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8 Bit Adder

A simple 8 bit adder from discrete transistors

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In this project I tried to build the simplest calculater possible from discrete transistors in order to learn more about digital logic on the most basic level

I started experimenting with single transistors on bread boards, building simple logic gates. The next step was to assemble the logic gates into a full adder. At first I used multiple different types of logic gates like in the following image.

I made the circuit on a breadboard and even build a second one to hook together. They worked as expected although the leds on the output were quite dim. But I didn't let that stop me and pushed trough with creating a PCB and ordered it. After hours of assembling half the board with 72 out of 144 transistors and twice as many resistors, I plugged it in only to see that the adders worked on their own but the carry out voltage was to low to turn on the next transistor. 

I created a simulation of the circuit with CircuitJS (see attached files) which showed me, that with my current architecture there are some cases where the voltage drop across the transistors is to great to turn on the next transistor. 

After some research I found an alternate design that only uses NOR gates that are very simple to implement with a single transistor and three resistors per gate. This effectivly cut the total number of transistors in half and made the design actually work. This time I tested everything in with simulations before ordering and assembling a PCB.

The stunning PCBs for this project were gerenously sponsored by PCBWay. The overall quality is very  good, they are made with very good tolerance and have a really good surface finish which was especially important for this project as it is supposed to be a show piece. This was my first time ordering from them and the process was very smooth and easy and the different manufacturing options are much better than with any other services I've tried in the past.

  • 72 × BC547b NPN Bipolar junction transistor
  • 144 × 10k Resistor
  • 72 × 1k Resistor
  • 25 × 220Ω Resistor
  • 25 × 3mm Led

View all 8 components

  • It's alive!!

    Robin Dorst6 days ago 0 comments

    The new transistors have finally arrived and after soldering one full adder on the board the output leds finally lit up and everything worked as expected.

    Now I just had to desolder the trasistors I've already put on and then solder the rest of the board.
    Thanks to my workplace, desoldering the transistors was a breeze with a metcal desoldering station powered by shop air. After that it was just a rinse and repeat for over 700 soldering points. 
    But all this soldering was quite enjoyable and worth it to have a functioning board that looks stunning. 
    After all that it's time for a case. I already have a really nice peace of wood which I want to mill out as well as a CAD-Design. The only missing piece is a CNC-Mill. 
    I also only now realized, that it would be nice if this board was battery operated. I will think of some way to add in a Li-Ion battery and charger circuit into the case.

    In the meantime I am thinking about the next project. I wanna do something more complex. 
    My Ideas so far are:
    - switch to ICs to design a more complex circuit getting closer to an actual computer
    - switch to SMD transistors and make a proper calculator
    - make modular PCBs that can be connected  in different ways to create different kinds of computing devices/functions
    If you would like to see any of the above feel free to drop me a comment, follow this project and my account.
    Thanks again to PCBWay for providing the stunning PCBs for this project.

  • First assembly with a bit of trouble

    Robin Dorst11/07/2024 at 14:22 0 comments

    After the first design failed I completly started from scrap with a new KiCad project and put way more effort into making the PCBs look nice.

    I got the PCBs and was very exited because I had never ordered ones with ENIG imersion gold finish and together with the purple it just looks amazing. This is a show piece after all and I am planning on building a nice wooden enclousure for it. 

    But I digress. I started the assembly progress by first soldering the USB C with some solder paste and a hot air gun and then continued on to the first full adder. After soldering 9 transistors and over 30 resistors I had enough components on the board to test basic functionality. But after flicking the switches in all possible combinations all the output led's stayed dim and unlit and the panic started creeping in. Of course it wouldn't work on the second try. But this time my mistake was much more basic and unnecessary as with the first PCB. After a quick probing session with the multimeter I turned to the schematic and board layout in KiCad to reference some things. I found the coulprit quite fast. I don't know how it happend or how I never noticed it but at some point I messed up the connection between footprint and schematic. The pin numbers didn't mach up correctly. In the schematic the base of the transistor is pin number 1. On the physical part of the BC547b transistor the base is the middle leg but on the footprint the pins are numbered from left to right making the the base the left most pin.

    So after a short rage I came back to my senses and researched if there is a npn transistor with similar characteristics to the BC547b but the weird pin out I accidentally created. The only one I could find that might fit is the SC945 but I have to rotate it 180 degrees to make it align with the correct pins. 

    I will post another update once I receive the new parts. Until then remember to double check your footprints even if they seem very simple.

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Dave's Dev Lab wrote 6 days ago point

awesome looking build!

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Ken Yap wrote 6 days ago point

👍 Excellent self-education project. It made you delve into details of the logic gates.

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