# Combine grain loss sensor questions

#### rhawley41

Joined Jul 12, 2023
10
Hello,

I am a farmer not an electrical engineer, but I have recently been trying to learn more about the electronics we use in farming. I have been trying to troubleshoot the grain loss system on our combine that hasn't worked in years. Keeping in mind that I am a farmer, as I understand it the sensors are just a piezo sensor glued to a piece of plastic over a piece of foam that isolates the sensor from the vibration of the machine (first picture). Any grain striking the plastic will send a signal to a monitor in the cab telling me that the machine is overloaded and dumping grain onto the ground with the chaff. There are four sensors on the machine @ $800 a piece. Could we fix these sensors for around ten bucks? The resistors seem to have tested as good. The pair on the bottom left (second picture) @ 46k ohm, top center @ 22k ohm, and pair on bottom right @ 1k ohm. The number on the transistor reads 2N 4401 422. I found a clip on YouTube on how to test small transistors using diode mode, and it showed a reading from base to emitter and from base to collector. I'm assuming it is good, but as I said, I am a farmer. Now the problem, I could not get any reading from the piezo or capacitor. As I understand (again from YouTube), a good piezo should flash a voltage reading when tapped. I could not get any reading. And a good capacitor should build a charge (?) in diode mode or build resistance when ohmed. I could not get either reading. The number on the capacitor is 473K 100N. Now my questions: 1) The piezo and capacitor are wired directly in series and both seem to be bad. Would one going bad cook the other? This sensor is original off a 1998 machine, so it is old. 2) The capacitor number is 473K 100N. I read that the top number on a small capacitor is usually the capacitance in Farad and the lower number is the maximum voltage it can handle. So what's with the N? Is it in Newtons? The closest I could find on Amazon is a 473K 250V metallized film capacitor; would that work? I forgot to mention that the power to the sensor is 8V. 3) Putting quality of manufacture aside, is a piezo a piezo a piezo? I couldn't find any product that suggested a difference in the voltage put off by a sensor when struck, but none look like the one I have on the sensor. Would any generic piezo do? This machine is old and the components are getting harder to find. Many sensors are getting ridiculously expensive. Do you think this is possible to fix? Maybe I'll have to bite the bullet and drop the$3200? Thank you all in advance for your help and insight. Thank you. Thank you.

#### LowQCab

Joined Nov 6, 2012
3,568
A Piezo-Element produces a tiny, very high-frequency, Voltage-Spike when something hits it.
It will also operate as a crude Microphone.

Placing a Piezo-Element in an environment where it will be subjected to
continuous bombardment by fast moving particles is a guaranteed way to destroy it.

Your Circuit-Board appears to have been subjected to a moist and dirty environment,
it's amazing that it's still reasonably intact.
The Circuit-Board is simple enough to be "reverse-engineered",
and a duplicate made, but the hours required to do this will come with a price-tag,
which could easily be over ~20-times the cost of the actual new components.

Piezo-Elements,
which could probably directly replace the existing ones,
but will probably have a short lifetime in a harsh environment.

Since Piezo-Elements are cheap,
it might be worthwhile to simply try replacing the existing ones with new,
but there are no guarantees that the "system" will perform as expected afterwards.

In my opinion, this is a stupid design,
and it has very questionable usefulness/accuracy,
but on the other hand,
maybe it's the only solution the Engineers could come up with.

Sorry that I can't be more helpful.
.
.
.

#### debe

Joined Sep 21, 2010
1,388
Here in Australia these grain loss systems have been around for many years, they work as far as i know with a filter that differentiates between the sound of grain & straw landing on the sensor. The unit on my family's farm harvester lasted the life of the harvester. The only problem i recall was the od cable connection fail due to vibration.

#### debe

Joined Sep 21, 2010
1,388
The voltage generated by a piezo device that size would be in micro volts when tapped. Tor testing i would use an oscilloscope to test its output. I doubt that capacitor is faulty as they are pretty reliable & it wont test using ohms scale unless it is short circuited internally. The ohms scale test on a capacitor is really only relevant on electrolytic capacitors.

#### MrChips

Joined Oct 2, 2009
29,809
Welcome to AAC!

What you have described appears to be simple enough for most of the experienced members here on AAC.

#### BobTPH

Joined Jun 5, 2013
8,073
Good job in the schematic!

I think you have the transistor upside down, recheck the pinout and connections. I think the 2n4401 is American style pinout which is EBC looking at the flat side with the leads pointing down.

The circuit is a simple high gain AC voltage amplifier with likely gain in the 100 to 200 region.

#### Dodgydave

Joined Jun 22, 2012
11,148
Here's what I came up with. If it doesn't make sense (I'm having to learn all this as I go) I can try to just trace it out exactly as it is on the board; it would just be slightly more convoluted.

Ya'akov: I'm a big fan of the idea of right to repair. That's why I'm trying to learn how to troubleshoot these problems. Seems a shame to spend hundreds on a new component and throw something in a junkyard that may just have a bad trace, blown resistor, etc. If something isn't working there has to be a reason that can be remedied if I can learn to find it. I have another \$400 sensor off a sprayer in the shed that would normally be thrown out. I still have a long way to go before I start making design improvements lol. A Massey Ferguson 8780 also isn't an especially common combine which might be part of why the parts are getting expensive.
View attachment 298264
Transistor leads are reversed C, E .
Everything else is right..

#### MisterBill2

Joined Jan 23, 2018
16,493
Checking a piezo element is not so complicated if you happen to have a high impedance headphone and good hearing. The headphones of an old crystal radio would be a perfect choice, even a crystal earplug phone would work. Connect the phone across the piezo sensor and tap the panel with a key or similar hard object. If you hear a sound in the phone then the piezo is working. Like others have stated, the wiring is the critical part because the signal is small. And shielded cable is easy to damage.

Several of my ancestors were farmers, some of them also brilliant designers of things to make farming more productive and less hard work.
The development of a grain loss sensor is brilliant, but it post-dates my last experiences with a threshing machine. (1964) WORKING ON MY UNCLES FARM.

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