Clock/sensor signal averaging

[Elandril]

For a small project with limited space (fixed enclosure) I have to monitor a larger number of fans that keep the installed hardware under somewhat acceptable temperatures. As I'm trying to avoid having to install additional equipment in the already quite full space, I would like to use the onboard fan monitoring of the mainboard to keep an eye of the fans. The mainboard offers three fan connectors which somehow have to monitor 12 fans. So I'd like to connect one row of 4 fans to each mainboard connector. I've designed a small pcb that powers the fans with +12 V and distributes the mainboard PWM signal to the fans. All of that works like a charm. :-)

Now I need to find a way to combine 4 square wave fan tacho signals into one for each channel. The tacho output from the fans is designed as a open-collector/-drain output where the mainboard contains the pull-up to +5 V. So the tacho signal is effectively like clock signal. Does anyone know of an IC that can do an average over 2/4 signals or a minimum clock frequency reduction? The frequency would be below 120 Hz (2 peaks per revolution) for the fans. My search on digikey and mouser was not successful. Otherwise I have to do it via a micro.

[thermistor-guy]

... Now I need to find a way to combine 4 square wave fan tacho signals into one for each channel. ... My search on digikey and mouser was not successful. Otherwise I have to do it via a micro.

I like your microcontroller idea. E.g.: measure the frequency of each tacho signal; calculate the minimum of the four frequencies; output a signal with that same (minimum) frequency. If one fan fails, or starts to fail, the output signal "passes on" the failure.

Or you could use a more elaborate relationship, between input frequencies and output signal, to suit your system.

[Geoff_S]

One hardware option - RC low pass filter the PWM to produce analog voltage and then average using an op amp adder circuit ?