[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 :cheers:
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[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

[vulcom1]

I am not sure what setup he is going to use but most that I read about where hall pickup with a 1/8" magnet imbedded in the hub. What I want is to do the same with mine and also incorporate a motor/clutch(Graupner 300 6volt) setup on the nose for starting. The nut is an acorn type with a rod for tightening instead of using a wrench. It is also not hard to balance as it is small. I also would go with an analog meter instead of digital as I have found it easier to watch the meter.
I have 3 digital tachs and a few years ago I gave my analog one away after quitting racing model aurcraft. Big mistake.
I appreciate the chip numbers and will have to get a couple for setting up. It does not look that hard.
Sorry for busting in on this but I have been looking for something like this for quite a while.
John

[lerman]

120,000 RPM is pretty fast. The forces involved may be non-trivial.

I'd use an optical technique an paint a stripe on the shaft. If the paint comes flying off, no one will be injured. ;-)

Ken

[Smurf208]

I think it will be a optical sensor. The LM2907 ic circuit stuff isn't looking promising. I read all the info and its not very clear to me anyways. Magnets would be a good option its as easy for me to install magnets as it is optical sensor. But the whole freq. to voltage stuff is o to confusing, the automotive tachometer on the site isn't clear on anything. Does anyone have any info on the electronics and how to build just a simple tachometer circuit, i don't think it will be too hard to find a optical sensor to buy somewhere, i'm pretty sure i could get my hands on one. Does any1 have the schematics for a simple optical sensor system??? Although the freq. to volt. system would work out a bit better cause i wouldn't have to build a screen it would just be a voltage calculation. But a screen would be fairly easy to do i have some laying around in my misc. electronic's bin. Is there any1 that could easily post a simplified freq. to volt. schematics for mee?? i get lost with the pictures on the site when i try to put the info together.

[Smurf208]

Or a simplified optical system???? Which ever you give me works for mee.

[NC Cams]

The LM2907/2917 IC's are, in an economy of words, the simplest and easiest and least expensive tachometer circuits that the DIY'er can obtain and program for their own specific application.

Basically, the 2907 taks a pulsed signal (frequency) that comes from something (magnets and sensors) and turns that into an analog signal (voltage) that is proportional to the excitation frequency of the spinning device (the magnets that trigger your hall effect sensor or whatever).

Since the electical signal is basically OFF or ON (whether it be from an optical sensor or a set of points or an iductive pickup) you have to figure out how to turn that ON/OFF time sequence into SOMETHING that relates to RPM.

Whreas a PWM'd drive signal takes a modulated pulse width of a given frequency and turns it into an averaged analog voltage signal, the 2907 does essentially does the same thing only in reverse.

The more pulses you get per second, the more the 2907 charges up (pumps up) the voltage on a storage capacitor. This chopped PWM'd voltage is smoothed out by the IC and the timing resistors/capacitors that are part of the circuitry and this become the analog RPM that you wish to read.

MANY tachometers use either the 2907 or an equivalent circuit that does essentially the exact same thing. Only in the premade tachometer, the math and scaling are already done for you.

Once you figure out the proper scaling factor, you simple read the voltage that is being "pumped" to the storage capacitor by the 2907.

Your options are quite few and limited:

1. Learn how to use the 2907/2917 or have someone show you how or do the necessary calculations for you.

2. Build and learn how to program and use a microprocessor and figure out the RPM via timing how many counts occure between the trigger pulses (a previous post outlined this) - far more complex than the 2907/2917 option.

3. Buy something that is premade from Futaba or someone else who supplies a tach for the hobby industry and adapt it.

The 2907 WILL work if YOU taket he time to learn what's going on, scale it properly and build the circuit. However, it would seem that your apparent abilities and/or lack of interest in learning how to use the IC relegate you to being more readily satisfied by option 3 above than by the other available options which have been cited so far.

[Smurf208]

I've already spent hours searching site after site on google. I simply can't find anything that shows or teaches me how to come up with any form of a device to build a tachometer. Thats the reason i found/joined this site. I found hardly any sites that even deal with tachometers and 90 percent of those sites are tachometers for automobiles which don't exeed 20,000rpm. I went to other peoples jet engine sites to see if they publish anything about there tachometers and so far all i've learnt is the great majority of people are using optical sensors but none of them share the electronics there using to translate the optical sensor into a form of reading a rpm. Now the freq. to volt. is a pretty good idea, i have a volt meter at home but the info they give me isn't straight. They don't give me a straight answer or anything thats clear on how to set up the system for rpm reading. They have lots of info but hardly any of it applies to tachometer and the info that is on tach is a set-up for automobile tachometers. So i've bin having too much problems already with this freq-to-volt rpm way. I'm still open to it if someone is able to simplify it but in the mean time i'm gunna try and focus my time on a optical sensor. So with that said does any1 have or know of information for building a optical sensor system/circuit? Any suggestions on other methods for rpm managment??? And yes i did search google on optical sensor rpm systems and found nothing, thats why i'm coming to you guys for help and thats why i joined this rare community with very expirenced opinionated website.

