Wiring a real C172 Ignition Switch

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Sling
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Re: Wiring a real C172 Ignition Switch

#11 Post by Sling » Fri Jun 11, 2021 8:58 am

Shouldn’t need to complicate it with power or resistors. I think just using gnd for the commons and utilising the built-in pull-ups for all 3 lines should work out fine. Adding extra external resistors will not harm though.

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Re: Wiring a real C172 Ignition Switch

#12 Post by jph » Fri Jun 11, 2021 9:57 am

Sling wrote:
Fri Jun 11, 2021 8:58 am
Shouldn’t need to complicate it with power or resistors. I think just using gnd for the commons and utilising the built-in pull-ups for all 3 lines should work out fine. Adding extra external resistors will not harm though.
Hi Tony,
I originally also thought that - but am thinking more of the starter circuit where - normally, the input to the solenoid is simply disconnected. The solenoid for the starter needs a Batt supply to energise.
Hence, if BATT supply is not connected to a logic high, then no action is taken on start position as in this position it connects BATT to Start - which is then - in normal use - passed to gnd via the solenoid
Normally it is completely O/C so here, it would not do anything (so it seems ?)
It appears that the only circuit in relation to 'start' is between START and BATT - in fact the only connection to start appears to be to BATT - nothing more.
GND - doesn't come into play.
If BATT were not connected or floating, then when start was engaged the output of the start terminal would not change ????
Also, when we connect BATT (as I 'think' we would need to ?) . then the start output would need to be pulled low via an external resistor for AM so it is always low, apart from when in start, where the signal from BATT (logic high / 5v) would cause the start terminal that was pulled down to go HIGH.
The issue is - as far as I can see - and without having a switch to hand - that there is only a single path connection between BATT and start so that needs to be controlled.
If we relied on the start to be pulled high by the weak pull ups, then on engagement nothing would happen as it appears to be only connected to BATT, and, unlike the other positions, is never grounded.
Hope that makes sense ? - it does to me - errrr - at the moment haha ;) .. note - subject to change as brain engages... ;)

You would actually - at first / second glance only need 1 resistor - a pull down on the start pin but would need the BATT connected to 5V (or logic high V Level) - this is for the start circuit only. 1 resistor is nothing really - 1k to 47k - non critical value - even... >1k to <100K - between start output and logic low - GND for pulldown - with BATT to supply - can be direct connection but a small current limiting resistor would be a good option - although not necessarily needed - say - 470 ohms - just to control max current in the event of an accidental short.

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Sling
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Re: Wiring a real C172 Ignition Switch

#13 Post by Sling » Fri Jun 11, 2021 10:44 am

You just connect gnd to batt and start to an I/O line. It’s that simple. When not in start the line is pulled high by internal pull-up, when in start position the line is grounded.

Remember this is only to tell AM and not for a real application. Pretty sure this is how we helped someone else do this way back.

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Re: Wiring a real C172 Ignition Switch

#14 Post by jph » Fri Jun 11, 2021 11:20 am

Sling wrote:
Fri Jun 11, 2021 10:44 am
You just connect gnd to batt and start to an I/O line. It’s that simple. When not in start the line is pulled high by internal pull-up, when in start position the line is grounded.

Remember this is only to tell AM and not for a real application. Pretty sure this is how we helped someone else do this way back.
That may well be correct. I would have thought - at first glance, that connecting batt to gnd would cause issues with the other 2 standard I/O pins ?
If not, nice solution Tony.
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bdarl357
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Re: Wiring a real C172 Ignition Switch

#15 Post by bdarl357 » Fri Jun 11, 2021 1:08 pm

You guys rock. I think Tony is on the right track. I did have it working through Simvim but that was over a year ago and I forget how it was coded and can't find my old forum post over there. I do have a jumper between batt and gnd. The way I have it connected is a pin for L, a pin for R, a pin for start, and batt jumped to gnd which is then connected to the rest of the grounds for my lower panel. So I guess at this point I'm just a little lost on where to start with the code for AM?

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Re: Wiring a real C172 Ignition Switch

#16 Post by Ralph » Fri Jun 11, 2021 1:43 pm

You will have to use digital inputs, and work out the magic combination. That's at least how I did it, worked fine.

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Re: Wiring a real C172 Ignition Switch

#17 Post by jph » Wed Jun 16, 2021 11:01 am

:D ;)
ABSOLUTELY.
This shows why a simple multi-meter check will do the job in all cases - and why they should always be used. You cannot ever presume that an unknown (internally) switch unit can be as you think
Simply get out the multi-meter and check, Then, list the results and combos, then sort out the logic required. How it originally was designed to work can help - although that may well only give you a clue. (only a clue!).
If someone doesn't own a multi-meter with basic continuity checking, or is unsure how to apply this - then it would be good to do a youtube search on multi-meter basic usage.
It is only AFTER that, that the code can be written - unless of course- there are comprehensive details of the actual item you intend to use. :)
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Re: Wiring a real C172 Ignition Switch

#18 Post by Ralph » Wed Jun 16, 2021 11:09 am

That's certainly true, a multimeter is key in all hardware projects. Preferably a digital one, with automatic range and which can make a sound (beep) when measuring ohm's (not sure what this setting is called in English).

Edit: Looks like it is called 'Continuity function'.

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Re: Wiring a real C172 Ignition Switch

#19 Post by jph » Wed Jun 16, 2021 11:20 am

Ralph wrote:
Wed Jun 16, 2021 11:09 am
That's certainly true, a multimeter is key in all hardware projects. Preferably a digital one, with automatic range and which can make a sound (beep) when measuring ohm's (not sure what this setting is called in English).
Hi @Ralph - it usually simply referred to as a 'continuity' or 'continuity check' (although continuity function also works great )-- often selected on the 'diode' / measuring selection. - which is usually the 'beep' setting.
I think anyone attempting hardware projects should have at least a multi-meter.
A useful link may be -

https://learn.sparkfun.com/tutorials/ho ... continuity
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Re: Wiring a real C172 Ignition Switch

#20 Post by jph » Wed Jun 16, 2021 11:53 am

Ralph wrote:
Wed Jun 16, 2021 11:09 am
That's certainly true, a multimeter is key in all hardware projects. Preferably a digital one,
Nowadays, I would definitely agree. But an analogue meter such as the AVO 8 was a requirement when working on valve (tube) circuits as all the measurement points tended to be around 20 K - ohms per volt - on the schematics and fault finding data.
Now, of course, the modern stuff is >1 M-OHM per volt - often > 10- M Ohm per volt so the impedance load on the circuit is utterly different during measurements.
I actually like the old analogue meter , but must confess, now, it is pretty redundant. - unless I work on really old gear where it is king.
Mind you, that is going off topic.. :shock: :roll: ;)
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