How to read RPM from Tachograph Pin (yellow) from 3-wire 12V DC fan

Projects General Guidance
1

Hi All

So, I have a 12V DC .15A fan that has 3 pins. +Ve (red), -Ve (black) and Tacho (yellow). I have a couple of questions before I connect this to my Arduino.

Using my Arduino, I would like to be able to measure the RPM of the fan from the tachograph pin, for no particular reason other than to learm etc, and so the questions I have are:

1 - I have measured the voltage at the tacho pin for different speeds of the fan and:

  • Full output on analog pin (255), fan speed max, tacho voltage is 1.1.V
  • Zero output on analog pin (0), fan stopped, tacho voltage is 11.8V
  • Voltage drops from approx 12V to 1V proportionally as I increase the fan speed.

Now, there is no way I want to connect a 0-12V signal to the input of my Arduino, so how to I reduce this signal so that it is safe to feed into one of the analog pins of my Arduino? Do I create a voltage divider circuit?

2 - I understood (probably incorrectly) that the tacho pin works off the hall effect, and as such a pulse (or two, unsure!) is generated for each revolution of the motor. Given that the voltage varies between 12V-0V (min speed - max speed) is this not the case and have I understood it incorrectly? or is it the case that the voltage measured by my DVM is in effect the result of the pulses from the fan?

I am loving all this coding and learning. Any help/advice greatly appreciated.

Thanks all.

2

theMusicMan:
or is it the case that the voltage measured by my DVM is in effect the result of the pulses from the fan?

Yes it is.

3

Hi,
You can use a capacitor and making the output like a ADC.

4

Do I create a voltage divider circuit?

Yes, use resistors: tach output->22K ->port pin->10K->GND to get about 3:1 voltage division.

5

aarg:
Yes it is.

Thanks, that's clarified that for me!

tauro0221:
Hi,
You can use a capacitor and making the output like a ADC.

I have no idea what this means, sorry!

jremington:
Yes, use resistors: tach output->22K ->port pin->10K->GND to get about 3:1 voltage division.

Thanks, I think I know what you mean... any chance you could sketch a schematic?

6

divider.gif

7

Hi,
Sorry I should give you an example and it is not adc should be digital to analog. here it is link so you can see I mean.

https://provideyourown.com/2011/analogwrite-convert-pwm-to-voltage/

8

tauro0221:
Hi,
Sorry I should give you an example and it is not adc should be digital to analog. here it is link so you can see I mean.

Arduino’s AnalogWrite – Converting PWM to a Voltage

No apology need sir, just that schematics help noobs like me! :DThanks for posting this though, really appreciated.