Understanding Open Collector Outputs in Smart Home Setups

In the realm of smart homes and automation systems, the terminology surrounding various device outputs can sometimes be confusing. One such term that often raises questions is the “open collector.tup” Integrators familiar with relays might find themselves puzzled by this term, wondering how it differs from the more common relay setup. This article aims to demystify the concept of open collector outputs, explain their functionality, and provide practical examples of their usage in smart home setups.

What is an Open Collector Output?

An Open Collector output, essentially an NPN transistor, allows current sinking to a common point in a circuit.

It behaves like a switch, connecting the circuit after the load to a common point. However, it requires an external power source to function.

Without a supply through a load, it acts as a dry contact, offering no change in voltage at the output.

To verify its functionality, voltage measurements should be taken across the output load.

This ensures proper assessment, emphasizing the need for an external supply to activate the open collector. It essentially operates as a switch, completing the circuit after the load to a common point.

Understanding the Change in State:

Consider a scenario where a transistor is inactive, and one end of a resistor is connected to the collector, while the other end is linked to 12 volts DC. In this state, the collector’s voltage with respect to ground remains at 12 volts DC. Upon applying bias to the transistor, the collector’s potential shifts to ground, causing a change in state. This alteration can be harnessed to trigger automation or security systems, logic setups, or even energize relay coils. Terms like “pull to ground” and “current sink” are often employed to describe the open collector circuit.

Examples of Devices with Open Collector Outputs:

Here are a couple of noteworthy devices that make use of open collector outputs:

Linear RF Receiver (LNDXSR1504/08):

The Linear RF receiver comes in two variants: the ’04 model with four outputs and the ’08 model with eight outputs. These devices work in conjunction with compatible RF transmitters to send notification or alarm signals. These transmitters could include door/window sensors, panic pendants, or handheld transmitters. When the RF transmitter activates, a bias voltage is applied to the open collector output, indicating a state change that can be integrated into your smart home automation.

ELK Products Doorbell/Telephone Ring Detector (ELK930):

ELK Products offers the ELK930, a detector designed to identify signals from doorbells and telephone lines. This device is equipped with open collector outputs that activate when the circuit detects a doorbell or telephone ring voltage. It’s a valuable addition to your smart home setup, ensuring that you never miss important notifications.


To wrap up, it’s evident that open collector outputs play a pivotal role as solid-state switches, characterized by their capacity to transition states via the application of bias voltage. While fundamentally different from traditional relays, these outputs present a dynamic solution within the realm of smart home automation. By leveraging their unique mechanism, a myriad of actions can be effortlessly triggered, adding a layer of intelligence to your home systems.

The practical implications of open collector outputs are vividly demonstrated through devices like the Linear RF Receiver and the ELK Products’ ELK930. These exemplary tools showcase how these outputs integrate seamlessly into the architecture of smart homes, paving the way for enhanced control and responsiveness. This integration is not merely an option but a necessity for integrators who aspire to construct homes that harmonize efficiency, convenience, and innovation.

In conclusion, open collector outputs stand as a testament to innovation within the smart home landscape. Their innovative solid-state approach, distinct from traditional relays, offers a new dimension of flexibility in automation. Through practical implementations, like the Linear RF Receiver and ELK930, the power of these outputs becomes tangible, reshaping the way we envision and design modern smart homes. As technology continues to evolve, embracing open collector outputs is a strategic move that empowers integrators to build cutting-edge, adaptable, and seamlessly interconnected smart home systems.

David Coleman
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