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
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.