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Mr.RC-Cam

Tachometer Interface

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I have decided to pull my old Telemetry project out of hibernation and completely revamp it as a video overlay instead of the RF com method I was using. One of the little things that I am waffling on is the tach input. Should I go Optical or Magnetic?

The magnetic method is a clear choice for heli apps and was what I had used. But long ago I moved on to electric planes. Adding magnets to the small props is not practical.

I am interested in keeping it simple. It would be implemented like a handheld tach -- natural light will illuminate the prop and a photo transistor will act as the sensor. Are there any commercial R/C telemetry systems that are doing this? If so, are they 100% reliable under all daytime lighting conditions without much fuss?

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I have decided to pull my old Telemetry project out of hibernation and completely revamp it as a video overlay instead of the RF com method I was using.

Will this be incorporated into the MAHI project?

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The MAHI technology was considered. But after looking at all that I want to do, I have decided to use the STV5730A OSD chip. That is if I can find some!

BTW, I will setup the screen display so that the MAHI graphics will integrate with it.

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Sounds interesting, I like the optical idea for simplicity. I was thinking of making an overlay with tach, airspeed, alt and batt volts but gave up because the board from Blackbox was a bit big. I would be interested if you came up with something like that.

Terry

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I stumbled across a laser tack in a surplus magazine recently and it got me thinking.... is there a reason you couldn't go with a bounced light idea? You could run a high frequency carrier on the transmitting LED to help filter out ambient light, and then the solution for your heli would be the same as a regular prop plane... a small splotch of white paint or a small sliver of silver muffler tape.

Regards,

Bill

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Yes, those laser tachs are nice gadgets. Whether it is laser, IR, or white light, the modulated beam method is great for indoor use since those tachs still will work with the "optical noise" from overhead shop lights. Since my app is all outdoors, I would like to avoid the extra circuitry.

But the question is, can I Muntz this, like I have in mind, and expect it to work well? It would be great to hear that other R/C telemetry projects did it the cheap way and had good success.

What does the old Sky-Spy and the new R/C telemetry systems use?

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I evaluated one once that used, of all things, a cadmium sulfide light sensor. It wasn't particularly accurate however... it was easily overwhelmed by the rapidly changing light conditions of 3D flight ;)

Regards,

Bill

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That is not good news for me. A CDS cell solution is not much different than my proposed phototransistor method. Thinking about all the lighting conditons that would need to be met, I have a sinking feeling that my idea will not work well.

BTW, I found some STV5730A video on-screen display IC's. So that headache appears to be solved.

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The TL100 uses a Hall sensor. Personally I would go for it, the needed circuitry will be easier than with the optical solution. No need for modulation etc. Just either count the pulses in a defined amount of time, or measure the time between 2 pulses if you need an extremely fast refresh rate. Of course you have to mount a magnet or 2 (for balance) somewhere, but if you can find the same ones that come with the TL100 (around 2mm diameter, 1mm thick), it won't be a problem.

I've used one of these old photoresistor handheld tachs, outdoors il really doesn't like light. You sometimes have to move a bit, or even go to the other side of the prop to have it working.

I know that for model jet engines they don't use any optical solution anymore in the new designs, because of the risk of ambient light disturbing the system. Even if the engine is oftn mointed inside the plane where it's dark...

But Hall sensors are cheap, and some have a digital output which will go high when the mag. field is above a certain value, else low. With such a thing you need no additional hardware than a free input pin on your microcontroller. If you choose a standard one you'll maybe need one transistor ;)

If you use brushless outrunner motors, you could maybe even consider eliminating the magnets and measure the field near the case, not sure about this but one might be able to see fluctuations even outside.

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Just thought I would post a follow up to this. I created an interface that connects directly to the brushless motor contoller (I'm using a Phoenix 25). I can measure the field commutation, which gives very accurate motor RPM data. I realize that with a gear drive the prop's RPM may differ, but accurate Motor RPM is just fine.

So, I can skip gluing magnets to props and such. Very sweet.

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If you use a motor where the magnets rotate, can't you just sense the magnets through the can with a hall effect switch? You could add or subtract (or is it multiply or devide) for extra magnets or gear reduction in the software to accurately determine prop rpm?

Dan

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Where did you connect it? Gates of the power transistors?

My interface connects across two of the three motor leads (any two work fine). It is optically isolated for EMI/RFI protection.

If you use a motor where the magnets rotate, can't you just sense the magnets through the can with a hall effect switch?

I tried that first since it would have been an ideal solution. But, there is sufficient magnetic bias, at all times, to prevent the hall effect sensor from working correctly. There may be a motor/hall sensor combo out there that will work well with this, but in my case it was a dead end.

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I created an interface that connects directly to the brushless motor contoller

I was wondering when somebody would do this. Why did you find it desireable to sense two of the leads, instead of only one?

--bluegill

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One sources the anode of the interface's optocoupler, the other is the cathode return. There are no other connections to the motor.

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One sources the anode of the interface's optocoupler, the other is the cathode return. There are no other connections to the motor.

What part type are you using for the optocoupler? Is there a schematic for how this was hooked up?

David

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Any common optocoupler should work fine. I use a PS2501. Be sure to current limit the LED portion with a resistor.

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