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Understanding Open Collector Outputs

Technical Notes

On several occasions, integrators have called asking about how to use particular devices that have open collector outputs. Those familiar with integrating relays with an automation or security system are sometimes baffled when, instead of seeing the Common, Normally Open and Normally Closed terminology associated with a relay, they read in the instructions that the device has an open collector, or solid-state, output.

So what is an open collector? How do you use it? Is it a relay output?

The open collector is a solid-state switch. It has no moving parts so, no, it is not a relay. It’s a transistor with the emitter at ground potential, and the collector “open” and accessible for use by the integrator. When a signal (bias voltage) is applied to the base of the transistor, the transistor turns “on” and allows current to flow from the collector to the emitter through the collector-base and base-emitter junctions effectively pulling the collector down to ground potential.

Here’s an example…when the transistor is off, and you have one lead of a resistor on the collector and the other lead tied to 12 VDC, the voltage present at the collector with respect to ground will be 12 VDC. Bias the transistor and the collector is now at ground potential. The output of the open collector has changed states, and this change in state can then be used to trigger the automation/security system, logic system, or energize the coil of a relay. “Pull to ground” and “current sink” are other terms used to describe the open collector circuit. Take a look at these diagrams of open collector circuits.

Here are a few useful devices available that have open collector outputs. The first is an RF receiver from Linear, the LNDXSR1504/08. The ’04 is available with four outputs, and eight outputs with the ’08. The other device with open collector outputs is a doorbell/telephone ring detector from ELK Products, the ELK930. The ELK930 can detect signals from two doorbells and one telephone line. When using the RF receivers, the bias voltage is applied when a compatible RF transmitter sends its notification or alarm signal. The RF transmitter could be from a door/window sensor, panic pendant, or a hand-held transmitter, just to name a few. With the ELK930, the bias voltage is applied when the circuits detects a door bell or telephone ring voltage.

Any questions? This will conclude the crash course on open collector outputs for now, and I hope you can utilize the above mentioned products in one of your upcoming projects.

6 Comments on Understanding Open Collector Outputs

  1. daniel mansbach // September 23, 2009 at 3:40 am // Reply

    we need to shift levels from 9vdc -:- 12 vdc cmos voltage level in the input of the level shifter and get a direct logic ttl level ( 5 vdc) @ the output

  2. I guess you are asking a question? Try using an opto isolator/coupler.

  3. Your link to the diagrams http://www.homecontrols.com/homecontrols/pdf/OpenCollectorDoc.pdf is no longer working… can you fix it and let me know…thanks

  4. Wow thanks for this! This is the most succinct practical explanation I’ve encountered for open collector circuits.

  5. is there a typical rating for voltage that can be sent across the collector and emitter. i know current is pretty low 100mA. i am looking at one that says permissible 24VDC Max, Can I use 5VDC instead? Thanks!

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