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?


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.

  • \$\begingroup\$ Your question is fuzzy ! Trigger with +/- or AC phase ? Input capacitance, resistance, limiting, external factors etc. got a lot arguments. A little trick : You need using current if haven't any idea which LEVEL is incoming. Logic and signal is very different points. Logic got standards (CMOS,TTL etc.) but signal got non-categorized shape. Allowable range(1-12V) > grounded to GND with diode(use a protection resistor but all values related your range) > use diode voltage as current for schmitt trigger converter = got a logic level, off-course you can use galvanic equipments... \$\endgroup\$
    – dsgdfg
    Mar 31 '17 at 7:09
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    \$\begingroup\$ I'm planning to use an optocoupler for receiving TRIGGER IN and a low power (1W or so) isolated DC/DC converter for transmitting TRIGGER OUT. \$\endgroup\$ Mar 31 '17 at 7:42
  • \$\begingroup\$ You already know signal frequency, but need use galvanic isolator if reached to Mhz frequencies. Most optocoupler got garbage delay, need use PS9115 if your signals is communication signal. \$\endgroup\$
    – dsgdfg
    Apr 1 '17 at 10:40
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    \$\begingroup\$ @dsgdfg MHz?! Seriously, 12V changes when a device is completely turned on or of. Like, frequencies of \$<10^{-2}\,\text{Hz}\$, not \$>10^6\,\text{Hz}\$. I'm absolutely not sure what you're referring to, nothing of what you say makes any sense in this context. \$\endgroup\$ Apr 1 '17 at 10:44
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    \$\begingroup\$ @EnricBlanco, short of finding a specification for this interface your best bet is to find some devices that already implement the interface and measure their actual voltages and output impedances. Then derate by a percentage and build to that. \$\endgroup\$
    – Trevor_G
    Apr 2 '17 at 13:08

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
  • 1
    \$\begingroup\$ +1 for the linked picture reflecting the frustration of having to deal with audio equipment triggers. Also, thank you for an useful answer. I'll probably consider these guidelines, at least as a good sanity check. \$\endgroup\$ Apr 5 '17 at 17:17

Best hint I could find:

Note: The VIC-12 requires a 100mA current from the projector trigger. It is not compatible with 20mA projector triggers used on some Sony, Panasonic, and Yamaha projectors.

So, I guess, your output should ideally be able to deliver 20 mA or 100 mA, at least (and probably not much more to protect faulty devices).

Then, I also found this "schematic" (look for the links on the right of the page) of a projection screen roll-up/let-down trigger:

schematic of the

My guess is that the capacitor labeled with 12V actually should be the trigger input, followed by some Low pass+voltage divider that have an effective DC resistance of 9.4 kΩ.

Since that thing (obviously designed by experts who totally didn't copy and paste together that schematic in photoshop!) probably reliably works, I guess you might be overthinking this – and that there's really no standard to this.

I do agree, it's not hard to come up with a receiving circuit, but seeing that there's really a lack of any usable link found via Google (and seeing that stackexchange Q&As rank high automatically), I'll take the chance to describe what I think would work, for future readers:

Build something that has 500 Ω to 10 kΩ input impedance (I guess it doesn't really matter, seeing that there's neither any chance for the signal to be of significant speed nor of your detection to interfere significantly with the power source), and just hook it up in parallel to the trigger port and any other devices to be triggered from the same signal.

  • \$\begingroup\$ where frequency point ? Who declared static values if work under non-pattern inputs(freq) and voltage levels. This is not an answer ! Who live "principle" life ? \$\endgroup\$
    – dsgdfg
    Apr 1 '17 at 10:23
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    \$\begingroup\$ @dsgdfg Sorry, I don't understand that. Can you please re-phrase it? If you're referring to the unspecified bandwidth of the system: These 12V signals usually only change when a central device (Big amplifier, FM tuner, projector) is turned on or off. So, signal bandwidth \$\ll 1\,\text{Hz}\$, inherently. Don't understand anything about "static values", "non-pattern", "principle life" from your question. \$\endgroup\$ Apr 1 '17 at 10:42
  • \$\begingroup\$ I agree with @MarcusMüller. At most, there could be a requirement for rising/falling times, just to avoid noise problems by having a transition as fast and clean as possible. But I don't think that's too critical either. \$\endgroup\$ Apr 1 '17 at 17:38
  • \$\begingroup\$ Observing things like ignition signal pulses on an oscilloscope would probably give people a heart attack. Nothing in a car is "ideal". Even the ignition pulses only get up to about 40 Hz. That gives several orders of magnitude tolerance for concepts like rise time, etc. \$\endgroup\$
    – Chris K
    Apr 9 '20 at 18:06

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