Step 3: Creating a Schematic for a Product

Picture of Creating a Schematic for a Product
One of the most important parts of reverse engineering an electronic device is to put together a schematic.  Depending on the complexity of the device, it may be unrealistic to put together a complete schematic, and you may have to settle for more of a functional block diagram.

Here are a few basic tips to help in the creation of a schematic or partial schematic from an electronic assembly.  It is by no means a comprehensive treatment of the subject, but it is a good way to start if you are new to the process.

In the case of this product, I was able to eventually put together a complete schematic. This product uses a double sided PCB, so all traces are visible.   If the PCB is multi layered it would be greatly more challenging to trace all the signals.

A good place to start is to try to determine the manufacturer and part number of as many of the components as possible.  It is usually possible to read enough of the manufacturer’s part number from ICs to use in a search for a datasheet. The datasheet will give you a good sense of the purpose of the part, from which you can further theorize about how the unit works.  The datasheet will also provide a pinout of the component, which helps immensely in the creation of a schematic.

If the component markings contain the name of the manufacturer or a recognizable logo, then it is usually possible to locate a web site for the company and search it for the datasheet.  Some manufactures sites are better than other for searching for datasheets.  Electronic component manufacturer’s web sites may also have application notes pertaining to their products and how to use them appropriately, and these can also be a wealth of free information.

If you have a part number but don’t know the manufacturer, you can always try to enter it into Google.  I have found that this will usually result in many sites, but they are often ones that specialize in datasheets and not the manufacturer’s web site itself.  It seems like many of these sites are from foreign countries, and it is often not straightforward to obtain the datasheet.  I usually try to see if these sites can at least provide the manufacturer name, and if they do then I try to go to their site directly to obtain a datasheet.

Some parts, like resistors, have the value written on them or represented with color codes.  From this information and the appearance of the part itself, the part can be identified. Likewise, most capacitors have some kind of marking on them indicating the capacitance and possibly also the voltage rating.

You may encounter some custom parts as well.  You may never determine all the specs for such parts, and will have to speculate or try to arrive at something close by indirect means.
jukees2 years ago
How can it be that there is a 16 electrolytic cap in mains, and also led's, i think your schematic is wrong
LargeMouthBass (author)  jukees2 years ago
Diode D2 rectifies the 120VAC input. Resistors R1 and R2, which total about 5k, are in series with the electrolytic cap, and limit to the current considerably. D1 is a 15V zener across the electrolytic cap, so it never sees a voltage greater than 15 V across it.
The LED is protected from reverse breakdown by D2, and the current is limited by R1 adn R2. (Refer to step 3 for details).

As I mention elsewhere, it is odd to see logic powered without any transformer isolation, but from the description I gave you can see how the circuit would in fact work. Its not the way I would design it, but it would be very cost effective.
deaks3 years ago
When looking for datasheets in Google, I find it helpful to type the part number I'm searching for, the word datasheet, then "filetype:pdf" so that it will ONLY return direct links to pdf's.

It saves a lot of time searching through those datasheet linking websites looking for the link where you can download. It has a secondary benefit of sometimes taking you directly to the datasheet on the manufacturer's website.
LargeMouthBass (author)  deaks3 years ago
Thanks for the tip! I'll definitely try that next time I make a general google search for a part datasheet.