OPTO-TACH Is not a "PLUG IN" product. It
requires the use of hand tools and must be modified for each
OPTO-TACH can be used to provide TTL level rotational
signals suitable for input into the usual 3 pin fan headers found on
PC motherboards. The required adjustment setting up and testing can
be done using an alkaline 9 Volt battery and the incorporated test
The OPTO-TACH requires exclusive connection to a spare
fan header that reads the "rotation" signal. Most
computers have 3 or 4 fan headers 2 of which usually have the
ability to monitor fan rotation. OPTO-TACH is not designed for
small fans such as CPU cooler fans which in any case provide
their own rotational signal outputs.
OPTO-TACH Is suitable for fans of 80mm and above. It is
ideal for use with AC powered fans.
Please read this document to the end before
THIS PRODUCT USES A SUPER BRIGHT LED LIGHT
STARE INTO THE LIGHT
- Theory of operation
A photo transistor monitors the reflected light
produced by a super bright LED. When this reflected light is above a
fixed level a second transistor turns on an indicator LED and the
open collector NPN transistor that sends out the rotation signal.
Power for the unit (40mA) is taken from the 12 Volts
available on the 3 pin fan header. This voltage is reduced to 9
Volts by the resistor incorporated in the supplied special header
lead. The fact that the OPTO-TACH circuit board requires 9 volts
means that it can conveniently set up when powered by an ALKALINE
9 volt battery using the included battery lead.
Installation Considerations and Testing
OPTO-TACH requires there be a change from white
to black on a rotating object. Before installation power up
the unit using the included battery lead and an alkaline 9 Volt
battery and test it under subdued room lighting by moving the light
head across the black and white test squares provided. You should
note that the green LED on the board turns off and on when the light
head is about 2 cm away from the paper and moved from black to
You must provide the white and black that OPTO-TACH
requires .Note some fans or software may require 2 white to black
changes per revolution, see software set up.
OPTO-TACH must be installed where it is protected ie.
inside the computer case.
The white area should be a minimum of 1.2cm Square.
If the fan is blowing air out of the case then the
centre hub of the fan is accessible and can be used by placing a "D"
shaped white sticker so as to cover 1/2 of the hub.
If the hub can not be used then a sticker can be placed
on one fan blade however just how secure the sticker will be is
unknown and painting a fan blade is an alternative.
If using the fan blades then the light head should be
directed so that the head only "sees" fan blades and not
the spaces between them or external light may cause problems.
Balancing the fan.
With the usual 7 bladed fan one spot on a blade can be
balanced by 2 half sized spots on the reverse side of the opposite 2
blades. If blades 1 and 4 are spotted then they can be balanced by a
single 4/10 sized spot on the reverse side of blade 6.
OPTO-TACH is installed and tested using a battery. The
equipment it is to be installed in should be disconnected from all
power. You should follow all recommendations pertaining to the
equipment including prevention of damage from static electricity.
The Board must be mounted so that no metal part of
the OPTO-TACH or any part of its components touch or is likely to
touch any other metal parts.
The recommended method is to attach the board to a
lower corner of the fan using a nut, bolt and shake proof washer. If
the fan body is metal then spacing the board away from the metal by
some washers will eliminate the possibility of shorting if the board
rotates around the screw.
- Do not use the corners of a mains (AC) fan near to where the
mains (AC) is connected.
The light head must be mounted on a different corner
of the fan to the board to ensure security of attachment.
white patch should be created and the OPTO-TACH connected to a
battery. The light head can then be test positioned by hand and the
fan turned to check for correct operation. Bend the support wire so
that it may be attached to a corner of the fan and secure with a
nut, screw lock-washer and flat washers. Needle nosed pliers should
be used to form a secure loop at the end for the screw to pass
through and this loop should be held under the flat washer. When
finally adjusted the light should come on when the white patch is
half into the beam and go out when it is half out, and there should
be some period when the white patch is fully in the beam. Photo
shows test position for blade use. Hub use would be preferable if
the light unit was to be on this side of the fan (This low cost 12 Volt fan had 3 wires but NO rotation signal).
If the light head has to look through a chrome fan
guard then blacken the chrome to stop reflections.
The best surfaces white and black, are mat. If the
black is shiny it may cause a problem if it "flashes" as
the curved fan blades pass. If using the hub where the light head is
directed at right angle to the surface the reflection may be reduced
by angling the light head slightly.
When satisfied with results remove the battery lead and
attach the header lead to the motherboard fan header.
Most fans provide 2 outlet pulses for each revolution
and putting 1 white spot on your fan means that OPTO-TACH will
provide half the usual pulses. If the number of pulses is below a
certain number no fan speed will show at all.
A comprehensive programme like Mother Board Monitor
(MBM) allows adjustments for this and many other things. With a
normal fan and a MBM fan divider setting of 8 a minimum fan speed of
662 RPM will enable the display of a reading. An OPTO-TACH equipped
fan with one white spot must be faster than 1324 RPM to display a
reading. Basically the minimum RPM as shown in the MBM help file
must be doubled when using OPTO-TACH. The fan multiplier that is
usually set to 1 in MBM should be set to 2*. This multiplies the
reading by 2 and compensates for halved output of pulses from 1
With very slow fans or to maintain compatibility with
less flexible monitoring programmes you should use 2 white spots
spaced as equally as possible.