120,000rpm tachometer help

[Smurf208]

Hi i'm building a very special jet turbine and its bearings are designed for max 120,000rpm. I've looked all over google for homemade tachometer circuits. I could use either 2 magnets for a magnetic rpm circuit or i could use a light probe? Does any1 know of a simple rpm tachometer i could build. Any circuit schematics or info would be great. I'm not the best with electronics but i've built a couple of small little circuit boards from parts from radio shack and simple designs off internet sites so i have some expirence with circuits. It would need atleast 4 digits in its screen and it could be digital, led, i just need anything that will do the job. I can play around with different designs and make them work as long as the electronics are taking care of. Whats have you guys found out with your engines?? Anybody have anything?? Thanks

[HuFlungDung]

How about using some vibration analysis equipment? That is only about 2 kilohertz, and I suppose that there will be some component of vibration at the operational speed of the shaft, plus various harmonic intervals.

[DennisCNC]

I was planning on building a mini jet also, there is a industrial light sensor availabe that I have seen used on jets to measure RPM by painting the front cone half black half white. The sensor has a fiberoptic light source and pickup. I don't remmember the name but I did see them on eBay for ~$50 or so. Keep searching through "Hobby jet related sites" .

[Smurf208]

Do you have the circuit schematics for any of these methods??? Any info on building them???

[NC Cams]

The LM2907 and/or LM2917 IC's can be used to make a simple and effective tachometer circuit.

You will need to find a way to generate a frequency from a rotating disk or the the like somehow. The circuit is easy, the sensing of motion frequency from a turbine is the hard part...

[Smurf208]

the sensing part is easy for me, i have many ways i can do it i just can't find the electronics and mainly the schematics (circuit drawings) of any sensing type. I have 2 magnets that i can put so it would have 2 pulses per rev but i need the electronics to translate that to a simple screen so i can read it. I can also paint black and white around 1 of the parts to give me a photo type sensor. i just don't know the electronics part. And what is LM2907 and/or LM2917 IC??? Are they circuits u build that will work or are the computer chips?? And if they are circuits do you have the schematics??

[NC Cams]

They are tachometer IC's supplied by National Semiconductor (NSC). Go to National's site and look for an application note for them. The app notes give the circuit diagrams and the calcs need to make a tachometer.

My old "written NSC catalogs" (circa 1988) have the chips and the application notes that you need. They should be available as PDF's by now as the chips are used in much more than tachometer's anymore....

[vulcom1]

I am also interested in what you are trying to do. I have spent quite awhile also as I am in the process of building a couple of KJ66's. I put everything on the back burner for the summer but it is time to start again. (It's snowing)
Question I have is why the need for 2 magnets in the front hub/nut? With one at 125k you get one rpm per revolution and with 2 magnets you get 250k which is double. I would also like it to go to 150k as alot of them are up there. My turbines are rated at 125k which are from Wren and certified.
I am going to check out the chips.
Thanks, John

[Smurf208]

Only 1 magnet would set the rotation out of balance, 2 keeps the balance. With the LM2907, what is the "charge pump" and where do i get it? Where is the material listings and where is the screen the tells you how fast its going? i see a circuit diagram but its not complete??

[NC Cams]

Did you read the application note or just look at the pictures? If you did not read the app note, the pictures are essentially meaningless.

You will have to do a bit of math to properly turn the pulse frequency from the sensor that is monitoring the spinning whatever to a voltage that is readable by a user supplied voltmeter. The LM2917 IS the charge pump.

Download the application note from NSC, read and study the app note, do the calculations, build the circuit AS IT IS SHOWN and ADD a voltmeter to it.

YOU provide the voltmeter, the IC ultimately provides a voltage that is proportional to the chopped signal that comes from the non-contacting sensor that you also supply.

You will have to figure out the resistor and capacitor values as they are using the IC to ultimately turn a signal wave from the motion sensor into an analog voltage.

[JRoque]

where is the screen the tells you how fast its going?

Does that imply that you're looking for a standalone system to measure and display the RPM? Or will the circuit be interfacing to anything else?

I have a laser based tachometer that reads up to 99K RPM, perhaps there are other models available that read more than that. If you decide to build your own, and want it to display on an LCD, your best bet is to use a microcontroller. You connect you sensor output to an interrupt on the uC and count how many hits you get in a second (so you don't have to wait a whole minute). Not sure if the uC option is viable to you.

BTW, rather than individual iron magnets, consider use a magnetized strip. You wrap half the shaft with it facing out and the other half facing in. That gives you two poles that can trigger a hall-effect sensor. A ring magnet would be great too and you can read it with this sensor

JR

[Mariss Freimanis]

Get a strobe-light tachometer. It will cure all your ills, impress all your freinds because it's so cool and you will be able to use it for a million other applications.:-)

I know. I have all sorts of tachs. The strobe is the first one I reach for when I'm in a hurry.

Mariss