Quick build – A transistor NOT gate


I had a few minutes to spare the other night so I decided to build myself a quick transistor NOT gate. I will be adding other gates to the breadboard and posting their functions on the site but this is something to keep you occupied for now.

First of all, let’s have a look at the schematic of the circuit itself.

not gateWith only 3 resistors and one transistor, it’s easy to build this small circuit into other circuits.

The NOT gate is often called an inverter. Basically, you give it either a 1 or a 0 (on or off) and the output will be the opposite of the input. e.g. if you connect and switch to the input and an LED to the output, when you turn the switch ON, the LED will be OFF. If you turn the switch OFF, the LED will be ON.

Here is a quick picture of the of the circuit I built on a breadboard. (Click the image to get a larger version)


Excuse the untidy wiring, I never have time to trim cables and make things look neat, especially on a breadboard. At the top left corner of the board I’ve got a 9v battery connected to a 5v regulator. That then feeds 5v into a 555 timer. The timer is used purely to supply a pulsing on-off signal. I could have easily used a manual switch but I had a 555 chip nearby so I thought I might as well use it. (Same for the voltage regulator). The transistor and resistors lower down on the board are the NOT gate.

When the 555 timer sends an ON signal, the red LED lights up. In the image, you can see that the green LED is currently lit. This is because the red led is NOT on.

When the red LED is lit, the green LED is NOT on. Simple.

Here is a truth table to show a NOT gate.

not-truthBasically, as a recap, if you give the NOT gate a 1 (on) the output will be 0 (off) and vise versa.

Found – Xilinx CPLD chip

I’ve been looking through a few old boards in the parts bin and came across this….









It was sat in this board…


It’s some sort of ISA CD-ROM card.

I’m now seriously considering building my own board with this chip and building a parallel cable to program it with.

You can never have too many development boards….right?

Logic Gate Simulator

One piece of software that I’ve found invaluable is Logic Gate Simulator.

It can be found here. It was written by a guy named Steve Kollmansberger.

There are probably plenty of other gate simulator programs but this is the first one I found and I love it.

It lets you plonk your gates onto the drawing board and then tie the inputs and outputs together. I highly recommend the program and I suggest you give it a try.



My journey into logic

I haven’t looked at logic gates since I went to college 15 years ago. I recently decided that it’s time to revisit the theory and get a better understanding of electronics using logic circuits.

After viewing a few youtube videos and reading some old college textbooks, I’m in a much better position to start some physical experiments. I have looked in my parts collection and while there are plenty of TTL chips, none of them are the simple AND, OR and XOR type.

Onto the internet I went, looking for bargain chips to get playing with……. unfortunately, I just don’t have the spare cash right now to go throwing at random chips.

Enter this ………..



This is a cheap CPLD board based around the Altera Max II chip.

I know I could have spent a bit more and gone for a larger chip or even an FPGA but I’ve decided that this  will suffice for now.

My aim is to design the logic circuits using the schematic view of the Quartus 2 software. I know everyone these days uses VHDL or Verilog but I’m not all that bright so trying to learn a new language is just going to send me over the top.

I’ve now got to wait another 20 or so days for it to arrive from China but I can’t wait to get started with my adventures into logic circuits.

Bring it on!!!

Page 3 of 3123