Please help an ignoramus

My knowledge of electronics is minimal but I hope someone can help. My principal interest is in old motorcycles, and I am looking for some way to build a device to tell me when the points open on a magneto. For those who don't know, the magneto is a self contained ignition system containing primary and secondary windings, both rotated with respect to a permanent magnet. There are points across the primary winding which are opened at a specific time by means of a cam, collapsing the primary current and inducing a high voltage in the secondary windings.

When the points are closed, there is essentially a zero resistance between them and earth; when they open the resistance seen is that of the primary winding which is of the order of 3Ω. So I need something which can detect this change in order to time the magneto accurately, preferably sounding a buzzer when the points open.

The old-fashioned way is by putting a cigarette paper between the points and checking when it becomes free, but I'm convinced this should be easy using electronics.

Any help would be very gratefully received.

The tool you need is a plain multimeter, choose one with a continuity beeper, and connect across the points.

He will have to use voltage or resistance or current. The continuity function will still sound off at 3 ohms resistance.

I agree he should have such a meter. They are very cheap ($5) and useful.

Thanks for the replies - I do have a meter but as flat5 says it sees the 3 ohms as "continuous", and as there is no external supply to the magneto I'd have to provide this separately. I was hoping to make a little self-contained box with a light & buzzer on it to detect the difference between 0Ω and 3Ω.

3Ω is not much to work with. You could make a constant current source drive the contacts and sense it with a comparator which could boink an LED.

That's what I want - but as in the title, I'm an ignoramus in these matters. What would a circuit to do this look like?

If you want to build something, check these links.
https://www.linear.com/pc/downloadDocument.do?navId=H0,C3,P2027,D2715

and here.
**broken link removed**

Maybe someone has a circuit already design to fit your needs. I have finals this week so no time to investigate for you. Sorry...

I think the second link has exactly what I'm looking for. Many thanks Mike.

The trigger point needs to be ≈1.5Ω.
Any 1.5v or 3v flashlight or penlight with this much resistance in series with the lamp will be visibly dimmer or just "out".
If you want a buzzer, you need to find a 1.5v or 3v buzzer. If it buzzes with more than 3Ω in series, put it across the lamp.

If you still need a trimmer resistance somewhere in the circuit, 10' of #30 AWG = 1Ω.

You still can use the multimeter. Use the resistance function.