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Old Apr 08, 2014, 05:19 PM
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Tachometer Calibrator

I was trying to find a schematic for this tach calibrator that I built about 15 years ago I believe EloyM was the designer. I was thinking of trying to remotely mounting the photo sensor for my hobbico tach so I wouldn't have to hold the tach and try to adjust engines at the same time. The Thumb Tach was built by my step dad around 1974. Amazing Heathkit stuff.
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Old Apr 08, 2014, 09:40 PM
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Heathkit Thumbtach, You simply point it at the florescent light bulbs, LED light bulbs, and maybe incandescent bulbs too (I forget now I went all green at home), then it will measure 3,600 RPMs and that is what you adjust it to, if it is off.

Your calibrator circuit is interesting too. I can see simply using a 555 Timer or a microcontroller to simulate a timer and have it drive a LED at different frequencies to test against the tachometers. of course you need a frequency meter or oscilloscope to double check the frequencies with just in case one of the tachs is off on you.
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Last edited by earlwb; Apr 08, 2014 at 09:43 PM. Reason: add more info
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Old Apr 09, 2014, 11:51 PM
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Incandescant works but not fluorescent.

I don't know about LED.

3600 is correct for 60HZ supply (USA) but if your power supply is 50 HZ (some countries) your tach will show 3000

Dave H
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Old Apr 10, 2014, 03:08 PM
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Huh, both my tachs work reading fluorescent fixtures right on at 3600. Never tried an incandescent light though I suppose it would also work. I do not understand why yours does not work with fluorescent bulbs. But it is not my problem if it can said to be a problem. Just strange to me. One tach is a Glow Be and the other is a Tower Hobbies.
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Old Apr 10, 2014, 10:30 PM
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Hmm, interesting,

Incandescent works because as the AC power fluctuates up and down the bulb glows brighter and dimmer which the tach reads as RPM.

In a fluorescent tube the electricity excites the gas inside which causes it to glow. I always get random readings when I point my tach at a fluorescent tube, but if it works for you that's great.

By incandescent I mean just an ordinary old fashioned light bulb.

Dave H
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Old Apr 12, 2014, 07:07 AM
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Here is my TNC about 4' from twin 8' fluorescent tubes. I have two Globee's and this TNC all read the same. Many accuse the Globee's of being not as accurate as the TNC but that is not the case. The fluorescent's brightness is also modulated by the AC, in an older non quick start tube you can see it plain as day as it warms up.
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Old Apr 12, 2014, 05:08 PM
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Quote:
Originally Posted by I TOBOR View Post
Here is my TNC about 4' from twin 8' fluorescent tubes.
I think that's correct ITOBOR.

The reason incandescent lights don't work, is that the filament material does not cool enough between AC cycles, so the tach sees a nearly constant glowing light source.

Fluorescent on the other hand is an excited gas that stops glowing almost instantly between AC cycles.

Remember the problems they had with industrial fluorescent lighting causing headaches, and other problems? That is like blinking your eyes at 60 cps!

Bill
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Old Apr 12, 2014, 06:09 PM
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Quote:
Originally Posted by Bill Adair View Post
I think that's correct ITOBOR.

The reason incandescent lights don't work, is that the filament material does not cool enough between AC cycles, so the tach sees a nearly constant glowing light source.

Fluorescent on the other hand is an excited gas that stops glowing almost instantly between AC cycles.

Remember the problems they had with industrial fluorescent lighting causing headaches, and other problems? That is like blinking your eyes at 60 cps!

Bill
That is like blinking your eyes at 120 cps

TF
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Old Apr 12, 2014, 07:37 PM
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When I first read about this technique (mid 80's IRC) the article said incandescent not fluorescent. My experience at the time confirmed that. Incandescent worked fine but fluorescent tubes gave a random reading.

Don't know what's changed since then, maybe Southern Hemisphere lights are different? Clearly fluorescent is working for you guys.

I haven't tried it in ages so I guess I'll be pointing my tach at the lights around my house when I get home. I've mostly got Eco bulbs now but maybe I can find an incandescent somewhere.

The point stands though, you can calibrate an optical tach against your local AC frequency.

Dave H
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Old Apr 13, 2014, 10:57 AM
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I think it has to do with certain types of fluorescent lighting. it may have to do with what types of ballast they use too, as certain lights flicker more with certain types of ballast. I remember with robotics how certain lights with certain light color spectrums would overload or jam or confuse the light detector sensors in use at the time. Then I do also remember how certain types would cause headaches and eyestrain with people too.
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Old Apr 13, 2014, 12:51 PM
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A low wattage incandescent will work better than a high wattage bulb as the filament in a high wattage bulb is thicker and doesn't cool as much between the AC pulses. Also optical tachs of different makes vary in the sensitivity to variations in brightness so some will work with incandescents and some will not.
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Old Apr 13, 2014, 01:04 PM
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my old royal digital tach reads just about everything: incandescent, fluorescent, computer monitors, TV.

have not tried it with the new style of screw in fluorescent bulb. pretty sure those operate at a much higher frequency than mains.
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Old Apr 13, 2014, 05:47 PM
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I have an old Ace TachMaster and there are two calibration marks (one for 0 - 25,000 scale and one for 0 - 5,000 scale) for fluorescent lights. Now my problem is I can point it at a light and the calibration in correct, however, when I point it at a prop all I get it 3,500 on a prop that I know is in the 6 - 7 k range. 2 blade prop and I even changed the battery. Not sure what is going on.............. Even tried a different engine and got the same 3, 500. I'm confused.........

John
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Old Apr 13, 2014, 07:58 PM
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Quote:
Originally Posted by Rubbernecker View Post
I have an old Ace TachMaster and there are two calibration marks (one for 0 - 25,000 scale and one for 0 - 5,000 scale) for fluorescent lights. Now my problem is I can point it at a light and the calibration in correct, however, when I point it at a prop all I get it 3,500 on a prop that I know is in the 6 - 7 k range. 2 blade prop and I even changed the battery. Not sure what is going on.............. Even tried a different engine and got the same 3, 500. I'm confused.........

John
Make sure you are not confusing the tach by trying to read engine rpm in an area where there is artificial lighting.
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Old Apr 13, 2014, 08:55 PM
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Also most all of the optical tachometers work in broad daylight. They normally don't work in the shade or during a heavily overcast day either.
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