Hall-effect RPM sensor for Jeti telemetry

[cbdane]

There are a number of cases in which you might want to use a standard magnetic sensor on your heli to measure RPM such as on a nitro or turbine powered helicopter. The only RPM solutions presently offered by JETI are for a brushless motor or use an optical sensor. At Kiba's suggestion, I installed an optical sensor on my turbine heli but was never quite happy since it often dropped out, either due to the sensor being blinded by the sun or just insufficient light modulation on the sensor. My nitro heli still has a flybar (sometimes vintage is fun) and the optical sensor doesn't work well at all since it occasionally "sees" the flybar (depending on lighting direction) causing high RPM spikes in the data.

I decided that I'd like a better solution therefore figured out how to adapt the very common Align Hall-effect magnet sensor to my turbine. First, I had to figure out what the sensor was. After a little research, I found that it uses a Melexis US5881 switch. A quick look at the datasheet (http://www.melexis.com/prodfiles/000...881_rev008.pdf) showed how it worked. It has an open drain output driver so it behaves like a transistor switch; The output pin is "shorted" to ground in the presence of the correct magnetic field. The use of a current limiting resistor (I chose 10kohms) provides a pulsed signal each time the magnet passes the sensor. Here's a photo of the Align sensor; relatively cheap and plentiful and it has a nice mounting bracket built into the circuit board.



The JETI brushless RPM sensor is an excellent candidate for this job since the firmware supports gear ratios (the optical sensor does not; just blade count) and by setting the number of motor poles to 2 (one pulse per revolution), it should work for a properly conditioned signal from the Align RPM sensor. Using a pulse generator on the bench, I found that the best results were obtained by applying the 0-5V (or whatever your receiver voltage is) pulsed signal to the black input wire and positive DC voltage to the red wire. Here is the hook-up schematic and this is the money shot. All of the new connections are shown with dashed lines. Really quite simple and easy to set up!



It is VERY important to note that the order of the Hall-effect sensor's pins have positive voltage on Pin 1 and ground on Pin 2 (the center pin). You need to make sure that the red wire from the Align sensor goes to ground and the black wire goes to positive voltage for it to work correctly. Rather than wire the sensor straight to the MRPM-AC, I used an inline servo plug so I just needed to make sure that the black and red wires were reversed in this plug as shown in the photo below.



Here's the Align sensor mounted below the fan. In a standard way, the fan has two magnets (for balance) but they are glued in with opposite polarity (north/south) so the Hall-effect sensor only counts one per revolution. Of course, you could use both magnets pointing the same way with the JETI MRPM-AC as long as you change the number of motor poles from 2 to 4 in the JetiBox setup menu.



If you want to use simple external electrical connections as shown in the schematic above, you can stop reading here. Carefully make the connections as shown and you'll be ready to go. Make sure that you set the number of poles to 2 (using a JetiBox or your transmitter) and enter the correct gear ratio. For my turbine which uses a 124T main gear (MA Whiplash) and a 16T pinion, the ratio is 7.8. You can display the RPM, record the RPM telemetry, and set alarms or actions based on the measured value.

I wanted to neaten up the wiring approach a bit so I decided to do more direct wiring to the JETI printed circuit board, including attaching the 10k resistor directly to the board. First, I carefully removed the existing shrink wrap around the MRPM-AC and de-soldered the two wire input lead (red/black). I connected the plus/minus voltage for the Align sensor to the back of the board as shown in the photo below. I've seen better soldering technique as I'm sure you also have but keep in mind that the board is quite small and the insulation on the supplied wire easily melts; you have to move quickly.



On the other side of the board, you'll see the sensor connections where the original black and red wires were attached. I should have taken a photo before I started but I show the original wiring alongside the new for comparison. If you look closely, you'll see the 10k surface mount resistor (labeled 103) that I added straight to the board between the two solder pads. Remember that you could have used a resistor with conventional leads external to the circuit without ever opening the shrinkwrap; this is just another approach.



Note that I chose to supply the positive voltage to the MRPM-AC input with a second red wire from the plug that I installed rather than bringing it around from the other side of the board. The wires are quite small and it worked very well to double-crimp the two red wires into the same servo plug pin. You can see this in the photo of the completed circuit below. Of course the plug on the left plugs into the JETI receiver (or expander) and that on the right plugs into the Align RPM sensor.



