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
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.
- Codes Capacitor.pdf
- Codes Resistor-Capacitor&Others.pdf
- Codes SMD Codebook.pdf
- Codes SMD Diodes.pdf
- Codes SMD IAEA.pdf
- Codes The SMD Codebook.pdf
Step 2: The Schematic of the SMD 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
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
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
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
I repeated this process until I had all the pin outs of the LED display.
Step 7: The Final Pin Out
Step 8: 4 Digit 7 Segment Display With Decimal Point
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
Step 10: Mystery Circuit Schematic
Step 11: Adjustable PC Power Supply
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.