633 Tachometer method?

[David D]

How does the 633 fan tachometer work, exactly? Does it measure voltage cycles? I'd like to use it to monitor my engine speed and vehicle speed (both signals pulsing between 0 and +5).

Thanks,
Dave

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[CF Tech]

The 633 measures the average pulse width over a 0.125 second period of time. Results are generated twice per second per channel (fan). You will want to set the corresponding fan power to 100% so the 633 will average over the entire 0.125 second interval.

It should be OK to feed conditioned 0-5v signals into the 633's tach pins. A car is a rogh place for electronics, so make sure the signals are clamped with protection devices and make sure that the power supply is clean. Also watch for ESD, ground loops and be careful with your shielding.

 

[David D]

Thanks!

I was figuring on connecting the signal wire to the gate of a transistor, and using the fan-power on the connector..

            +12  (fan connector)
             | 
             |  /
              /
speed     | /
(car) --- |
          | \
              \
             |  \
             |
             |
          fan tach

That should be pretty safe, right?

Dave

 

[CF Tech]

Where does the "speed" signal come from?

If it is 0-5v, the 633 should be able to see it without any modification (like the transistor), you just want to make sure nothing above 5v or below ground comes in. You can use a couple of back-to back schottky diodes to do this.

 

[David D]

The speed signal comes from the car''s computer -- but I don't trust car voltages very much (my car's positive voltage ranges from around +9 to +16, I think) and I've got a ton of spare resistors, so I figured I'd use the +12 from the fan power -- can the tach. pin only take +5, or will it handle +12 as well?

Thanks,
Dave

 

[Grinder]

The tach should be able to take 12v, but I'd feel better if you used a 1K series resistor feeding into a 5.1v zener to ground, then we know all will be well. Still would not need the transistor.

 

[David D]

Ok. Thanks! What voltage does the tach. switch on? 1.5? 3?

Dave