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Changing the sense of a signal

Hi All,

I have two situations on our Pioneer 300 where I need to flip the sense of a 12v signal which is normally closed (NC) to normally open (NO).

The first is to drive a hobbs (millimaps) from my generator warning light. I need the hobbs to work when the light is off. I would also like to rig a switch to detect airspeed for our Becker transponder (BXP6401). This will enable it to switch between GND and ALT without touching it. The Becker has a squat switch input which when grounded will they claim will auto select Mode S GND. I see Peter has a pic of a differential pressure switch on another thread that looks ideal. In the case of the pressure switch I need it to be on when the aircraft is on the ground and go open circuit in the air but they seem to seem to be NO switches not NC. I can wire a NC automotive relay to do these jobs but as I am only switching signal levels at 12V that seems over kill and relays are noisy spikey things.

I am totally cr*p at electronics. Can any of the electronics guys advise a simple circuit to invert the sense of these 12V signals pls?


Last Edited by SteveN at 26 Dec 09:50
Gloucester UK (EGBJ)

I am not sure what you mean by N/O or N/C and 12V. Relay contacts are voltage-free and can be connected to anything (subject to contact ratings etc etc).

If you mean you have a 12V signal coming out of somewhere when “something” happens, and you want it to come out when “something” doesn’t happen, the simplest way is a relay with a 12V coil, driven off that signal, and then you wire its contacts to do what you want. You need to make sure the signal has enough power to drive the coil of the relay. Relays are available in a huge range of coil resistances. Do you have a circuit showing where the 12V signal is coming from, so one can establish how much current is available?

There is a zillion suitable relays, but perhaps the easiest are the automotive ones with push-on terminals

These are a bit cheap and nasty (which is why everybody uses them) and if I was fitting a relay into a plane I would use a hermetically sealed one so (a) it isn’t affected by humidity and (b) it cannot ignite fuel vapour if there is a leak. The general styles of sealed relays are these

and in your case (non CofA) you can choose the technically best one without legal issues. The first two are milspec and that’s what I would use to do a decent job. A lot of plastic relays are “sealed” (including some used in cars now) but they are not as sealed as they could be, for aviation where the ambient pressure could be 50% (FL180) and then back to 100%, forcing the muck in nicely. You can get the hermatic ones on Ebay – search for milspec relay.

That diff pressure switch is in this writeup and the P/N is given in there. It’s a very good solution. If you want to reverse the way it works, the supplier may do the opposite style, or again you can use a relay to reverse it (drive the relay coil from the switch and then you use the relay contact(s) as you wish.

However it may also be possible to connect the pressure switch the other way: instead of going between GND and the switch terminal on the transponder, one could connect it between +12V and the switch terminal on the transponder, probably with a pulldown resistor to GND. I would definitely want to see the circuit diagram of the transponder before doing this however. Or it may be software configurable which way the switch is sensed – I vaguely recall it is in some transponders.

Relays are spiky things when it comes to switching the coil current, and you should put a diode across the coil to stop most of the arcing on the contacts of whatever is switching the coil current. The diode needs to be reverse-biased i.e. the cathode goes to the positive side

If the relay coil is switched by say a transistor then the diode still goes in the same place

A suitable diode for a 12V or 24V aircraft would be a 1N4001 which is common as muck. I can pop some in the post

The relay contacts themselves don’t generate interference. What they do is they bounce i.e. they don’t switch cleanly but spend some milliseconds bouncing around on and off and back on. I don’t see this being a problem in anything you are doing here.

One can also use MOSFET (or even ordinary) transistors to switch stuff, but that needs a lot more information (circuit diagrams) about what exactly is being switched.

Last Edited by Peter at 26 Dec 10:32
Shoreham EGKA, United Kingdom

Hi Steve,

It could be done with either resistors or a small relay. An automotive relay would be overkill, you could use a minature relay. I would use the differential pressure switch to swith both the hobbs and transponder (via resistor or relay) or use a SPDT differential pressure switch.

The differential pressure switch are often SPST normally open, but are also available in SPDT. With a SPDT you wouldn’t need anything else to switch both hobbs and transponder based on air speed.


Thanks for the advice gents. I did not know about the miniature relays and will use them. I have some 1N4004 diodes to kill the spikes.


