Retro Electronics: DIY Resistor-Transistor Logic Gates

By Robin Mitchell

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.

 

Required Materials

There are no specific number of parts listed here because this depends on how many you want to make. Also, all resistors are the same value and all transistors are 2n3904 bipolar junction transistors. This is one of the reasons why this logic family was very popular (due to the simplicity).

 

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.

 

NOT Gate Schematic

 

The NAND gate consists of two NPN transistors in series, each having a base resistor (for current limiting) and a single collector resistor. When both inputs are off (0V), both transistors are turned off and therefore the output is connected to VCC through the collector resistor. If either input is turned on (but not both), then the transistor that is turned on will conduct, but the transistor that is turned off will not conduct current. Therefore, the output remains connected to VCC through the collector resistor. If, however, both inputs are connected to VCC, then both transistors are turned on. This means that the output is connected to 0V through both transistors and therefore the output is off, thus realizing the function of the NAND gate.

 

NAND Gate Schematic

 

The NOR gate also consists of two NPN transistors which are arranged in parallel. If neither input is turned on then none of the transistors are turned on and therefore the output is connected to VCC via collector resistor. If either input is turned on, then the corresponding transistor turns on and thus connects the output to ground via that transistor. Even if both transistors are on, the result is still the same and the output remains off, thus realizing the NOR function.

 

NOR Gate Schematic

 

Constructing Your RTL Logic Gates

Building RTL logic can be built using any construction technique. However, when building RTL systems, you need to consider the size of the circuit that you wish to construct. For example, building a single gate can easily be done on breadboard or stripboard but if you plan to build a computer then you need to consider methods for saving space and time. So, the RTL NAND gates demonstrated here are built on a PCB that holds four NAND gates in the same pinout as many popular 4000 series chips. This means that this larger package can be mounted and then wired to other logic clusters which makes wiring much easier.

 

NAND Gates on PCB

Four NAND gates on a single PCB

 

This four-gate package could be made much smaller if made on a double-sided PCB. If surface mount parts are used, then the size could be reduced even further and make large scale projects easier. I hope you learned a little about logic gates. If you’ve made your own logic gates, please share them in the comments!

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Showing 6 comments
  • Robin Mitchell
    Reply

    Hi,

    As sad as it is that is an impossible task! Any load on any output will affect its fanout. However, to have minimal impact use a CMOS device such as any 4000 series chip such as the 4096, 4040, 40106, or a discrete CMOS NOT gate using an NMOS and PMOS transistor.

    All the best,
    Robin

    • A Nerbman
      Reply

      I apologize. I did not explain myself very well. If an RTL gate output where to be connected to an LED driver such as this one http://apps.usd.edu/coglab/psyc770/images/trans5.gif, with an LED and current limiting resistor in series with the transistor’s collector, the difference in base current supplied by the RTL gate output would be much smaller than the collector current of the LED driver’s transistor. In this situation, the RTL gate’s fan-out would be significantly reduced to probably one or two. My idea was to place an RTL inverter between the RTL gate output and the LED driver, allowing the RTL gate output to maintain its typical fan-out when dealing with just RTL inputs. If I didn’t have access to the complemented version of what I was trying to display, I could use a double inverter. I believe I have seen something similar used in the Apollo Guidance Computer. The reason I asked about it here was to see if there were any other solutions that didn’t require as much additional circuitry while still using RTL exclusively.

      • Robin Mitchell
        Reply

        Hi,

        The first thing you can do is significantly reduce the size of the collector resistor on the gate. My designs show 10K but you could change this to 100 ohms and it would provide a lot more current when the transistor is off. However, this means that the transistor will get warm when fully saturated.

        Another way around would be to use an NMOS as your LED driver instead of a BJT since they are voltage activated devices and NOT current activated devices. See here

        http://www.pretzellogix.net/wp-content/uploads/2011/03/LED-MOSFET.png

        All the best,
        Robin

        • A. Nerbman
          Reply

          Thanks for taking the time to respond and providing some alternatives. I think I’ll stick with what I came up with for now while my project isn’t entirely permanent.

          • Robin Mitchell
            Reply

            Hi,

            No worries! Hope your project turns out well =)

            All the best,
            Robin

  • A. Nerbman
    Reply

    What kind of circuit would you use to drive an LED with the output from one of these without effecting its fan-out?

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