More overclocker circuits: A simple, easy to build and cheap fanbus that allows several fans to be monitored by a single fan connector at the motherboard. The design is based entirely on discrete components that are easy to find and are low in price.

Design features:

* Simple, discrete design - no IC's are used.
* Cheap, easy to get standard components are used.
* Allows detection of fan failure of several fans on one motherboard fan connector.
* Follows standard - open collector output for the fan pulses.
* Easy to expand - modular design and option for external fan power.

This circuit (due to it's simplicity) have one drawback - the shown RPM from the fanbus is from one of the connected fans. I can't find a good way to show several fans RPM on one single monitor connector. The primary function of this circuit aren't affected by this limit - a failure in any of the connected fans will be detected.

WARNING: Don't exceed the current limit of Your motherboard's fan connectors!
Connecting several fans to one single connector could easily exceed the capacity of the power feed from the fan connector, specially if large and/or high RPM fans are used. If this is the case - use the external power feed connector that is included in the design. Power can be taken from a harddrive connector. The failure detection aren't lost if external power is used (the circuit still have to be connected to the motherboard for monitoring).
I will not take any responsibilities of any damage to motherboards from this circuit! Use it at Your own risk - play safe in case of doubts and use external power for Your fans!

Schematics and functional description:

Download the complete schematics here!

The desing is rather simple - the pulse detection (one for each fan) is the same as for the fan failure detection circuit I have made. The outputs from the detectors are connected together with resistors, forming an AND-function that will shut off the RPM-pulses from one of the fans.

There are two jumpers in the circuit:

J4 - Allows selection of which fan's RPM should be presented to the motherboard.

J5 - Selects type of power. Position 1 is for external power, position 2 is for internal.

As You can see, the circuit is rather simple and build in a modular way. This makes it easy to add more pulse detector stages for more fans. Just copy the circuit in the gray box shown at the drawing. Connect the output resistor from the pulse circuits together and to the base resistor of T6. There is only one critical point in this circuit that have to be checked in case of adding more fans - the voltage at the base of T6. It should always be above 1V when all fans are running to ensure proper operation. When one of the fans stops due to some failure, the voltage should drop down to zero (groud level).

A picture of the 2-fan prototype. This one is specially made for my computer and the optional external power aren't used.