container, fill it with some ferric chloride solution, enough to cover the whole pcb, then dump the board in the solution. Once the board is in the solution, you’ll notice that the ferric chloride starts changing color. From the initial brown color, it starts becoming darker and darker. This happens because of the copper on the board that starts dissolving in the solution.

While the etching process continues, try to agitate the solution periodically, which will speed up the reaction. A warmer room will also help. Every now and then, check the status of the board and remove it from the solution as soon as you don’t see any more copper on the surface of it.

Once the etching is completed, remove the PCB from the solution and start rinsing it immediately, to stop the reaction that would continue to attack the remaining copper on the surface.

You now need to remove the toner film from the copper traces, otherwise you will not be able to solder the components on it. To do so, use a Lacquer thinner on a piece f paper or cotton and work slowly a little bit at a time. Do this in a well ventilated area. Solvent vapors are both unpleasant to breath and harmful.

Third step is to drill the holes. It is only necessary if you use pass through components, of course. If you use surface mounted components, this step is not necessary, unless you need holes to hold in place the board.

Finally, the fourth and last step is to do another transfer, on the components side of the board, to transfer the drawing for the silk layer. The procedure is exactly the same, but this time the toner will be lay down directly on the board support, not on the copper.

You can see how this process allows you to quickly repeat the whole procedure on as many boards as you like. You just need to print multiple copies of the layouts on the thermal paper and go through the previous four steps.

Hope yo liked this procedure, and don’t forget to go watch the corresponding video, so you will see exactly how this procedure works.