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Open-Collector Logic

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An open-collector logic gate is a digital logic gate with an output terminal that can be disconnected from its power supply. The open-collector output is either connected to the ground or left open (disconnected), allowing the output voltage to be pulled low by a resistor. Open-collector outputs are used for interfacing with other digital devices, such as sensors, that require a low voltage level to indicate a logical “0” state.

Open-collector logic is a digital electronics term that refers to a particular type of electronic circuit. In an open-collector circuit, the output transistor does not have a connection to the ground. This means that when the transistor is turned on, it can pull the voltage at the output low, but it cannot pull it high. Electronics, such as phones and computers, are found in nearly every aspect of our lives these days. The underlying principles of electronics are relatively simple, but the applications can be very complex. Digital electronics is the use of electronic components to process and manipulate digital signals. These signals can be thought of as 1s and 0s that make up binary code. By stringing this together, we can create an instruction set on what a computer or other digital device needs to produce. The heart of any digital electronic device relies on two main components: logic gates and flip-flops. Logic gates take in input signals and produce an output signal based on a pre-determined rule. An AND gate will only output a 1 if both of its inputs are 1. A NOR gate, on the other hand, outputs a 1 unless both of its inputs are 1. Flip-flops are bits of data that are stored in memory. A flip-flop can be used to create anything from a simple counter to complex machines. 

Open collector logic

Open collector logic


Working of Open-collector logic:

In open-collector logic, the output is either connected to the ground or left open (disconnected). When the output is connected to the ground, the voltage at the output is 0 volts; when the output is left open, the voltage at the output is equal to the power supply voltage.

Open-collector logic is used in applications where it is necessary to control a device that is powered by a voltage source other than the logic circuit power supply. For example, an open-collector output can be used to control a device that is powered by an AC line voltage.

The main advantage of open-collector logic is that it can be used to interface two circuits that have different power supplies. The disadvantages of open-collector logic are that it cannot be used to drive more than one load and that special care must be taken to prevent damage to the transistor due to excessive currents.

Working of Open-collector logic:

Working of Open-collector logic

All primary terminologies

In digital electronics, the two voltage states of a signal are represented by zero and one. The number 10 can be represented as 1010 in binary form. A digital circuit is an electronic device that uses a series of discrete voltages. Digital circuits are made up of logic gates, which perform Boolean logic operations on the inputs and output based on those operations. Boolean algebra is a mathematical system that uses a binary code to represent logical relationships. It’s used in virtually every digital circuit and has an important role in designing and analyzing them. Logic gates are digital circuits that have Boolean logic operations on their inputs and produce an output based on those operations. The most common types of logic gates are AND, OR, NOT, NAND, NOR, XOR, and XNOR. A truth table is a chart showing the outputs of a digital circuit for every possible input combination. Truth tables are often used to design and analyze digital circuits. 

Applications of open-collector devices:

Open-collector devices are often used in logic circuits because they can pull the output voltage of a circuit low. This is useful in digital electronics because it allows for the creation of circuits that can have multiple outputs that are all pulled low by a single open-collector device. There is one common application for open-collector devices: TTL circuits. In a TTL circuit, bipolar junction transistors (BJTs) are the active element. When a TTL circuit is powered on, the collector-base junction of the BJT is forward-biased and the transistor is turned on. This allows current from the collector to the emitter and causes a drop in voltage at the collector to less than one volt below that at the emitter. The transistor then turns off by reverse biasing the collector-base junction which cuts off the current flow and pulls the voltage at the collector back up to that at the emitter. Open-collector TTL devices are used when it is necessary to have multiple outputs and one input. For example, an AND gate may have two inputs and one output. If both inputs are high, then the output will be high as well. However, if either input is low, then the device will pull the output low. This means that several inputs can be read together even with a single device.


Open-collector logic is a type of logic where the output signal is open-drain. This means that the output can sink current, but cannot source current. Open-collector logic is often used in situations where multiple devices need to share a common signal line. 

  1. Improved system performance
  2. Lower capacitance
  3. Larger loads may be connected to the collector without the danger of crossover breakdown 
  4. Open collector outputs are relatively insensitive to electromagnetic interference

Multiple open-collector outputs can be wired together to create a logical OR function. For example, if you need an OR function with two inputs, you could wire one of these outputs to A and the other to B. This can be useful if you are trying to figure out which is the better option for your system, for example, whether to use type A or type B motors. Open-collector outputs can be pulled up to a higher voltage with a resistor, whether you’re using an AT mega or a 3.3V device. 

Open drain outputs have a high impedance when left undriven, making them ideal for use as status indicators. One disadvantage of open-collector logic is that high voltages are required to turn off the circuit. 

Open collector outputs cannot source current, so it’s not possible to drive more than one load from a single open collector output. The maximum voltage output of a device is limited by the driving device’s VOH (voltage of high). For example, if an open collector is being driven by a 5V device, the maximum voltage that could be applied to the load will be 5V.


The main advantage of open-collector logic is that it’s easy to interface with other digital devices. The open-collector design also doesn’t suffer crosstalk or noise as much as other digital designs. However, the disadvantage of open-collector logic is that it isn’t as fast as other digital logic designs. An open collector is a type of output found on some integrated circuits (ICs) that cannot sink as much current as standard output. When used as an input, the open collector gives the same result as a switch: it can be either ON or OFF. The advantage of using an open-collector output is that it can be connected to other open-collector works to create a wire OR connection.

The main advantages of open-collector logic are:

  1. It can be used to create a wire OR connection between multiple outputs.
  2. Open-collector outputs can sink more current than standard outputs, making them ideal for driving LEDs and other low-current devices.
  3. Open-collector logic is often used in applications where Electrical Interference (EMI) is a concern, as the lack of an internal connection between the output and ground reduces the amount of EMI generated.


When using open-collector logic, one of the main disadvantages is that there can be a potential for contention. This happens when two or more devices are trying to pull the output low at the same time. In order to avoid this, proper design and layout techniques must be used. Another disadvantage is that open-collector outputs can only sink current, they cannot source it. This means that if you need to drive a high-level signal, you must use an external pull-up resistor.


There are many digital electronic devices that use open-collector logic. Open-collector logic is a type of digital electronic circuitry where the output stage of the circuit is open. This means that the output stage does not have a fixed voltage, but instead relies on the voltage of the load to determine the output voltage.

Open-collector logic is often used in real-time applications because it allows for a faster reaction time to changes in input voltages. For example, if an open-collector output is driving a load that is slowly changing voltage, the output will also change slowly in response. This can be an advantage over other types of digital logic circuits, which may take longer to respond to changes in input voltages.

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Last Updated : 09 Nov, 2022
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