User Instructions.


OPTO-TACH Is not a "PLUG IN" product. It requires the use of hand tools and must be modified for each installation.

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 indicator.

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 commencing installation.



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 white.

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.

The 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.
Software set-up.

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 white spot.

With very slow fans or to maintain compatibility with less flexible monitoring programmes you should use 2 white spots spaced as equally as possible.

Test Spots.