Open Collector Outputs — Technical Reference

Technical reference from Home Controls, Inc. explaining Open Collector outputs and how they interface with alarm/automation panel zone inputs and relays. Useful for installers wiring sensors, controllers, or transmitters that use open-collector transistor outputs.

What Is an Open Collector Output?

An open collector output is a transistor whose collector is left "open" (unconnected internally) so external circuitry decides how it's biased and pulled up. This lets the same output interface with a range of voltages and circuit configurations rather than driving a fixed voltage/current itself.

Transistor Basics

• No base bias applied: The transistor is OFF — high resistance exists between collector and emitter, blocking current flow.
• Bias voltage applied to the base: The transistor turns ON — the collector is pulled down to ground potential through the now-conducting collector-base and base-emitter junctions.
• Unbiased, with a pull-up resistor: Voltage at the collector (relative to ground) sits at the supply voltage (e.g. 12VDC) — the resistor pulls the collector up.
• Biased, with pull-up resistor: Voltage at the collector is pulled down to (near) zero.

Connecting to an Alarm/Automation Panel Zone Input

• No bias applied: The zone input reads across the resistor; current flows from + to – terminals. The open collector acts like a Normally Open contact switch — the panel sees no change.
• Base biased (transistor ON): The resistor is shorted out by the conducting collector-emitter junction. The panel sees a change in resistance across the zone input and reacts accordingly — circuit completion happens through the ground path.

Connecting to a Relay Coil

• Unbiased Open Collector connected to the minus side of a relay's solenoid: The relay is NOT energized. Common (C) is connected to Normally Closed (N.C.) — use the relay's contacts as desired for your application.
• Open Collector biased (transistor ON): The relay IS energized and its contacts change state — Common (C) is now connected to Normally Open (N.O.).

Practical Takeaway for Installers

When wiring an open-collector output into a zone input or relay coil, remember: the transistor being OFF (unbiased) generally corresponds to a "no change / Normally Open" state as seen by the panel, while the transistor turning ON (biased) pulls the collector to ground, changes the resistance the panel measures, and — in the relay example — switches the relay's contacts from N.C. to N.O. Always verify the specific device's polarity and pull-up/pull-down configuration against its own wiring diagram before connecting to a panel zone.