Automotive ignition pulse sensor

j0rdan

New Member
On an old vehicle with no ECU, but with Hall Effect "points replacement" device:

I want to count ignition pulses, and I gather a method is to use an inductive or capactive sensor with a spark plug lead.
However, I'm wondering if I could use the low tension wire at the ignition coil terminal?
I just think it would be neater.
Assuming it's possible, any suggestions for some filtering circuitry?

The project is to be a "starter motor inhibit", to prevent it from trying to engage once the engine is already running.

Thanks folks,
Old Newbie
 

premelec

Senior Member
Welcome to this forum... In past tests I found that putting a coil taped to the outside of an ignition coil gave a good signal; almost any pickup coil will do - old headphone or relay coil etc.... then put a 5 volt clamp on the signal - zener diodes etc to protect your microcircuitry - this method also isolates you from all the high voltage spikes associated with ignition coil primaries or secondaries... If you don't want to try this use the primary signal with a lot of filtering through resistor, capacitor, zener - if you have an oscilloscope that will help a lot to see what you are working with [usually a pretty complex ringing decay signal].
 
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Pongo

Senior Member
There's a very old (but still available) chip, the LM2907, that includes all the functions you need for this. I have used it fed from the coil primary on several older technology vehicles back in the day.
 

premelec

Senior Member
I think OP just wants to know if engine running or not... You can find the LM2907 in many OLD floppy drives for salvage... [If I recall correctly - been a while since I was into my piles of old drives... ;-0 ].
 

Janne

Senior Member
Hi, if it's recent enought to have an alternator, one and perhaps easier option would be to sense the engine's running state from the charging light.
 

Pongo

Senior Member
I think OP just wants to know if engine running or not... You can find the LM2907 in many OLD floppy drives for salvage... [If I recall correctly - been a while since I was into my piles of old drives... ;-0 ].
Interesting, I didn't know it was used for that.

OP's circuit needs to recognize the difference between being turned over by the starter and actually running. The 2907's switching function is ideal for that - or you could do it with a picaxe. I get nervous with pic micros around auto electrical systems, especially in a function that could leave you stuck on the side of the road <missing chicken emoticon>

Hi, if it's recent enought to have an alternator, one and perhaps easier option would be to sense the engine's running state from the charging light.
Another good idea, as I recall even pre-alternator engines had a charging light.
 

j0rdan

New Member
Thank you for the welcome and replies.

LM2907 is interesting, but I'd prefer to try a Picaxe first. I'll keep it in mind though.

I did consider a charge indicator for sensing, but if that circuit failed the starter inhibitor wouldn't work, so not fail-safe.
There is no oil pressure switch on the engine, on a motorcycle with low-pressure oil system.
There is a backup starter system - the kick starter.

Taping a sensor coil to the ignition coil is something I hadn't considered. It should be a bit neater than attaching to a spark plug lead, and I like that it might avoid need for complicated filtering. May I have more detail about "zener diodes etc"? I'm afraid I'm just out of my depth when it comes to selecting component values.

By the way, PIC makers Microchip advertise that their products are used in road vehicles. Is it that only some of them are suitable?
 

tommo_NZ

New Member
20 years ago, I found a gadget at work which I believe was an over rev indicator. It was in a small plastic box about the size of a matchbox. The pickup was 3 turns of average size wire around a high tension lead. It only had 3 or 4 components and it turned on an LED when the engine revs reached a set value. This pickup method of detecting a running engine seemed so simple . Worth a thought?
(See this https://www.eng-tips.com/viewthread.cfm?qid=320604 )
 

j0rdan

New Member
There's lots to read on that link, thanks.
I also found an old thread on this forum with some filter circuits to try.
 

mortifyu

New Member
From what I recall, the ignition circuit in older vehicles was very basic. If I were looking to achieve your goal, I'd add 1x 47K resistor and 1x 5V1 zener diode for protection.

When the distributor points are OPEN, there would be +12V on the NEGATIVE terminal of the ignition coil. When they are closed, there would be 0V. Making use of a reasonably high value current limit resistor (47K) and a 5V1 zener diode to limit the voltage going to your PICAXE to 5.1V MAX, you would capture the data of every spark sent to every cylinder. This circuit would basically send 1's and 0's to your PICAXE input pin.

Here is a diagram of my suggestion...

OLD CAR IGNITION.png


Hope this helps.


Regards,
Mort.
 

tommo_NZ

New Member
From what I recall, the ignition circuit in older vehicles was very basic. If I were looking to achieve your goal, I'd add 1x 47K resistor and 1x 5V1 zener diode for protection.

When the distributor points are OPEN, there would be +12V on the NEGATIVE terminal of the ignition coil. When they are closed, there would be 0V. Making use of a reasonably high value current limit resistor (47K) and a 5V1 zener diode to limit the voltage going to your PICAXE to 5.1V MAX, you would capture the data of every spark sent to every cylinder. This circuit would basically send 1's and 0's to your PICAXE input pin.
Hope this helps.
Regards,
Mort.
The back EMF generated in the primary circuit when the secondary's magnetic field collapses across the primary windings can be as high as 500v, that's why there is a capacitor across the points, to absorb this spike. With the 'old style' points system (known as a Kettering Ignition system) the High Tension voltage usually max's out at 12kv whereas a modern system with electronic triggering can be as high as 60kv. I still think detecting the HV spike by induction is safest. It would be handy to put a scope on an induction pickup.
Regards,
Kevin T.
 

mortifyu

New Member
The back EMF generated in the primary circuit when the secondary's magnetic field collapses across the primary windings can be as high as 500v, that's why there is a capacitor across the points, to absorb this spike. With the 'old style' points system (known as a Kettering Ignition system) the High Tension voltage usually max's out at 12kv whereas a modern system with electronic triggering can be as high as 60kv. I still think detecting the HV spike by induction is safest. It would be handy to put a scope on an induction pickup.
Regards,
Kevin T.
This is all very true, back EMF (effectively a reverse voltage generated from the collapsing magnetic field of an inductor) can be problematic if not understood and allowed for. Similarly, this is why it is very important to place a diode across the coil of any relay's used in circuits. However, in this case, the zener would keep this back EMF under control. Perhaps for even more robust protection (and it wouldn't alter the pulses going to the PICAXE), increase the resistor value to maybe 150K and use a transorb in place of the zener. He'd still get the desired 1010101010... result.

Transient voltage suppressors or Transorbs for short, are very robust protection devices. See here:
https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

For an understanding and appreciation of back EMF, See here:
https://en.wikipedia.org/wiki/Counter-electromotive_force


Regards,
Mort.
 

premelec

Senior Member
Don't know if OP has resolved this... I have at times used an opto isolation structure made with neon tube [Ne2] shining at a distance on an LDR in dark sleeve or pipe insulated casing - with fiber optic or lucite light pipe if long.. The neon tube can take the HV stuff ok -Slow response ... if you use an LED it needs filtering / clipping circuits...
 

j0rdan

New Member
I'm still here, folks.
Just heavily distracted with other things recently.
I've been taking note of the many suggestions, have obtained some hardware, and look forward to when I can spend a couple of days trying out various options.
I have a simple CRO to help me see what's going on.
Thanks all, I will post if/when I do something interesting.
 
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