Eeek - how do I get started?


For this series of blog posts, I'll be sharing how I make this circuit into a PCB:



The circuit will turn on an LED when it gets dark I am going to make it with a 3V  CR2032CR2032 battery so the finished PCB can be put in a jam-jar and left outside to mark a path or a tentpeg or put in a pumpkin at halloween or used as a nightlight Or anything else where you want a light to come on when it is dark


I prototyped it on a breadboard to make sure everything works together as I expect them to and to select the resistors that made the LED bright and the phototransistor work how I wanted it.:





The photo-transistor is sensitive only in one direction (straight up), so the light from the LED does not effect it.


Parts: (I have included a full Bill of Materials (BOM) later in this post, along with the Farnell part numbers, Eagle library etc.)

Battery 3V  CR2032CR2032

Battery Holder CR2032CR2032

Resistor 330 ohms

Photo Transistor  TEPT5600TEPT5600

Transistor BC548

Resistor 75K ohms

LED amber

Having installed CadSoftusa Eagle PCB design software, the front page you see when you open it is the Control Panel. If you have not loaded the software, have a look at the Intro here.


The Control Panel is where you can load and save projects.


Under "Projects", there is a folder called examples. By clicking on some of these you can get an idea of various PCB's that can be made. There are also tutorials you may like to work through on the CadSoftusa website.


(This is an example from the tutorial)


The .sch files (above) are the schematics - the circuit diagrams very similar as you would draw them in a notebook.


(This is an example from the tutorial)


The .brd files (above) are the actual board layout, including the tracks.


File, New, Project lets you start a new project (un-surprisingly). I have saved my new project as "OnWhenDark". Other projects I have already worked on include the Open Collector (Thingatron) (Blog here) and the Inputs Optoisolator (blog here) - and one called RpiTrafficLight.


Control Panel Screen Shot

Open the Project

Highlight "OnWhenDark"

Right Mouse Click

"Open Project" (This can also be done using the menu options File, Open, Project, "directory", "OnWhenDark".

A green spot appears next to the project name, as shown in the image above.


Create a new Schematic

Highlight "OnWhenDark"

Right Mouse Click - New, Schematic (Or from the menu File, New Schematic)

This brings up a new window:

Screen Shot Add a Part.png

Now to insert the parts, using the "Add a Part" icon:

Add a part.png

This icon can be found half way down the tool bar on the left hand side of the window.

Clicking the "Add a Part" icon brings up a whole library of parts, which I find a little daunting:

Search Term.png

However, there is a search box, which helps a lot.

To find the Transistor BC548 type "BC548*" into the search box. Then click OK.

The device is then highlighted. It can be found in the "transistor" library, filed under "NPN".

Screen Shot 2015-08-03 at 15.14.52.png


Make sure it is highlighted. Click OK. This takes you back to the schematic but now with the BC548 icon attached to your mouse. Click where you want to drop it. If you want more than one, keep clicking. I'll do that for the resistors. Right clicking turns the part through 90 degrees. The escape key brings you back to the library. You can cancel out of the library to see the schematic.


The "trash can" next to the "add a part" icon can be used if you've dropped too many onto the schematic.

It doesn't matter what order the parts are added. They get linked together in the next step.

Clicking the "x" next to the search box clears the search and brings back the full library list.

For resistors I use the device "R-EU_0207/10 (R-EU_)" from the "rcl" library (use R-EU_0207* as the search term). There are many resistor libraries to chose from. Experience and asking advice (thanks @rocketengines) has shown this one works well. I suspect others do too. Two of them are needed on the schematic.

The battery holder and the photo-transistor are not already in the Eagle Library, so I had to make them. Here's a blog post on how I made the battery holder For the photo-transistor I copied a 5mm LED package and a LPT80A(opto-trans-siemens library symbol and made my own device called  TEPT5600TEPT5600


Here's a list of the parts, the Eagle Library they are in, what they are called and their Farnell Part Numbers.



My DescriptionEagle LibraryDevicePackageValue (User input)Farnell Part NumberFarnell Link
Resistor: 330 ohmrclR-EU_0207/10 (R-EU_)0207/10330R9341730

Resistor 75K ohmrclR-EU_0207/10 (R-EU_)0207/1075K9342230

Transistor BC548transistorBC548-NPN-TO92-CBE (*-NPN-)TO92-CBEBC5481574381
Adapt OneAdapt OneAdapt OneTEPT5600TEPT56001497673
LED ledLED5mm (LED)


Battery Holder  CR2032CR2032Adapt OneAdapt OneAdapt OneHU2032-LFHU2032-LF1319749
Battery 3V  CR2032CR2032N/AN/AN/ACR2032CR20322065171


Once all the parts are added, they can be rearranged and then linked together using the "Net" icon Screen Shot 2015-08-17 at 14.40.53.pngin the menu.

Screen Shot 2015-08-17 at 14.39.30.png

The schematic is now done. The next blog post will discuss moving the items around on the "board" view to make a neat PCB.


Remember to keep saving your work! I put version numbers on every time I save, in case I want to go back.