The Tacho is electronic and picks up the electrical pulses through the ignition points as they open and close to fire the sparkplugs via the coil.
So it smimply "counts" the number of times the points open and shut per minute - which is twice per each revolution of the motor.
Exactly what simplemind7 said. Since it is an analog device (and not a digital to simply count the pulses) it would be interesting to know the circuitry involved in turning the pulses to an average voltage and eventually an RPM measurement.
It looks like a linear actuator. It has a field attached to the needle around a rotary ring with a small magnet on one side. 8 resistors, 4 capacitors, 2 diodes, 2 transistors and 1 potentiometer. It looks like it fires the transistors to convert to a steady voltage. From there it must excite the field and force it away from the magnet depending on frequency....I took one apart. To see the actual curcuit you would have to take it completely apart as the traces are on the guage face side of the board.
The tach uses a relatively simple analog circuit with two transistors.
- The first transistor detects the pulse from the points being opened. The pulse from the points is actually an oscillation that lasts a couple of milliseconds. It starts out with an AC component of about +/- 50V that damps down to a DC voltage (about 24V) in a couple of milliseconds (ms). Once the spark is done the voltage drops to about 12V DC and stays there until the points close again.
- This is not an easy signal to deal with, so the first transistor is used to convert it to a simple square wave pulse with about a 2ms duration. The output of this stage is then a set of pulses of 2ms duration on and the rest off, with one pulse for each time the points open.
- The second transistor acts as a current amplifier so this pulse train can be used to drive the meter. The higher the duty cycle of the pulses, the more the needle moves.
- The meter will go to full scale with about 10ma of current.
- Interestingly, the tach actually works on 6V internally. Part of the circuit drops the 12V input to 6V to operate the tach.
I guess I never labeled D1 in the schematic. It is near the meter. It provides a current path for the back emf generated by the meter when the pulses end. If it is not there, when the pulse from Q2 ends, the current running through the coil in the meter will generate a significant voltage pulse since it has no where to go. With D1 in the circuit, the current can just loop through the meter, until is is dissapated by the resistance in the coil. This helps keep the needle steady between pulses and protects Q2 from the same pulses.
The only capacitor that really matters is C3, since it (with R1) determines the length of the pulses (2ms). C2 is just there to smooth out the 6V DC created by D2. C1 is a high-voltage, low value capacitor that gives Q1 DC isolation from the points/coil and keeps down the total energy in the pulses that trigger Q1. It's voltage rating is more important than it's capacitance.
I am new here and not as knowlegable as you, but it seems like this is a schematic for a tachometer with a connection to the (-) pole of the ignition coil and the other two connections for the (+) and (-) poles from the battery. My vehicle shop manual shows that I should use a tachometer like this, but I have a tachometer that only has one connector to the (-) pole of the ignition coil and another one for ground. My question is about the compatibility between my tachometer and my vehicle ignition. My car is a 1989 Toyota MR2. I could not get any answer from the MR2 forum. Maybe the Germany car forum has more technical info than the Japanese car forum.