Introduction: Reverse Engineering
Many of the members here at Instructables ask about datasheets or pin outs of a device or display in answers, unfortunately you cant always get a datasheet and schematics, in these cases you only have one choice reverse engineering.
Reverse engineering is a skill that is not taught in college or university electronics courses and yet it is a valuable tool in an engineer’s tool belt. Many people working on electronic equipment search to no end for schematics for a device when if they knew how it in all probability it would take less time to make their own schematics and parts lists.
Take the flash circuit in this Polaroid Camera, if you wanted to model a capacitor charger after the flash circuit in this camera, unless you worked for Polaroid your chances of getting a schematic are next to impossible. Other than the resin covered custom IC the rest of the circuit was easy to trace and make a schematic of taking me just four hours.
You can hand draw your schematics, build them in paint like I do, or build your schematics a circuit simulator like 123D Circuits where they are easy to read and you can test them making sure you made the schematic correctly.
I am going to show you some of the things you need to know to reverse engineer.
Step 1: Reverse Engineering Surface Mount Device (SMD) Circuit Boards
Although through hole circuit boards and SMD circuit boards look quite different they work the same way and they are reverse engineered the same way. Through hole components are the first components engineers learn to read, however surface mount devices, (SMDs) are not as easy to read because of the small aria for a part number or color codes. To identify components correctly you need SMD code books and SMD codes I have attched these in PDF format.
SMD resistors codes are alphanumerical however they are not that different than resistor color codes, the first number is the first digit, the second number is the second digit, the third number is the multiplier, and the R is the decimal place. So 103 is 10 kΩ and 4R7 is 4.7 Ω almost like the code on ceramic capacitors only with capacitors the letter is the tolerance.
SMD ICs have more information on them like 74HC595 or HC595. The through hole device is SN74HC595 and most of the time the datasheet for the through hole will do the same thing however not all the time. To get the exact datasheet for most SMD ICs use the part number prefix MM, so SN74HC595 is MM74HC595 and you can get the datasheets at these web sites.
For SMD transistors and other semiconductors you need code books like the ones in PDF format here, on the semiconductor you will see a code like 2X F. When you look up the code in the code book it gives you the SMD part number MMBT4401 and the through hole part number 2N4401.
Step 2: The Schematic of the SMD Circuit Board
Now that you have the values and datasheets for your components you can make a schematic of the circuit board. For this part sometimes I remove the components, photograph the circuit board, and trace out the conductors in paint. Start at one pin and follow the conductor to the first component draw the pin and wire and the component if the conductor branches off draw it to the next component. Once you have drawn all the branches of that conductor go to the other side of the first component and draw the next conductor and its branches to the components it goes to. Continue this until you have drawn the schematic of the circuit board.
Three capacitors 10 nf
Two-4 bar 103 is two-4 x 10 kΩ resistors
Two-4 bar 75R0 is two-4 x 75 Ω resistors
Two x 392 is 3.9 kΩ resistors
Two x 51R1 to 2 LEDs is two 51.1 Ω resistors
Three x 82R5 to 3 LEDs is three 82.5 Ω resistors
Four x 68R1 to 4 LEDs is four 68.1 Ω resistors
One x 42A04F8, HC163 is MM74HC163 or SN74HC163 IC
Two x 42A74HT, HC595 is MM74HC595 or SN74HC595 IC
Two x 2X F is MMBT4401, SO4401, or 2N4401 transistors
Kingbright SA43-21GW SA43-13GW one digit 7 segment LED display
Step 3: Reverse Engineering a Component
Sometimes you need to reverse engineer a component because you can’t find a datasheet for the component. This is common with components that don’t have part numbers on them like Chip on Glass Liquid Crystal Displays (COG LCDs) and Light Emitting Diode (LED) displays.
Many people salvage parts from used electronics like the display in this alarm clock and when they go to use it in a project they can’t find a datasheet for the display. All you can tell is it is a four digit seven segment display and they spend hours trying to use a meter to find the pin outs. Before removing a component from a circuit board find the datasheet for the component or reverse engineer the pin outs.
Weather a COG LCD or a LED display they are reverse engineered the same way, follow the conductors from the display to the driver IC and look up the IC.
Step 4: Cleaning the Circuit Board
Quite often the circuit boards are dirty with debris and scuffs on the components making the circuit board and components unreadable. To clean them I use a rag, a paintbrush, and methanol or rubbing alcohol, to clean the parts. This makes the components part numbers and color codes more readable and it is easier to see where the conductors go.
Do not smoke or clean the circuit board near an open flame, alcohol burns.
Do not rub to hard or printed on part numbers can be removed.
Step 5: Start Engineering
Now that you can read the part numbers, color codes, and conductors, it is time to start reverse engineering the component you cannot get a datasheet for.
Start by following the conductors from the component you cannot get a datasheet for to its driver, in this case a LM8560 IC.
Look up the IC and get the datasheet, you can do this by looking up the LM8560 in a data book or by looking it up at these two web sites.
Once you have the datasheets find the pin outs and start tracing out the conductors.
Step 6: Tracing the Circuit
Like I did in the SMD circuit board I photographed the circuit board and traced the conductor’s one or two at a time and drew the conductors and components in a partial schematic since I only wanted to make a datasheet for the LED display.
I repeated this process until I had all the pin outs of the LED display.
Step 7: The Final Pin Out
After I made partial schematic of the LED display and its connections to the IC and the rest of the circuit board I checked the circuits in the display with a meter and made a schematic of the display. Now when I go to use the LED display in a project I already have the pin outs.
Step 8: 4 Digit 7 Segment Display With Decimal Point
Some 7 segment LED displays are hard to get datasheets for and common anode and common cathode cannot be determined by just looking at the display. But when you look at the circuit board the transistors indicate which pin is the common cathode or the common anode by going from the pin to ground or Vcc. The SN74HC595 IC makes it a simple matter to decipher the LEDs segments a, b, c, d, e, f, g, & DP.
With the SN74HC595 QA connects to segment a, QB connects to segment b, and so on until you get to QG to segment g. QH connects to decimal point, and QH’ connects to the second SN74HC595 to drive the anodes of the individual digits.
This LED display is a serial input parallel to the common anode 4 digit 7 segment LED display with decimal point. Now I can make my own datasheet for the Common Anode 4 Digit 7 Segment LED Display.
Step 9: Mystery Circuits
I look for things to do so I salvage, on one of my salvaging forays I came across this circuit. At first I thought it was an audio amplifier for a PC but when I looked at it more closely it only had power in and 4 outputs, no audio input. It had to be some kind of adjustable power supply, so I reverse engineered it.
Step 10: Mystery Circuit Schematic
As I explained in the earlier steps of this Instructable I started by looking up the components and making a parts list. Once I had a parts list I started at one pin and traced the conductors to the components and from the components to the outputs. As I traced the conductors I made the schematic.
Step 11: Adjustable PC Power Supply
Once I made the schematic and the parts list I built the circuit in a circuit simulator. Faster than doing the math a circuit simulator can give me test points and values I can use in the real world to determine if the circuit is working as designed.
This circuit is a 160 watt adjustable power supply for inserting into a PC beyond that I can only guess at its purpose, it is ether a supply for specialized accessories or converting a tower to a complete electronics lab with adjustable power supply.
Reverse engineering circuits and components gets you schematics for electronics you cant find schematics for and tells you pin outs for components you cant find datasheets for, a useful tool in your tool belt.