We all love our modern devices ranging from smartphones to smart cars and all of it is made possible thanks to the transistor! The technology we use has only been around for a few decades, so how did engineers in the past create complex digital circuits? This also brings up the question “how did NASA build a computer that was small enough to fit on the Apollo modules?” The answer lies in RTL which stands for resistor-transistor logic which for a while was one of the most popular logic families.

Logic Gate Schematics





While there are many logic gates that exist, the three gates that will be shown here will be the fundamental gates (those that use the least number of parts and can be used to make every other logic gate). These gates are the NOT gate, NAND gate, and the NOR gate.

The NOT gate consists of a single NPN transistor, a collector resistor, and a base resistor. When the input is connected to 0V, the transistor is switched off and does not conduct any electricity. This means that the voltage at the collector will be equal to the supply (if the output is unconnected). If the input is then connected to VCC, the transistor fully saturates (i.e. switches on), and effectively connects the output to ground (0V). Hence, the operation of the NOT gate is performed.