Transistor Logic Gates 1

I have been trying to build some transistor logic gates for a whlle using my old 2n2222a transistors.  Having sent for more (50) I have been having a new attempt.

Today I was successful

The circuit is from a book called Getting Started in Electronics – By Forrest M Mimms III, who will be familiar to anyone who remembers Tandy or I would guess any readers out there from the USA

So to start off with I have built an AND gate.  I took a photo of the page for inclusion here.  (which I guess is ok)

So far this has been built up and tested on a breadboard.  It works as the LED only comes on if BOTH switches are pressed.  I have now started to work out how to solder this up.  Once built I will have at a digital logic circuit which in theory could be used in a classroom to help teach digital logic,  this would fit in with the new national curriculum for IT / Computing.   I can then move on to building the other circuits that go along side this.

Issues:  The circuit sort of works,  however if I press button 2 the LED comes on this aparently us due to the LED drawing current from the circuit,  if I press button 1 the LED glows brighter.

It seems I need a resistor, between the LED and ground.  Hopefully then the led will stay off when both switches are off .   I didn’t notice this before on the breadboard as the LED was still glowing very dimly. so only really noticeable in lower light.

So you learn by experience,

If I understand my chat with Speccyman on IRC properly to calculate the resistor you

Vs = Supply voltage

Vd = voltage drop of component

Vc = Vs – Vd

Vc should be the sum of voltages drops across various components I think.

You can then calculate resistor by using ohms law

R= V/I


I am happy to share what I am doing with schools and I would really like to support this new curriculum,  some of my other projects should be easy enough for primary level.

If any Torbay based schools have a need for a Teaching Assistant with a more specialist interest in ICT / Computing / Science.  I have more experience at primary level but may consider secondary schools too.