Hi,
Although I'm quite old I am new here. I found this forum after looking for books and electronic circuits on Amazon etc, as I couldn't find anything that I thought would help me. I'm not totally ignorant of circuit principals although my knowledge is severely deficient on how to assemble practical circuits out of semiconductor components. (I date from the valve era!) Part of my difficulty is that there is nowadays such a diversity of special-purpose building blocks.
My problem is that I would like to build a simple device to detect when a magneto is opening and closing its points. The device would be used to set the ignition timing on a classic motorcycle engine, when the crankshaft is turned slowly by hand to detect precisely when the points open. When the points are open the circuit resistance is about 4 ohms and when they are shut the resistance drops to about 0.3 ohms. As a practical point, it is quite possible that even when being rotated quite slowly, that some sort of voltage spike would be induced when the points open, after all that is the point of a magneto.
I had thought to use a circuit based on a Wheatstone bridge, where the corresponding arm on the opposite side of the bridge would have a resistance of say 2 ohms, and the two remaining arms would have equal resistances. One could conceive of a circuit where a centre zero milliameter linked the two bridges, a small 1.5 V battery drove the circuit. The equal resistances could have values of say 10 ohms, and I think it would probably work, with the needle flipping from one side to the other as the points open or close.
Having thought that, I then thought that it might be possible to put an LED in place of the milliameter, but I don't think they start to emit light at a particularly low voltage. So I suppose what I'm looking for is a circuit that can turn current or voltage on and off as the bridge voltage goes through zero. The circuit would also need a reliable means of disposing of overvoltage spikes (preferably in either direction). If the current or voltage being turned off was sufficient to drive one of those little 1.5 V sounders, that would be rather good, as when one is fiddling with magneto's one has to keep an eye on the magneto itself and audio information on the points would be really helpful.
I would very much appreciate knowing whether I'm thinking on the right lines in the first place and also knowing what sort of devices are the right ones to use. Thanks for any help.
Although I'm quite old I am new here. I found this forum after looking for books and electronic circuits on Amazon etc, as I couldn't find anything that I thought would help me. I'm not totally ignorant of circuit principals although my knowledge is severely deficient on how to assemble practical circuits out of semiconductor components. (I date from the valve era!) Part of my difficulty is that there is nowadays such a diversity of special-purpose building blocks.
My problem is that I would like to build a simple device to detect when a magneto is opening and closing its points. The device would be used to set the ignition timing on a classic motorcycle engine, when the crankshaft is turned slowly by hand to detect precisely when the points open. When the points are open the circuit resistance is about 4 ohms and when they are shut the resistance drops to about 0.3 ohms. As a practical point, it is quite possible that even when being rotated quite slowly, that some sort of voltage spike would be induced when the points open, after all that is the point of a magneto.
I had thought to use a circuit based on a Wheatstone bridge, where the corresponding arm on the opposite side of the bridge would have a resistance of say 2 ohms, and the two remaining arms would have equal resistances. One could conceive of a circuit where a centre zero milliameter linked the two bridges, a small 1.5 V battery drove the circuit. The equal resistances could have values of say 10 ohms, and I think it would probably work, with the needle flipping from one side to the other as the points open or close.
Having thought that, I then thought that it might be possible to put an LED in place of the milliameter, but I don't think they start to emit light at a particularly low voltage. So I suppose what I'm looking for is a circuit that can turn current or voltage on and off as the bridge voltage goes through zero. The circuit would also need a reliable means of disposing of overvoltage spikes (preferably in either direction). If the current or voltage being turned off was sufficient to drive one of those little 1.5 V sounders, that would be rather good, as when one is fiddling with magneto's one has to keep an eye on the magneto itself and audio information on the points would be really helpful.
I would very much appreciate knowing whether I'm thinking on the right lines in the first place and also knowing what sort of devices are the right ones to use. Thanks for any help.