To conclude, here's a couple of photos of the complete installed system. Note that in an effort to be clever, I applied the MRPM-AC sticker back on the unit after shrink wrapping but accidentally stuck it on backwards. Oh well; at least it still reminds me what it is.



Here's a final standback shot showing the sensor, the MRPM-AC, and where it plugs into my 2-channel telemetry expander. If you're interested in learning more about this heli, info can be found here: https://www.helifreak.com/showthread.php?t=598675



I hope that this is helpful to someone wanting to add RPM telemetry to your nitro or turbine-powered helicopter! Let me know if something doesn't make sense or if you have any questions.

[buttles]

Many thanks for this. I can now take down the flag pole my Opto sensor is sat on!!

[Puttputt maru]

Very nice thinking outside of the box.

I have to get a turbine helicopter one of these days, they look so cool.

[Kiba]

Thanks Brent! Very straightforward and simple conversion.

The most difficult part of the installation on my Magnum 44 is going to be deciding where to locate the sensor and fit the magnets, especially since the output shaft on the turbine only has the hub and lacks the fan.

[cbdane]

Originally Posted by Kiba

Thanks Brent! Very straightforward and simple conversion.

The most difficult part of the installation on my Magnum 44 is going to be deciding where to locate the sensor and fit the magnets, especially since the output shaft on the turbine only has the hub and lacks the fan.

Check one of Cletus' posts over in the Bergen forum Troy. He mounted a small cylinder with magnets imbedded onto the shaft on the top of the heli were some folks install the Jewel. https://www.helifreak.com/showthread.php?p=2302216

[Kiba]

Originally Posted by cbdane

Check one of Cletus' posts over in the Bergen forum Troy. He mounted a small cylinder with magnets imbedded onto the shaft on the top of the heli were some folks install the Jewel. https://www.helifreak.com/showthread.php?p=2302216

Thanks Brent, the picture is nearly the exact idea I came up with while staring at the heli last night-- the shaft protruding above the clutch block is a perfect spot.

I was also considering mounting the magnets on the main gear or using a collar similar to Cletus' on the main shaft to capture only true rotor RPM; mounting the magnets to the stub of the clutch shaft will obviously show the second stage idling RPM before the clutch engages but that's not really an issue once the clutch is engaged and the heli is in the air.

Guess I need to get some parts on order...

[Laurens]

Thanks for sharing!

[rameshtahlan]

So actually i could use the Jeti RPM sensor for brushless motors on my 50cc gas plane to measure the RPM.

The hub already has a magnet, so i could screw on the Align Hall Sensor to the body with the sensor about 1 mm above the magnet, This magnet is used for Ignition Unit. I really don't have to drill a hole and add another magnet.

Is my understanding correct. I already have the Jeti sensor, and all i have to do is order the Align Sensor.

[TeroS]

Originally Posted by rameshtahlan

So actually i could use the Jeti RPM sensor for brushless motors on my 50cc gas plane to measure the RPM.

The hub already has a magnet, so i could screw on the Align Hall Sensor to the body with the sensor about 1 mm above the magnet, This magnet is used for Ignition Unit. I really don't have to drill a hole and add another magnet.

Is my understanding correct. I already have the Jeti sensor, and all i have to do is order the Align Sensor.

Yes. I did this on my Logo once, worked really good.

[cbdane]

Originally Posted by rameshtahlan

So actually i could use the Jeti RPM sensor for brushless motors on my 50cc gas plane to measure the RPM.

The hub already has a magnet, so i could screw on the Align Hall Sensor to the body with the sensor about 1 mm above the magnet, This magnet is used for Ignition Unit. I really don't have to drill a hole and add another magnet.

Is my understanding correct. I already have the Jeti sensor, and all i have to do is order the Align Sensor.

Yep; what Tero said!

But, there may be an even simpler way. On my (one and only) gasser (Aeroworks 30cc Bravata), I just tied the DLE digital RPM signal output from the ignition module (they call it the tachometer lead) straight into the Jeti RPM unit. It's already a signal-conditioned 5V signal. If your ignition system has one of these outputs then you're done! Works beautifully.

