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General Category => Your projects => Topic started by: sgking2 on February 07, 2012, 16:24:07 PM

Title: Multiplexer or decade counter?
Post by: sgking2 on February 07, 2012, 16:24:07 PM
Hello,
I was hoping you could help me determine the best way to accomplish this little goal.
I have a nice graphical color LCD screen that feeds back temps and fan speeds, I want just 1 channel to read the tachometer fan speed of all 10 fans. No single fan failure is going to cause a catastrophe but I would like to know and this device alarms if the fan speed detects a stall or no connection for more than a couple seconds. So...what I was thinking was using a decade counter of some sort to switch each of the 10 tachs into this input for 3 seconds each and just keep cycling through that loop, reading all 10 fans 2x/minute. They are all the same type of fan and while their speeds are variable, they should all vary together. So the RPM won't even appear to be changing more than 100rpm from fan to fan - any one that stands out more than a couple hundred rpm is having a problem.

Any thoughts on the best way to cycle between the 10 tachs into 1 input? Decade counter, multiplexer, flip flop or some other device? Switching time is not critical as the unit looks for a long gap of no pulses before alarming (like 1.5 seconds).
Thanks!
Title: Re: Multiplexer or decade counter?
Post by: kam on February 07, 2012, 20:52:58 PM
it is multiplexing what you are looking for. there are multiplexing chips to do exactly that. how are you going to test if a fan is working?
Title: Re: Multiplexer or decade counter?
Post by: sgking2 on February 07, 2012, 21:34:33 PM
I have the NZXT Sentry LXE fan controller which I am using for other purposes, however, I can use one of its channels to receive the tach signal from a single fan input. It displays the exact rpm of the pulse train fed into it and if the pulse train ceases for a couple seconds it alarms. I justy want to be able to switch between 10 fan tachs periodically into that 1 input. I do not need live monitoring of each fan, but if it is checking the rpm twice per minute that is fine. That is cycling between each of 10 tach inputs for 3 seconds each and then starting over in an infinite loop. Even each fan for 6 seconds, meaning each fan is checked once per minute and displayed would be more than enough. If the fans are remote (out of sight) this periodic check lets me know all fans are operating. Do you know of any multiplexing circuits that route 10 pulse trains into 1 output, one at a time, for a specific amount of time (r/c time constant or reference)?
Title: Re: Multiplexer or decade counter?
Post by: kam on February 07, 2012, 21:44:27 PM
i do not know any circuit, but you can easily cascde 2 8-bit multiplexers, such as the 14HC151 or 14LS151