Gloucester UK (EGBJ)

I would use the differential pressure switch to swith both the hobbs and transponder (via resistor or relay)

In general one may not be able to use a single switch to switch both the hobbs and the transponder, because these devices generally have internal resistive pullups to their internal supply rails. These rails may not be the same voltage e.g. one could be +12V and the other could be +5V. Also if the power supply on either device is removed (by the pilot pulling a CB, or via a failure) then the non-functional device could pull down the shared signal, permanently, or result in intermittent behaviour.

Without having the circuit diagrams of the two instruments, one cannot be sure whether the control line can be simply interconnected and operated with a single shared switch contact.

In my article I show how one can use two diodes to solve the problem. That particular installer told me he had been wiring up stuff like that for years, mostly in rather high-end piston aircraft, and he says it works, which is true – until something stops working…

OTOH, if you are switching it with a relay, you may as well get a 2 pole relay and use 1 pole per instrument… easy.

Shoreham EGKA, United Kingdom

As Peter writes (cautiously between the lines) semiconductors might be preferable as switches – no spikes, no noise, no sparks. But some more analysis would be required. If you could post a schematic of what you intend to do, I’ll be glad to suggest a solid state equivalent.. However, use of semiconductors would involve soldering – as you state electronics is not your cup of tea you might well prefer the push-on connectors..?

EBZH Kiewit, Belgium

Among the issues to watch with semiconductors might be bus spikes e.g. a 12V bus could go to 300V due to starter motor inductance.

If you try to start the engine with avionics ON, it is somewhat likely that some boxes will get blown up.

Now, if you used a MOSFET to switch something, and its Vgs(max) is say 20V, you will blow it up. So you have to put in a means of clamping that, so you end up with a few components around it. All easy, but it isn’t as trivial as popping in a small relay.

One could debate whether it is OK to bundle a small relay into a harness. It’s not ideal but if there is good access, it is properly insulated, has no sharp edges (or has overall clear heatshrink), and you have documented it, then it should be OK. With a MOSFET, you can’t be sure it isn’t going to overheat under fault conditions… which may mean mounting it separately somewhere.

Shoreham EGKA, United Kingdom

Peter, you will remember I am unacquainted with the world of IR-equipped high-end private planes. But didn’t I understand that
-) starting the engine(s) with the avionics master switch on is considered deadly sin, almost on par with a wheels-up landing?
-) to protect the precious avionics against said error, a crowbar circuit on the avionics bus is a self-evident necessity in this category of plane, because we are all only humans after all?

BTW you’ll excuse me for not being impressed with the heat argument regarding mosfet’s – surely today’s mosfet’s with their very low Rdson will dissipate less power than any kind of relay coil? Apologies to the non-electronics members…

Last Edited by at 26 Dec 18:29
EBZH Kiewit, Belgium

Jan I can solder components and have made up simple circuits on vero board but have little idea how they work.

It looks like Maplin here in the UK have a suitable SPDT miniature relay:


I should be able to seal it up with a dab of superglue on the case joints :)

I’m afraid I didn’t explain things well above. This is for a Rotax 912S engine. It has a Ducatti voltage regulator with a “L” terminal that goes to ground when the alternator is not running but the master is on. The charge warning light is normally wired to 12v and the “L” terminal makes the circuit but goes open circuit when the alternator kicks in. I plan to wire 12V to the hobbs positive and use the relay/circuit in the ground wire. The hobbs must measure engine run time as bizarrely that is what Rotax specify for service intervals not airbourne time.

Gloucester UK (EGBJ)

Well, the general approach seems to be to run the Hobb’s timer off an oil-pressure detector, the same that actuates the “low oil pressure” light… Some even have the Hobb’s timer only active when ASI comes above stall speed or such, not sure how that is implemented.

On a general note: distrust the Ducati voltage regulator, mine blew out after less than 50 hours TT and it wasn’t the first; I replaced it with a German free-market alternate which has done fine up till now but that is only a short while. That is on a 912UL, 80HP so not the S variant. In all honesty I must add that my airframer, though generally quite knowledgeable and acclaimed, was less than impressive on the electrics, mounting the Ducati regulator in the horizontal, thus disabling all and any airflow along its cooling ribs. Sure enough I mounted the alternate vertically against the firewall, on the cabin side.

Last Edited by at 26 Dec 19:09
EBZH Kiewit, Belgium
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