Electronic – Which are the input/output requirements of a 12V trigger interface

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For a personal project, I'm building a simple I/O control board for an audio module. This board has an MCU that implements all the I/O control logic, and some interfacing electronics as well. It will be integrated in the audio module.

One interface that I need to handle is 12V level-triggering. The control board must keep powered up my audio module as long as the TRIGGER IN signal is HIGH (+12Vdc), and power it off when it's LOW (0V). I also need to generate a TRIGGER OUT signal (or several of them) in order to pass this "command" to other external audio modules.

This rudimentary triggering mechanism has been around for decades (and is still widely used) in consumer audio gear. It's very common, like a de-facto standard, but I can't find any reliable source of either formal or informal specifications for this interface, like:

  • Maximum/minimum input/output impedance.
  • Maximum/minimum input/output current.
  • Acceptable input/output tolerance on the 12V level.
  • Maximum rise/fall time, if applicable.
  • Any other relevant specifications: isolation requirements, inrush current, etc.

So the question is: which are the input/output requirements of a 12V trigger interface? Is there an specification in place for it?

N.B.:

This question is NOT about how to build the interfacing circuits, as I believe I already have a pretty good idea about the route to go (an optocoupler for receiving TRIGGER IN and a low power 1-2W isolated DC/DC converter for transmitting TRIGGER OUT). What I actually want to know is which are the operating conditions that my interfacing circuits must deal with.

Best Answer

There doesn't seem to be any official standard for this, in fact, exact voltage and current values are not consistent even for products of the same manufacturer. Variations of this picture appear more than once in discussions regarding audio equipment triggers.

Here's what I would do.

For inputs:

  • tolerate at least up to 15V
  • treat anything above 3V as logical "1" and anything below 1V as logical "0"
  • draw between 1 mA and 10 mA of current for any input you interpret as "1" (1.5-3 kOhm input resistance)

For outputs:

  • Output logical 0 as below 0.5V
  • Output logical 1 as 10-12V
  • Provide at least 100 mA of current.
  • Handle short circuits gracefully
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