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PICList Thread
'tachometer output fans'
1999\01\14@134410 by Lee Jones

When they fail, a lot of fans will warn you with a bearing
whine.  Often though, they silently just stop.  Frequently,
new PC motherboards include a 3rd wire on the fan connections
for a tachometer input and the hardware to monitor RPM.

Problem I'm having in finding fans with tachometer outputs.
Two questions:

Does anyone know of a source of supply for low volumes (qty
1 to 10) of fans (sizes 80mm, 92mm, & 40mm square) having a
tachometer output?

Anyone know of where to get a specification detailing the
tachometer signal?  It seems that all the PC motherboard
manufacturers have settled on a common standard -- I'd
like to know what it is.  For example, Intel's N440BX wants
to see 2600 RPM +/- 600 RPM with 2 "pulses per revolution".

[Yes, I can try to reverse engineer it, but I prefer to
design to a published standard if one is available.   ]

Seems to me that this is a very nice niche for a small PIC
as a retrofit to existing fans.  Have the PIC use a photocell
or phototransistor to "look" through a fan at outside ambient
light.  As the blades pass, it counts them and simulates the
tachometer output.

Tricky part seems like it would be handling the vast change
in ambient light level.  Say a basement computer room going
from bright flourescent lighting (pulsing at 120 Hz in US)
to nearly pitch blackness.  Maybe a grain of wheat bulb on
the PIC sensor would be a good idea...

But first I need to find out what the tachometer signal
is before I can simulate it with a PIC.

Thanks in advance.
                                               Lee Jones

1999\01\14@140039 by Brad Stockdale

picon face
At 10:41 AM 1/14/99 -0800, you wrote:

>Tricky part seems like it would be handling the vast change
>in ambient light level.  Say a basement computer room going
>from bright flourescent lighting (pulsing at 120 Hz in US)
>to nearly pitch blackness.  Maybe a grain of wheat bulb on
>the PIC sensor would be a good idea...

Well, an idea that comes to my mind is to modulate the photodiode (say at
40 Hz or something common like that -- that's how the IR remotes for TV's
work). Then, you use a discriminator type circuit to look for the 40 Hz
'signature' and go from there.

Just my two cents, for what it's worth.


1999\01\14@141908 by Greg Holcomb

This sounds like an interesting product.  I think you would need a light
source.  Many people leave a PC on all night.  Also, your worst case
would be when the PC is running with the lid off, I do that too

I would look into a reflective sensor module.  I use a HOA-1401 by Omron
in a product.  It would work well for your aplication too I think.  It
has an infrared led and a phototransistor.  Using a PIC12C671 with the
analog input, you can pulse the infrared led and read the
phototransistor with the analog input.

Problem is, add the cost of the sensor ($5) with the cost of a pic and
you could buy a new fan.


{Quote hidden}

1999\01\14@142531 by Howard McGinnis

Try a Papst web site search. It's a German made fan with a tach output. We
have a device that monitors 128 fans with tach outputs. Tach output is TTL
with the fan using 12 VDC ( I believe it's a Hall effect device for the tach

The P/N is 4212/12H, puts out about 110 transitions per second - or is it
pulses per second? I can't remember - we're switching over to use a PIC
based system.
Howard McGinnis                   Electronic Visions, Inc            1650 Barrett Drive
(407) 632-7530                      Rockledge FL 32955
FAX (407) 632-3396     

1999\01\14@142540 by Harrison Cooper

We use LARGE fans with a hall sensor output.  I would imagine that there are
small fans (we also use some on our graphics chips.....Diamond Fire4000,
Accell, etc) that I don't remember seeing a wire for a hall sensor.
Probably are some tho..

1999\01\14@170110 by Morgan Olsson

picon face
Dont go over the river to fetch water...

Small fans electronic cinsist normally of only hall sensor and drive stage.
Some are possible to open.
Probably the only interface you need is a series resistor.

Another way is to monitor the transients in the supply current, which for
some fans are high, but very low for other ones.

I believe small Sunon fans have high supply variation as they only have one
winding, actually.  Readable by a transistor, a cap and a few resistors.