[Puttputt maru]

Originally Posted by cbdane

Yep; what Tero said!

But, there may be an even simpler way. On my (one and only) gasser (Aeroworks 30cc Bravata), I just tied the DLE digital RPM signal output from the ignition module (they call it the tachometer lead) straight into the Jeti RPM unit. It's already a signal-conditioned 5V signal. If your ignition system has one of these outputs then you're done! Works beautifully.

Yep; what cbdane said!

You might have to play with the wiring to get it to work. This diagram might help:

https://www.rcgroups.com/forums/show...ostcount=13443

[cbdane]

Originally Posted by Puttputt maru

Yep; what cbdane said!

You might have to play with the wiring to get it to work. This diagram might help:

https://www.rcgroups.com/forums/show...ostcount=13443

Nice! Good that Jeti is reading my posts. But I knew that already after the release of the MBar!

[rameshtahlan]

Originally Posted by cbdane

There are a number of cases in which you might want to use a standard magnetic sensor on your heli to measure RPM such as on a nitro or turbine powered helicopter. The only RPM solutions presently offered by JETI are for a brushless motor or use an optical sensor. At Kiba's suggestion, I installed an optical sensor on my turbine heli but was never quite happy since it often dropped out, either due to the sensor being blinded by the sun or just insufficient light modulation on the sensor. My nitro heli still has a flybar (sometimes vintage is fun) and the optical sensor doesn't work well at all since it occasionally "sees" the flybar (depending on lighting direction) causing high RPM spikes in the data.

I decided that I'd like a better solution therefore figured out how to adapt the very common Align Hall-effect magnet sensor to my turbine. First, I had to figure out what the sensor was. After a little research, I found that it uses a Melexis US5881 switch. A quick look at the datasheet (http://www.melexis.com/prodfiles/000...881_rev008.pdf) showed how it worked. It has an open drain output driver so it behaves like a transistor switch; The output pin is "shorted" to ground in the presence of the correct magnetic field. The use of a current limiting resistor (I chose 10kohms) provides a pulsed signal each time the magnet passes the sensor. Here's a photo of the Align sensor; relatively cheap and plentiful and it has a nice mounting bracket built into the circuit board.



The JETI brushless RPM sensor is an excellent candidate for this job since the firmware supports gear ratios (the optical sensor does not; just blade count) and by setting the number of motor poles to 2 (one pulse per revolution), it should work for a properly conditioned signal from the Align RPM sensor. Using a pulse generator on the bench, I found that the best results were obtained by applying the 0-5V (or whatever your receiver voltage is) pulsed signal to the black input wire and positive DC voltage to the red wire. Here is the hook-up schematic and this is the money shot. All of the new connections are shown with dashed lines. Really quite simple and easy to set up!



It is VERY important to note that the order of the Hall-effect sensor's pins have positive voltage on Pin 1 and ground on Pin 2 (the center pin). You need to make sure that the red wire from the Align sensor goes to ground and the black wire goes to positive voltage for it to work correctly. Rather than wire the sensor straight to the MRPM-AC, I used an inline servo plug so I just needed to make sure that the black and red wires were reversed in this plug as shown in the photo below.



Here's the Align sensor mounted below the fan. In a standard way, the fan has two magnets (for balance) but they are glued in with opposite polarity (north/south) so the Hall-effect sensor only counts one per revolution. Of course, you could use both magnets pointing the same way with the JETI MRPM-AC as long as you change the number of motor poles from 2 to 4 in the JetiBox setup menu.



If you want to use simple external electrical connections as shown in the schematic above, you can stop reading here. Carefully make the connections as shown and you'll be ready to go. Make sure that you set the number of poles to 2 (using a JetiBox or your transmitter) and enter the correct gear ratio. For my turbine which uses a 124T main gear (MA Whiplash) and a 16T pinion, the ratio is 7.8. You can display the RPM, record the RPM telemetry, and set alarms or actions based on the measured value.

