12V On/Off Inputs

PaulSS

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Feb 21, 2019
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My searches have helped educate me a little but I just wanted to confirm what I think I know by asking the question, without suggesting an answer.......this is my way of saying I have done some research :)

I have two, separate, units that 'produce' 12V output when triggered. One is a low fuel switch. When the fuel gets low the float switch makes contact and, hey presto, 12V would run through a normal warning lamp. Also, a start power relay. When start power (to a Rotax 912iS) is available through the relay it sends a 12V 'signal' that would normally go through a lamp to ground.

You know what's coming next: I want to get rid of the two lamps and have lights on the SkyView D1000 instead. One would be a red light with 'Low Fuel' and the other a green light with 'Start Power' written underneath it.

I get a bit concerned when I see '0-5v' written on the EMS General Purpose Inputs, so would like to ask how I go about achieving the above. Please be as specific as you like; it only helps to make this numpty understand things better 🤪

Thank you.
 

airguy

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My searches have helped educate me a little but I just wanted to confirm what I think I know by asking the question, without suggesting an answer.......this is my way of saying I have done some research :)

I have two, separate, units that 'produce' 12V output when triggered. One is a low fuel switch. When the fuel gets low the float switch makes contact and, hey presto, 12V would run through a normal warning lamp. Also, a start power relay. When start power (to a Rotax 912iS) is available through the relay it sends a 12V 'signal' that would normally go through a lamp to ground.

You know what's coming next: I want to get rid of the two lamps and have lights on the SkyView D1000 instead. One would be a red light with 'Low Fuel' and the other a green light with 'Start Power' written underneath it.

I get a bit concerned when I see '0-5v' written on the EMS General Purpose Inputs, so would like to ask how I go about achieving the above. Please be as specific as you like; it only helps to make this numpty understand things better 🤪

Thank you.
The installation manual (free download on this site) is your friend - you are looking for section 7, starting page 13 - a description of all the various possible sensor types and pins to hang them on, with voltage and current limits.
 

PaulSS

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Thanks Airguy. I had seen the tables in my research but didn't want to pre-empt anything. As I understand it, I can input those 12v on/off sources to any of the Enhanced General Purpose inputs (C & blue in the table). Although receiving 12v the EMS records them as 5v. I then set up a widget to turn red/green when it receives 5v from EGP input XX.

Does that all sound about right?
 

Rhino

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Correct. It doesn't measure past 5 volts, but will allow up to 15 volts continuous.
 

djones

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Just a note here, any 14 volt contact inputs into the EMS should have 10K resistors installed. We found that if you have several contacts that all go
to 12+ volts at the same time, it will X out all of the widgets on the EMS screens. Not exciting when you take off and retract the gear to lose all your instruments.
This was originally documented on 28 volt aircraft, but we found it caused problems with 14 volt as well.
 

RV14_TD

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let's say I'm connecting the Dynon heated pitot probe. Do I understand correctly that the pitot status white wire should be connected to EMS via a 10k resistor?
 

n144sh

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Just a note here, any 14 volt contact inputs into the EMS should have 10K resistors installed. We found that if you have several contacts that all go
to 12+ volts at the same time, it will X out all of the widgets on the EMS screens. Not exciting when you take off and retract the gear to lose all your instruments.
This was originally documented on 28 volt aircraft, but we found it caused problems with 14 volt as well.
Is this documented in the manuals or sample wiring diagrams? I haven't seen this before and it seems like kind of an important change in recommended wiring!
 

BlueCH750

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let's say I'm connecting the Dynon heated pitot probe. Do I understand correctly that the pitot status white wire should be connected to EMS via a 10k resistor?
Install insructions state to NOT to use a resistor when connecting thte status wire to the EMS
 

djones

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let's say I'm connecting the Dynon heated pitot probe. Do I understand correctly that the pitot status white wire should be connected to EMS via a 10k resistor?
The Dynon heated pitot status is a different case and does not need the resistor installed. It does not put out voltage from our pitot heat control, it only grounds the 5 volts sent by the EMS. The resistor is only needed when connecting a GP input to something that puts out aircraft voltage.
 

cbretana

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So, to clarify that I understand this, the contact pins on the EMS can operate in two modes:

1. Ground Mode - like for Canopy locked, or Gear/Flap position indicator). Where a general purpose "CONTACT" input pin to the EMS is connected to an on-off switch which is then connected to aircraft ground. In this case the EMS applies a very low mili-amp 5V signal to the pin and detects whether it is connected to ground or not.

2. Powered Mode - like for fuel level sensor, or pitot heat On/Off indicator. Where a general purpose "CONTACT" input pin to the EMS is connected to a aircraft power source, (12V/24V), which is being used to power some other aircraft function through a switch. In this case the wire to the EMS pin must be connected to the existing aircraft circuit between the load and the switches controlling whether power is applied to the load, and have a 10K resistor in line to the EMS. The EMS then detects whether the voltage between the pin and ground is zero (switch closed), or 12V (switch open).

