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Piezoelectric sensors, Fm trancievers and more.

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Wigo:
Hello Community,

I am fairly new to electronics. I started my learning expierience 4 months ago. And while i am making good progress, the information on the internet is just too much to choose the right spots. Even my google skliis have forsaken me as i am out of ideas of what terms to search for. So i hope you can help me in designing my first own project and bringing it to life  :scared:

So what do i want to design?
Good you ask. Basically i want to meassure inpatcs on wooden tragets and transmit the "data" to a base station where it will be stored and displayed.

So what is the purpouse of this "gadget"?
Well in short it should save me time. If i can meassure the inpacts (or better said the number of inpacts) on the targets and transmit it back i can save up to 2hrs.
The maximum range for the transmitting would be up to 1km but with line of sight.

What have i achieved till this point?
Ok this will need a little bit of room to outline this all.


* I came up with the idea to use 2,57cm piezoelectric disks (or better said guitar pickups) to sense the inpact. As i have no idea how much of the inpact energy will be transferred i needed some way of  setting the sensitivity for the meassurement.
* Some googling later i tried to use the piezo directly on the analog inputs of an Arduino (mit a 1M resistor in parallell) and set the sensitivity in the code. This got me the first hint that i need something different. The signal was too small to be meassured if the inpact was more than a few mm away from the sensor.
* ok my next thought was that i need some kind of amplifier. Said and done i found a little tutorial ( http://davidhoulding.blogspot.com/2014/02/high-sensitivity-vibration-sensor-using.html?m=1 ) online that uses a LM358 as amplifier and comparator to puls a logic high signal if vibrations are sensed by the piezo and the sensitivity can be set via a 100k potentiometer.
* through Google i found that i can use LoRa to transmit the data from my Nodes to the Base.
* All Sensors and Nodes should be powered by battery and should work for 48 hrs. But i have not had the time to think about power consumption jet
Ok now to the guts of my problem(s)/questions.


* As i have up to 50m between the sensores and my transmitting nodes i had the idea to establish an Fm connection between teh sensors and my nodes. But is it possible to toggle a Fm signal without a microcontroller? The easiest way i can think of achieving this would be some sort of Fm relay that toggles to logic high when it gets a Fm puls from the sensor. The Node will be a Micro to handle the LoRa transmission with the base.
* As following tutorials is nice and easy but it does not have a greast value in understanding why and how this things work i want to design my own amplifier circurit. But i have no clue where to begin with looking for an OpAmp or better said i do not know exactly what the OpAmp should be capable of. Maybe some one can give me a hint on what to look after?
I hope someone can give me a little hint. Even tipps on what to google after are highly appreciated.

The first picture is of a simplified positioning of the Sensors and other things. Green are the sensors, Red is the Node consiting of a Microcontroler and the LoRa module and Red will be the Basestation made of a Microcontroler and LoRa module.

The second sketch is of the dimensions of the target and the position of the sensors.

Best regards,
Chrsitian

Wigo:
Ok i made a few steps forward.

I kept the amplifier circuit based on the LM358 and made a successfull test with real life conditions. It works nice but it is a little bit too sensible. Good that i can adjust the sensitivity.

As this is working i moved on to the next part. The Fm transmition of the signal. I found cheap 433mhz receivers and transmitters. And as i have to have a way to tell the sensors appart i will add HT12E/D pairs to the transmitters and the reciever. Now i can set an individual adress for each sensor and i can tell them appart within the microcontroller on the receiving end.

Attached is the schematic for the sensor and transmitter circuit. Maybe someone can have a look at it. i am sure there are mistakes in it that i have not spotted yet. Or maybe there is room for improvement?
Please let me know if so.

Best regards
Christian

jeremy:
LoRa should easily do what you need, how much data do you need to transmit and how often? It should be pretty easy to get 48h of battery life.

profdc9:
A piezoelectric element has a high impedance.  You may get better results using a JFET op-amp which has a high input impedance such as the TL081/TL082, especially at high frequency.  The TL082 is a drop-in replacement, more-or-less, for the LM358.  I typically use JFET op-amps on piezo disc guitar pickups and the like.

ThomasDK:

--- Quote from: profdc9 on May 27, 2019, 03:12:56 pm ---The TL082 is a drop-in replacement, more-or-less, for the LM358.

--- End quote ---
Well, mostly 'less' in this case. A TL082 will NOT work in this circuit.

LM358 has common mode range to ground, while TL082 is 3V to VDD. In other words, the piezo needs to output at least 3V before the TL082 'sees' it.

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