Introduction to Logic Gates

Open Collector and Open Drain

As we said before, “1” means 5 V. Sometimes you need a higher voltage for controlling a device that doesn’t work with 5 V. You may want to control a 12 V relay, for example. Also, sometimes you may want to control a 5 V circuit but it drives more current than a standard integrated circuit can deliver. In those cases, you can use open-collector configuration.

Integrated circuits from the 74xxx series (all integrated circuit examples we are giving on this tutorial) are based on a technology called TTL, Transistor-Transistor Logic. Open collector means that the transistor used on the output of the gate doesn’t have its collector internally connected to the integrated circuit’s VCC (voltage). So, you have to do this connection by yourself. This means that you need to install an external resistor (called “pull-up”) between the output and VCC to make the circuit work. The good thing is that VCC doesn’t need to be the +5 V power supply. You can install it to a + 12 V power supply and feed your 12 V relay, for example.

The term open-drain is used for CMOS integrated circuits and is exactly the same thing.

Open collector or open drain gates are usually marked with an asterisk.

Figure 26: Example of open collector circuit.

Open collector configuration has far more applications. The most common is a technique called wired-AND, where the junction works as an AND gate. See the example in Figure 27. The output Y will be equal to (A OR B) AND (C NAND D). The junction will work as an AND gate.

Figure 27: Example of wired-AND using open collector logic gates.

Some open collector integrated circuits include 7403 (NAND gates, same pinout as 7400), 7405 (inverters, same pinout as 7404), 7409 (AND gates, same pinout as 7408) and 7433 (NOR gates, same pinout as 7402), just to mention a few examples.

We will stop here otherwise we will be leaving the original subject of this tutorial.