rpm meter of cars

[mostafa zaky]

hi guys
i am searching for a photo that explains how to tab from the distributor of the car in order to make rpm meter but i could not found any thing or a circuit can some body provide me with a photo or an accurate explanation for that topic?
here is a photo for the ignition system of the car

[cliffforrest]

The connection from the points to the coil will be connected to ground once every engine revolution, by the points closing.

So there will be a 12V "square" wave at this point.

Because of the inductance of the coil there will be a large positive peak when the points open. This is what the "condenser", normally a 0.1uF capacitor is there to suppress, which id does, to a degree.

Depending on what the tacho requires, you need a series R (maybe 50K or so?) from this point (for isolation) to a 5V or 12V zener diode to ground. The voltage across the zener will drive the tacho, but you may need to experiment with the values.

Be careful! There is a huge inductive kick from these things!

[star882]

Modern cars actually have much more complex ignition systems (essentially high voltage switching power supplies). If there isn't already a tachometer, you can derive it from the crankshaft position sensor or camshaft position sensor. Deriving it from the ignition control circuit might not always work since the ignition might be disabled while coasting in gear.
http://techno-fandom.org/~hobbit/cars/tachack/

[Iain McNeill]

if your car is less than 6 yrs old (or so) you most likely have ODB2 interface on your car. Get a cable and CAN decoder board and download the scantool software (or several other useful apps)

This can be read and graphed by a laptop, tablet, PDA in real time. Keep your eyes on the road

[dangus]

Take a look at the LM2907/2917 frequency-to-voltage converter chip. The app notes include a tach circuit for point ignition systems.
LM2907
I'm sure you could pick up a usable signal somewhere on a car with electronic ignition, though.