How to detect if stop button on magneto coil is pressed?

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Futterama

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Hi forum,

I have this magneto coil on a gasoline engine:

https://www.davesmotors.com/s.nl/it.A/id.1402/.f

There is a stop button mounted on the coil and when pressed, the button shorts the 2 terminals which are connected to an internal coil which then destroys the magnetic field inside and prevents the main HV coil from creating the voltage for the sparkplug.

When the engine is not running, I would like to detect if the stop button is pressed. So basically I wish to detect whether there is a short between the coil terminals or not.

I have done a few measurements on the coil connected to the terminals. It seems that the resistance is only 1ohm. Since there is also resistance in the wires and also in the button, I'm not sure measuring for resistance is a reliable approach.

Could I somehow measure for inductance? I have a PIC available but don't know how I could use that to measure for inductance so I would like to hear your suggestions.

Thanks.
 
The dual magneto timer for my aircraft engines do this. They detect if the magneto points are open or closed by using the inductance of the primary coil vs the short circuit if either the points are closed, or if the magneto kill switch is shorting the coil. If you connect a DC Ohmeter between the "p-lead" and ground, you see ~1Ω either way, but the magneto timer can tell if the points are closed or open...

I would make a Colpitts Oscillator where the inductor is the ignition coil primary. The oscillator may oscillate even if the points are closed due to the wiring inductance, but the inductance when they are open is likely to be hundreds of mH, while the wiring inductance is likely to be a few tens of uH.

I have an Inductance Meter. A simple measurement would show the actual numbers.

ps- I notice that SkyGeek has a **broken link removed**.
 
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Thanks Mike, I will try the Colpitts Oscillator, maybe I need help with component values but I'll try to build the circuit first.
 
I haven't built the circuit yet, but could another approach be using the coil as a inductor in a boost converter design? Of course the current through the inductor (or button when this is pressed) should be limited. If the converter generates a voltage, the button is not pressed, this is easy to measure from the PIC. The PIC itself could also be controlling the switching using a MOSFET.
 
I have an **broken link removed** which is based on a PIC, so we know that works

You could also just make an AC Ohmmeter. Apply ~1Vpp at 1Khz to the coil and measure the current. High current = short, low current = inductor....
 
Be aware of the voltage spike that occurs when the breaker points open, and the effect that may have on your monitor device, whatever it is, if it is left connected while the motor is running.
 
ccurtis, i just thought of that last night, there are voltage spikes around 150V on these terminals when the engine is running, the circuit has to withstand that and also not consume any of the current from these spikes since this could have a negative effect on the magnetic field that the main HV coil depends on.
 
If the engine is running, you know that the kill switch is open

If the engine is running, a dwell meter circuit knows if the points are open or closed. (See post #5 in this thread)

You only need the inductance detecting circuit I linked to if you are in effect "timing" the engine, when it is not running...
 
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Futterama said:
What are those points you are talking about?
Click to expand...

I am talking about the switching element in a typical Kettering ignition system. If you have a solid-state ignition, the "points" are the switching transistor.

The only difference between the figures above and what is done in a magneto ignition is that there is no battery; the current for the coil primary comes from a magnet swinging past the coil just before the "points" open.
 
The magneto ignition I have is only 2 coils in the same package, when the magnet swings by, the HV coil generates the voltage for tha sparkplug, the other, the one I'm trying to work with here, genereates around 150V p-p and is only for stealing the magnetic field or energy from the HV coil when you need to stop the engine. There are no movable parts other than the flywheel with magnets.
 
When the engine is not running, I would like to detect if the stop button is pressed.
Click to expand...
I have to ask....why? If the sole purpose of the button is to stop the engine, and the engine has indeed stopped, can't you just assume the button was pressed then released?
 
alec_t, yeah, let me explain then. The engine is for a RC car and I have developed a remote activated killswitch some years ago which I now wish to update with additional features. The user should be able to adjust some settings, but I don't want to put a button on the PCB for several reasons I won't get into now. So I would like to use the stop button on the engine instead so the user could press the button during powerup and then activate setup mode ect.

That's why I need some way to measure whether the button is pressed or not even though the engine is not running at all.
 
Perhaps something that detects the presence/absence of a voltage across the kill (stop) switch as the engine is cranked (started) is suitable?

Edit: Nevermind. I posted before reading the previous post just above.
 
Is either end of the coil which the kill switch shorts connected to anything else? If not, the solution will be simpler.
 
Yes, one end of the coil is connected to the iron core which is a common ground for the sparkplug too.
 

Why not monitor the signal that activates the killswitch instead of the rather hairy task of trying to monitor the killswitch contacts that can have rather high voltage across them that can't be loaded down much by a monitor.
 
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ccurtis, what? I want to use the stop button as an alternative to putting a button on the PCB on the remote killswitch device.

The remote device I'm working on, I call "killswitch".
The physical button on the engine that shorts the terminals on the coil, I call "stop button".
 
Here's a suggested circuit:
U1a forms an oscillator of ~700kHz. Its output is applied via R2 to the magneto coil. Assume the engine is stopped. If the stop switch is open the reactance of the coil results in a small voltage across the coil which is amplified by J1 and Q1 then buffered by Q2. Q2 drives a charge pump to build up a voltage on C6 which turns Q3 on. If the stop button is pressed the voltage across the coil is shorted out and hence Q3 turns off, so the Out line goes high.
If the engine is running the ~150V (peak) AC generated by the magneto coil causes the Out line to toggle on and off at and about each zero crossing of the AC waveform.
Resistors R2,R3 and diodes D1-D4 protect the remaining circuitry from the high voltage (150V) and also present negligible damping to the AC coil voltage.
 

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  • KillSwitchSensor.asc
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Thanks! I don't understand everything about your circuit but it gives me some ideas.

In it's current form, the killswitch device is about 24x17x15mm and all those components will make it a lot larger, so I don't think I will be using the circuit as it is. Could U1a be replaced by the PIC toggling an output pin? Could the internal comparator in the PIC somehow replace J1 and Q1? And could the detection be done in software instead of using a charge pump?
 
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