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With 100hz supplied from an old battery boiler, we begin in earnest, if you're elsewhere you could have 120hz to play with. At 50 hz AC mains frequency, with full wave rectification you end up with 100hz. So that's 6000 per minute. Now the more pedantic of you will point out that it ain't exactly 50hz, the frequency varies throughout the day. That said due to parallax error, the pointer isn't always where it appears either. So in short, if you need a better reference source, buy one.
100hz x 60 sec = 6000 lumps of electric every minute.
If we need 2 lumps for every turn of the crank on a 4 stroke, 4 cylinder, that's 3000 rpm with conventional single spark technology. If you're running coil packs, get a responsible adult to work out the maths for you, same with a V6, I can't be arsed to do the maths.
This should work on most tachometers, mine has inputs for contact breakers, sink and source ECU signals and AC from the W terminal of an alternator. A bit of common sense should tell you what to use. The instructions are always handy and will aid you build a rest rig. For example, if it were meant for the W terminal of an alternator, pick off an AC from the secondary of the transformer. If you don't know what I just said, don't go messing with it. Adjust the maths if you're on 60hz.
The scope was just to show the ripple for those who haven't seen it before, you don't need it. Just a battery, a battery charger and the tacho are all you need.
Obviously if you end up blowing the smoke seals, killing your first born or get confused beyond the point of no return, it's not my fault.