100,000 rpm tach

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100,000 rpm tach

Postby aaron87 » Thu Feb 10, 2011 10:30 pm

Hey guys,

I am currently studying mechanical engineering and I am working on a gas turbine engine (jet engine) for my senior design and need a way to measure rpm. I saw the other tachometer on the website but could use something more simple. Basically it needs to measure 100k (99,999) and output a voltage to log the rpms. I was wondering if there was a way to do this by just simple components not using a microchip.

If it is too hard to output both, than one of the other would be fine. I know an IR sensor is probably the way to go..


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Postby ThePyroElectro » Thu Feb 10, 2011 11:54 pm

Hi Aaron,

The tachometer for most devices is usually a sophisticated digital counter. For the tutorial you saw, a microcontroller counted internally each time a beam of light was interrupted by a fan blade.

If you want to cut the microcontroller out of the circuit and move for something more simple, a digital counter is where you will want to look. Digital counters are hardware IC's that increment every time it receives a pulse.

Here's an example hardware 4-bit counter:


Datasheet Link: http://www.futurlec.com/74LS/74LS93.shtml

Since your design only cares about counting to 100,000, you would need at least a 17 bit counter (combine (1) 16 bit counter +(1) 4 bit counter), which could count up to 2^17 -> 131071.

When the output of the counters reaches 100,000 (binary: 11000011010100000), then you can use standard logic AND/OR'ing to double check this number has been reached and then send a single +5v pulse signal wherever it needs to go.

This type of a system would tell you how often 100,000 rotations have occurred via a +5v pulse, with that information you could calculate current RPMs. Is this along the lines of what you were looking for?

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Postby luis901004 » Fri Feb 11, 2011 5:39 am

I wonder if the wiring diagram of the circuit is the same we use to mount and that is fitting, comply with the microcontroller in a different pcb and also the potentiometer connected LEDs is different

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