PS  any body want Fan controllers regualtors etc?  I produce some types and
special variants.  DS

       Morgan Olsson                   ph  +46(0)414 70741
       MORGANS REGLERTEKNIK            fax +46(0)414 70331
       H€LLEKS           (in A-Z letters: "HALLEKAS")
       SE-277 35 KIVIK, SWEDEN     

1999\01\15@082744 by Mark Willis

I've thought of using those little wires the stores use to stop
shoplifters as a very light, glue-on or melt-on, sensed tag.  Haven't
figured out what they are yet (Not in a big hurry on that project.

 On this, if you put a White/Black sticker {radial pattern, say split
in quarters of alternating colors} on the fan's hub, and a photosensor
inside the power supply case looking at the rim of that sticker, (Or
atop the CPU fan hub for that matter) that'd give you a pretty balanced,
controllable sensor retrofit, if you cannot find fans such as these...
Dust could be a problem, of course (That's why I thought of the
anti-theft wires, though come to think of it most any magnetic sensor
could be made to do this, I'd bet, use magnetic ink not colored ink?)

 If a positive pressure sensor were cheap enough, you could use it like
a pitot tube & detect the AC component from the fan blades' passing, can
a Piezo disk do this, I wonder?  <G>


Lee Jones wrote:
{Quote hidden}

1999\01\15@095644 by Peter L. Peres

picon face

On Thu, 14 Jan 1999, Brad Stockdale wrote:

{Quote hidden}

In fact, nearly all electronically commutated fan motors can be tapped for
rotation signal off any phase, using an AC coupling and a single
transistor to shape the signal. This requires 3 * R, 1 * C, 1 * Trz. They
are usually biphase so the signal should be within the Intel spec. AC
driven fans are a different matter, but they can be sensed using a SMT
Hall sensor IC and an inexpensive SMT op-amp near the rotating permanent
magnet edge. Many muffin fans have a special hole in the outer rim for a
reflective photocoupler. This is for photocoupler sensing as you are
trying to use.

hope this helps,


1999\01\15@095651 by Peter L. Peres

picon face

On Fri, 15 Jan 1999, Mark Willis wrote:

>   If a positive pressure sensor were cheap enough, you could use it like
> a pitot tube & detect the AC component from the fan blades' passing, can
> a Piezo disk do this, I wonder?  <G>
>   Mark

Now, why would you ask this ? Hehe. In fact, I've tried, and it can't. Not
even with a good-sized tab of copper foil soldered to it as turbulence
sensor. You can't tell whether it's the fan or a disk drive vibrating
near-by. But a pair of ultrasound transducers can. They are mounted to
bounce the beam off the fan blade (one at a time). The output is both
frequency and amplitude modulated and at a good level (5Vpp drive -> about
0.1Vpp output at 0.5 meters). This is a little bit expensive for muffin
fans but it can be used for large fans (A/C, dryer, you name it) and it is
not sensitive to lint/clogging within reason so it can be retrofit.

Don't bait me with piezos anymore please. I can still remember when I
tried to pay with a piezo disc for candy late at night, by mistake (same
size as a certain coin here). Didn't have the proper change either. Yes,
of course I had already swallowed 1/2 of the candy.


1999\01\15@102311 by Mark Willis

One solution I've seen, come to think of it, for a PC power supply

 Put a thin (mylar?) flap outside the case, with an outward-blowing
fan.  Put an opto-interruptor on a frame below it;  If the flap drops,
the opto is interrupted.  Fan spindown then causes a cheapie alarm to go

 (One place just has little ribbons tied to each fan's protective grid,
but that requires an occasional glance to check.)

 Once in a while a fan's blade assembly will split and pop off the
hub.  One Co. told me they lost a old 386 server due to this, the motor
was still spinning (they say - ??) - but the blade assembly had split
and managed to not contact the hub any more (More usually the blades hit
the housing & make this awful racket.)  In a way this is a better
solution; if you power the cheap alarm off the AC output of the power
supply it handles turning it off automatically, and it's a LOT simpler
than watching blade RPM's etc. to just assure that enough air flow is
happening for sufficient cooling of the PC.

 Could also use a magnet & reed switch (magnet on the flap of course


1999\01\16@075246 by wwl

picon face
On Fri, 15 Jan 1999 05:19:14 -0800, you wrote:

{Quote hidden}

It would be a lot easier to measure the AC component of the current

1999\01\17@000449 by Derek Bischoff

Ok, there were lots of responses to this, I happen
to know that there is a circuit out there that can
monitor your fan.
check out
110 alert +
Works well with nearly any fan, just fine with their fans.

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