[NC Cams]

No matter how you cut it, you have two problems that need to be addressed:

1. Generating a non-contacting signal to use to determine the speed of the shaft that you're trying to determine the RPM of

and

2. Converting that signal into a "number" that is recognizeable as an RPM.

I'll leave you to your own devices on how to solve the problem listed in #1 above.

A simple "tachometer circuits" search on Google found over 1 million hits of which the following were on the very first page or two (and, strangely enough, the 2907 was part of the more obvious solutions that showed up) besides these:

www.discovercircuits.com/T/tachometers.htm

www.hobbyprojects.com/circuits2.html

www.aaroncake.net/circuits/tach.htm

http://www.niksula.hut.fi/~mdobruck/...cho/tacho.html

www.shumatech.com/dro-350/add-ons.htm

www.vermontficks.org/dtachd.htm

Check them out and see if something can be adapted.

Unfortunately, you're probably NOT going to find a "bolt it together this way" circuit that will directly interface to/with a jet engine, especially one that is buzzing along at 120K rpm.

Thus, you'll have to use some creativity and some math so as to adapt an existing circuit to suit your needs. This is part of the charm of DIY projects - namely, learning to do NEW things with NEW technologies.

For example, 120,000 rpm is a speed of 2000 hz - pretty slow in digital technology speak. Even if you come up with 2 pulses per rev, you're only talking 4000hz.

Thus, you could take your input frequency from you motion sensor (optical or whatever) and divide it before hand to facilitate scaling as well as to 'square up' the signal to make it more stable and easy to subsequently process. Some 4000 series CMOS IC's might work fine in this regard.

Since this is not an electronics course, I won't go into the specifics but this can be done (dividing and "squaring up" the input signal) a number of ways, For example, you can have your sensor trigger a 555 'one shot' timer that then feeds the the signal to a flip-flop divide by 2 circuit which then feeds the signal to a Schmidt trigger. Maybe even an optical sensor that outputs a CMOS or TTL level signal might be lurking out there.

By dividing of the raw signal would produce a lower frequency that would be easier to sense and subsequently process via digital methods or via the F to V converter.

Using the 2907 as the example, I'd first consider feeding it with 24 vdc - it will run on that voltage. I'd then use the formulae in the 2907 application note for a "minimum parts count tachometer" (it is in the app note, look for it)and program it to generate 12vdc at 120,000 rpm or 2000 hz for a single pulse per rev signal. The app note should tell you how to do that.

If you use 2 pulses per rev raw signal, program it to generate 12vdc at a 4000 hz output (two pulses per rev signal).

If this is still too fast for the 2907, take a 2000hz signal (one pulse/rev) and divide it by 2 with a flip flop to give you 1000 hz signal - or divide by some other TTL or CMOS divider IC and create whatever pulse speed that the 2907 can handle.

These 2K and 4K frequencies are still a bit quick perhaps for the 2907 but there are CMOS chips that will divide a signal by 2, 4, 10 whatever. You simply scale the raw counts down to a factor that you can work with and then do some filtering and compromising as needed to affect the F to V conversion that you want/need.

THis is exactly what the users of the 2907 do when they adapt it for use with a 4, 6 8 or 12 cyl engine.

Or, you just might try spending 2-3 days learning/working with the formulae in the 2907 application notel. I did exactly this to create a tach for a 60,000 rpm R/C car motor. Coincidentally, it had a raw pulse from the flywheel of 1 and then 2 pulses per rev (which coincidentally is 1 pulse per rev of what you proposed to do with a single magnet trigger).

You can then spend another week or two breadboarding and playing with values to get the damn thing to work at that ungodly speed. With the right experimentation, you too can learn how to take something that wasn't meant to work at that speed and make it do exactly what you want/need it to to. It is doable if you want to take the time to experiment and learn.

Yes, the folks on this "experienced opinionated website" are known to provide GUIDANCE on how to solve problems. They also will readily point to locations where solutions can be found directly or ones that can be adapted. They even provide hints to somewhat obscure sollutions.

However, there are those of us who will NOT "simplify it" even further by doing your DIY calculation problems for you. That's what calculators and breadboards are used for.

[gv71]

take a look at http://www.5bears.com/gt.htm


Greg