Is this correct?
 
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djones

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That is correct, although technically, even the 12 volt inputs are working exactly the same way. When the 12 volt powered device is switched off,
the item (like the pitot heater) is actually grounding the 5 volts out of the EMS. It really doesn't care about the 12 volt being applied to the pin at all. Only the grounding of the pull up voltage when power is switched off.
 

PaulSS

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I apologise for returning to this subject but just wanted to double-triple check my understanding is correct and that it is safe to input a 12v voltage into the EMS D37 as per the drawing below. The input would be used as a 'Contact' input to display a red light when triggered by the input voltage.

My primary concern (and reason for checking) is that the installation manual states: "You must ensure that when closed, the contact connects to a ground common to the SV-EMS-220". I assume the 12v input would 'flow' to Earth having triggered the voltage input in the EMS. In other words; 12v comes into the EMS, is seen as 5v by the 'Contact', triggers the red light to come on and then goes to Earth through the EMS internal connection to Earth.

SO, wise ones, have I got it right or do I need to wire it differently?

Thank you.

tempImageLPasp3.jpg
 

Rhino

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No. Ground needs to be applied when the voltage is turned off. A simple open switch won't do that. Try looking here:
 

PaulSS

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@Rhino. Excellent! Thank you very much for your pointers and especially your final diagram. I think the use of resistors will be a lot easier than the relays.

From vLittle's comment, I gather using 20K resistors from the sensor would be better for multiple inputs. I will have 3 x 12v inputs, configured as Contacts, so I THINK I should wire each input as per your slightly amended circuit below.

I think I might, finally, be there. Cheers 👍

tempImage3hIFat.jpg
 

Rhino

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20k should be okay. As long as you have at least 2 volts reaching the pin, it should work fine. You should have around 4 volts to the pin with that setup.
 
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vlittle

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This circuit was designed to provide 5V on the input with a 15V signal. Typically, you'll see 12-14.5V, so this won't overload any of the SV inputs. It can be used with both normal and enhanced inputs, just check the voltages on the debug page with the switch open and closed and set the thresholds appropriately.

1668391528136.png
 

Rhino

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I want to clarify something a bit further. When you measure the voltage you're getting from an on/off indicator circuit, like I mentioned in post 15 above, you may need to make that measurement with the EMS connector disconnected. The EMS has a constant internal bias voltage applied to those pins (about 4 to 5 volts) that could confuse the voltage measurement you're getting from your on/off indicator circuit if you leave the connector hooked up. If you're utilizing a ground connection through that connector for your on/off circuit, you may need to supply a different ground for the test, since removing the connector will likely also remove your ground connection. You shouldn't need to do this often, if at all. It's mainly used as a troubleshooting method when an on/off indication isn't working properly.

The diagram Vern posted above is electrically the same as the last hand drawn diagram PaulSS posted, except that Vern's version uses a plug-in D-sub connector to build his circuit. That has distinct advantages if you like to tinker, or if you envision future changes or additions to your on/off circuits. Instead of digging into the wiring harness again, you simply unplug the D-sub connector, reconfigure or modify the resistor connections, and plug it back in. Hard to beat that kind of convenience. He even provides pictures in that other forum thread mentioned in post 13.
 

Guenin

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The diagram Vern posted above is electrically the same as the last hand drawn diagram PaulSS posted, except that Vern's version uses a plug-in D-sub connector to build his circuit. That has distinct advantages if you like to tinker, or if you envision future changes or additions to your on/off circuits. Instead of digging into the wiring harness again, you simply unplug the D-sub connector, reconfigure or modify the resistor connections, and plug it back in. Hard to beat that kind of convenience. He even provides pictures in that other forum thread mentioned in post 13.

I just came across this, and I think this technique might be useful in another application I'm working on which is unrelated to this one. Is there a name for this sort of technique? I'd like to read more about it, including tips, cautions, etc. I saw the photo you mentioned that was referenced in post 13.
 

Rhino

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It's essentially a resistor block. I'm not aware of any specific name for doing it on a DSUB connector, but maybe Vern would know.
 

vlittle

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About 20 years ago, when I was building my RV-9A, I recognized the need to develop custom avionics devices. Many of these are still sold commercially.

The two largest costs in an electronics product are often packaging (cases) and shipping. This lead me to select a particular type of off-the-shelf d-subminiature backshells as the casings for these designs.

They are very light and low cost, but limited the size of circuit board that could be mounted inside. So I decided that I would focus on designs that would fit the cases, rather than make cases that fit the designs. By using the d-sub backshells, I also got the benefit of using standard connectors that could easily be mounted anywhere with zip ties.

The end products were so small and light that I would ship them anywhere in the world for free! Times have changed, but shipping costs are still very low.

So whenever I need some special function, I naturally gravitate to using this technique.

Note: it sure helps to focus a designer on key features and functions when the number of available pins and circuit board volume is limited. A great discipline. Compare my audio mixers to the competition, for example.


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