Analog Tachometer Input Multiplier

[skrazo]

I have an older 0-5000 RPM analog tachometer that I'd like to install on an old points type ignition farm tractor. Am looking for a simple device to multiply (double or triple) the tach input, so 0-2000 RPM (actual) will indicate closer to full scale on the faceplate. Full output RPM of the tractor is 1750. The most ideal solution would be to find a simple device to place between tractor output and tach input. I'm new to the forum, so if this question as already been addressed, I apologize for the redundancy. :grin:

 

[pradeep.g.belchada]

if it is not a electronic device then you have to use the gear ratio

 

[crutschow]

What is the input to the tachometer? Do you connect it directly across the points on the engine?
How many cylinders is the engine?

 

[1/2dime]

There are many frequency doubler circuits (both digital and analog) floating around on the internet. The simplest is an Inverter tied to a XNOR gate. Triplers are a tad more difficult. E

 

[skrazo]

The tach I want to use is electronic. Connections are: 12V power supply, one input lead to ground, other input lead connects to the negative side of the engine coil, and a slide switch tells the instrument how many cylinders the engine has (in this case =4). The tach just counts the pulses, knows the number of cylinders the engine has, and provides a readout to the analog meter. My thinking is, if I could just "double" or "triple" the number of pulses the input to the tach "sees", it will indicate a higher RPM on the scale (thus indicating 4K RPM for an actual 2K RPM from the engine, making the instrument more sensitive). The engine typically runs at 1200 to 1400 RPM at full load. If this does not make sense, let me know.

P.S. ~ Simply doubling the output reading would be fine.

 

[chuckey]

So its a petrol fired tractor with sparking plugs then? I would open up the tacho and trace out what the switch does. It probablely changes the time period of a timer, so replacing a resistor would change its calibrations. I do not believe for your application a frequency doubler will work as you are dealing with a 20 micro second pulse which varies is in speed from 6.6 HZ (800 RPM) to 66 (8000 RPM). You can synthesize a "second" spark to fool the tacho, but the problem is then either turning it into a 400V pulse or modifying the input circuits of the tacho to accept much lower amplitude pulses. One thing that might work is to put a piece of insulated wire on the "coil -" terminal and tape it temporarily to a spark plug lead, depending on the design of the tachos input circuit there might be enough pick up for it to work. If it does just use a second pick up wire on another spark plug. Use tyraps or some thing more substantial for the final arrangement!
Frank

 

[crutschow]

Does the slide switch have a setting for 2 cylinders? That should give you a double reading on the Tach.

 

[skrazo]

Does the slide switch have a setting for 2 cylinders? That should give you a double reading on the Tach.

The switch is only for 4, 6, or 8 cylinders. When I get some time (very busy with farming right now), I'll pursue Chuckey's ideas. The electronics are enclosed in a metal case which has a crimp around the dial holding the whole thing together.......it will take some effort to get the crimp removed. I'm hoping once the printed circuit board is exposed, there will be an obvious way to change the "scale factor" of the "meter driver".

Years ago, I had a tachometer from "Heathkit" that was dual range, by using a slide switch on the back of the case, the display could be changed between 0-2500 RPM and 0-5000 RPM.