Logic Gates: Digital Logic Gates Just Using Transistors

Logic Gates: Digital Logic Gates Just Using Transistors

4 digital Logic Gates Just Using Transistors, AND,NAND,NOT,NOR, and a simple flip flop circuit

There are times when you want to incorporate some basic logic into your designs but your available pcb space is limited, or maybe you would like a cheaper alternative to the more costly IC option.

Transistors used in these circuits can be any npn transistor such as BC547, BC548, BC549, 2N3904 etc.

Diodes can be almost any type so long as they can handle the reverse voltages in your design. LED's were only used to provide a visual reference to show these circuits in action.

Experiment on breadboard first before you use any of the circuits shown, All were tested before publishing and All of the component values can easily be played around with to meet your needs.

inputs can be virtually any signal source from timers to oscillators and more, or even push buttons and reed switches and so on.

I came up with these circuits back in 1995 when i studied Electrical Engineering in college, i quickly moved onto microprocessors so i never got a chance to put these designs into practice apart from a test probe below which uses my NOT gate design, so i would love to hear any ideas you have or maybe you are putting one into practice right now, please let me know.

2 input AND Gate

Both inputs need to be high to switch the output on.

2 input NAND Gate

Both inputs need to be on to switch the output off, if both inputs are high then the output is low. Both Diodes are needed to prevent feedback, The input resistors could be lower values if used in a lower voltage circuit or input voltages are less than 9v. The 220 ohm and 500ohm resistors are required to make the circuit switch on and off.The 1K Base resistor could be changed for a variable resistor to create a variable output voltage.

NOT Gate

The result is the opposite of the input, if the input is high then the output is low, somtimes called an inverting buffer, although buffers generally have a high output voltage compared to standard logic gates.

2 input NOR Gate

Output is high until one or more of the inputs are used. More inputs can easily be included, as you can see the inputs share the same transistor base, you may need to add diodes to each input if you need to prevent any feedback or if you come across any problems, if there are switching errors you could also change the resistor values for each input, the 2 existing diodes are there to prevent basic switching errors.

:: A Simple Practical Useful application with the NOT Gate above ::

:: Test Probe / Continuity Tester ::

Every now and again you may need to test a circuit board for short circuits or broken tracks so this handy little gadget should help you out.

The probes can either be a pair of crocodile clips or purpose made probes which you should be able to source from any good component supplier, if you wanted you could even integrate them onto the PCB with the rest of the circuit.

I designed this circuit to run on 2 AA or AAA batteries but you could also use a 3V button cell commonly used for car remote key fobs, do as much testing as you can before any sensitive components are fitted, although there is only around 800mv to 1.5v running through the probes, its better to be safe than sorry.

When the circuit is live, the red LED is lit but when the probes come into contact with a solder bridge or a short circuit is found, the red LED switches off and the green one lights up. Other fault checking could include broken tracks, in which case the led will remain red and aid in tracking down the problem areas.

How it works is fairly simple.

The transistor connected to the red LED will only stay on while there is no contact between the probes, as soon as the second transistor is switched on, there isn't enough power for the first one so it switches off, The 220 ohm resistor helps block the power to the first base while it flows into the second transistor which is now switched on.

I am working on a simple cheap Analog to Digital Converter at the moment so if you would like to see it and some other great stuff please click the follow button to keep an eye on my future instructables. Read More