I wanted to neaten up the wiring approach a bit so I decided to do more direct wiring to the JETI printed circuit board, including attaching the 10k resistor directly to the board. First, I carefully removed the existing shrink wrap around the MRPM-AC and de-soldered the two wire input lead (red/black). I connected the plus/minus voltage for the Align sensor to the back of the board as shown in the photo below. I've seen better soldering technique as I'm sure you also have but keep in mind that the board is quite small and the insulation on the supplied wire easily melts; you have to move quickly.



On the other side of the board, you'll see the sensor connections where the original black and red wires were attached. I should have taken a photo before I started but I show the original wiring alongside the new for comparison. If you look closely, you'll see the 10k surface mount resistor (labeled 103) that I added straight to the board between the two solder pads. Remember that you could have used a resistor with conventional leads external to the circuit without ever opening the shrinkwrap; this is just another approach.



Note that I chose to supply the positive voltage to the MRPM-AC input with a second red wire from the plug that I installed rather than bringing it around from the other side of the board. The wires are quite small and it worked very well to double-crimp the two red wires into the same servo plug pin. You can see this in the photo of the completed circuit below. Of course the plug on the left plugs into the JETI receiver (or expander) and that on the right plugs into the Align RPM sensor.



To conclude, here's a couple of photos of the complete installed system. Note that in an effort to be clever, I applied the MRPM-AC sticker back on the unit after shrink wrapping but accidentally stuck it on backwards. Oh well; at least it still reminds me what it is.



Here's a final standback shot showing the sensor, the MRPM-AC, and where it plugs into my 2-channel telemetry expander. If you're interested in learning more about this heli, info can be found here: https://www.helifreak.com/showthread.php?t=598675



I hope that this is helpful to someone wanting to add RPM telemetry to your nitro or turbine-powered helicopter! Let me know if something doesn't make sense or if you have any questions.

Hi,

I just opened up the Jeti MRPM AC EX, at it seems that the Red and Black wires on the right side are reversed in my MRPM.
So what should I do.
Follow your colour code, OR, use the colour code on my MRPM.
In your image, the bottom is Black,
and in my image bottom is Red.

Also, I got 2 magnets with my Hall Sensor.
Actually i will be using it on my 50cc gas engine, which already has 1 magnet. So do I need to install one Magnet with opposite polarity to the one already installed on the opposite side
OR will the Hall Sensor work with just the 1 installed on the gas engine

Regards.

[rameshtahlan]

So finally i followed the colour coding of the wires that you have actually soldered.

Today i installed on my Gas engine, Evolution 33cc.
Worked perfect.

I attached the sensor to the engine body, and used the same magnet that is used for Ignition.

Many thanks for the info and how to do it.

Will have to do same for my 50cc gas ending also.

[cbdane]

Originally Posted by rameshtahlan

So finally i followed the colour coding of the wires that you have actually soldered.

Today i installed on my Gas engine, Evolution 33cc.
Worked perfect.

I attached the sensor to the engine body, and used the same magnet that is used for Ignition.

Many thanks for the info and how to do it.

Will have to do same for my 50cc gas ending also.

Oh my. Sorry I missed your original post but I'm very glad it worked well for you!

[Drumski]

Hi! I am new here.

I would like to made to make a RPM sensor for the glow fuel engine that works with FrSky Tandem X20S transimitter and telemetry system of the FrSky.

I can't find it ready-made anywhere, so I have to do it yourself. I found a promising wiring diagram here! But I don't fully understand it, plus I should figure out how to implement it with the FrSky Smart Port RPM and Temperature Sensor.

Could someone advise in more detail how to implement that wiring when using the Melexis US5881 Hall-effect switch and the FrSky Smart Port RPM and Temperature Sensor? I made the attached picture. Could someone explain to which pin I connect what?

Melexis US5881 Hall-effect switch have three pins:
1 = VDD (supply voltage)
2 = GND (ground pin)
3 = Out (output, open drain)

FrSky Smart Port RPM and Temperature Sensor have these pins:
Smart Port S + –
A and B

Take a look at the drawing I edited, where I have clarified things. Link to the picture that is in my Dropbox:
https://www.dropbox.com/s/x1bwnig03z...by_ED.jpg?dl=0

(Because I am new here, I can't put image to this message, sorry.)