PC-Fan Silenser and Guard

All PC overclockers out there - here is a small, useful circuits for You! This design is a small and easy build fan silencer (by reducing fan voltage) and guard (monitors rpm and indicate failure). This circuit have been designed to be simple but with good features - it is entirely based on cheap standard components and could be very compact built.

Design features:

* Simple, discrete design - no IC's are used (except for a voltage regulator).
* Cheap, easy to get standard components are used.
* Transparent - it can be connected between fan and motherboard without loosing RPM monitoring in BIOS of the PC (fan pulses are transmitted through the circuit).
* Silences the fan (and make it live longer) by lowering the voltage - default to 6.5V but it can easily be changed by modifying a few components.
* Monitors the RPM of the fan - if the fan stops in any case, the indicator LED will change from green to red, indicating something is wrong.
* Lowering the current need of the fan - a lower voltage will make the fan draw less current.

Schematics and functional description:

Download the complete schematics here!

I will give a guide to the design so You can easily modify it to suite Your needs. It is handy to have the drawing visible when You read this - I will refer to it.

The Silencer part - very simple, it is a standard 7805 3-pin voltage regulator (U1) and two diodes (D1, D2). The purpose of the diodes it to level the reference ground of the regulator - they add the missing voltage to the regulator (there are no 6.5V reg's avaiable). Without the diodes You will have 5V from the regulator. The capacitors (C1, C2) around the regulator is just to filter out HF-disturbances, mainly from the fan engine. If You plan to run the fan at higher voltage, just change the numbers of diodes (a 1N4148 adds about 0.5V) or change type of regulator (from the 780X series). If You don't use any diodes, then the ground pin of the regulator have to be grounded (U1, pin 2). The regulator could need a small heatsink, depending of the size of fan used. Note that a fan running at lower speed will take less current - I ran it with a 0.3A 90mm fan and at 6.5V I got a current draw of about 180mA including the guard circuit.

The RPM failure detector/guard - this part is more complicated and does not allow any heavy modifications. Anyway, it is always interesting to know how it works. We start in the fan end (connector J2). The RPM pulses from the fan (white lead) enters a buffert stage built around T5. Resistor R11 is required due to the open collector output of the fan. Next stage is a pulse detector built around C4, R6, D3 and C3. The output of this circuit is a small DC-voltage that goes to zero (0V) when the fan stops due to failure. The remaining part of the circuit is the LED driver stage and level detection. Transistor T4 will respond at the output of the pulse detector. When pulses arrives from the fan, a voltage at around 1V will appear at R1 and the base of T4. That will shut off T3 and turn off the red part of the double LED (D4). At the same time, transistor T1 will turn off, allowing T2 to open and light the green part of D4. In case of fan failure, the pulses will disappear an the base of T4 will go to 0V - this turns off T4 and allows T3 to lead - the red part of D4 will shine and at the same time, it will also activate T1. The active T1 will block T2 to lead - and the green part of the LED will stop shine.
A modification likely to be done is the LED - any kind of double LED (the 3-pin type with common catode) or two separate LED's can be used. Resistors R7 (for the green LED) and R8 (for the red LED) may be necessary to change to suite selected LED's current needs.

Pulse Pass-through - also very simple, an additional transistor (T6) taps the buffer output (connected to the collector of T5). This will feed the pulses of the fan to the motherboard side of the circuit (connector J1, white wire). The T6 is open colletor - the same as the fan itself. If pulse pass-through isn't needed, the T6 and R12 could be left out without affecting the rest of the circuit.

The prototype of the complete circuit. The white connector at top of circuit board (right of regulator) is where the 2-color LED is connected.
Here a 92mm fan with circuit mounted - with glue :-). The complete unit can be used as a regular fan - just remember that the RPM is much lover and monitor software have to be adjusted to avoid low-RPM alarms appearing.
A simpler variant - just a regulator (5V with three diodes) for 7V. This is a safer way than tapping 7V from the PSU by connecting the fan between the +12V and +5V leads. For big fans - don't forget to put a small heatsink on the regulator (up to 100mA can be taken without cooling, 1A max can be delivered with proper cooling) - or put the regulator in the airflow of the fan... ;-)