jim-p You are absolutely correct considering the Common mode rejection ratio of the amp. I have found this: If I use a quality Op Amp and you have to deal with the Common mode rejection ration, I need to find some other solution. Plus I did not like the large resistors in the NVE diagram. I found using a instrumentation amplifier on the output worked much better than the NVE circuit. Maybe someone can show how a digital potentiometer can be controlled by Arduino to set the range of the output to a zero to 100mV? Which includes hookup diagram and code. That would help me understand how to use Arduino and digital potentiometers.
The circuit schematic I displayed is the schematic I worked with. The only difference TomGeorge is the following: I tried putting an Op Amp on the input to feed a stronger signal to the GMR. The Op amp arrangement was non inverting and Unity gain. The only other change I made to the schematic was to use standard instrumentation amplifier. The gain was controlled by one resistor on the instrumentation amplifier. I am sure you understand what a non inverting arrangement of an Op amp is. I am also sure you should know what a simple arrangement of a instrumentation amplifier is. The main schematic is (WHAT I POSTED). Above I explained about experimenting with a Op amp added to the front input, and a instrumentation amplifier added to the out output. So the entire schematic is what I posted. After trying the Op amp on the input, and the instrumentation amp on the output, I went back to the NVE circuitâthat is the schematic posted. The original NVE circuit, and any and all of my experiments ended in the same resultâmeaning I could not get the circuit to go totally off. I am still wondering if someone could post how to make the circuit go to zero with a digital pot and Arduino. Maybe you could post that TomGeorge, because I know you want to help.
Give me a couple of days and maybe I can.
Can you do surface mount part soldering?
Can you buy parts from Digi-key or Mouser?
Exactly what GMR part do you have, please provide the datasheet,
Confusing thread.
There seem to be two problems.
- volume control of some undefined sound source.
- some poorly defined human sensor to control that volume.
First one can be solved with a specialised volume control chip, like the PT2257 (eBay).
A common HX711 seems a much easier approach to process a Wheatstone-type sensor.
Leo..
Wawa, and TomGeorge: Both of you have asked what my input signal source was: Here is my signal source: BK Precision signal generator set on Sine Wave. Amplitude set at 100mV. You can buy BK Precision signal generators on the internet. So Wawa, that is my previously undefined sound source! It is defined for you now. Wawa, the poorly defined sensor is a magnet moved near the NVE Corporations GMR sensor AA005-02E. That is the GMR sensor in the circuit diagram I posted. It is a GMR sensor arranged in a Wheatstone bridge. One of the experiments I performed was putting a OPA134 Op amp on the front of the circuit diagram posted earlier. The purpose of the Op Amp put on the input was to input a stronger signal. The OPA 134 was Unity gain, and non inverting arrangement. Another experiment I did was put a AD620AD instrumentation amplifier on the output of the circuitâafter removing the Op Amp in circuit diagram posted earlier. Wawa, thanks for bringing up the PT2257, I will check that out. Nowâjim-p Yes, I can do surface mount part soldering, no problem! Yes, I buy parts all the time from Digi-Key and Mouser. I listed the exact GMR part I used above, again it was: AA005-02E. You should be able to use that part number to look up the data sheet easily on the internet. If not go to NVE and email them, and they will send you the data sheet. I hope I have answered all the questions people have, if not shoot me more questions. Now help me figure out how to use a digital pot and Arduino to make the range of my circuit zero to 100mA
One final thing: The gain resistor value I used on the AD620AD instrumentation amplifier was a 2.23K. Circuit was powered by a 9 volt battery, but could have been powered by lower voltage.
Did you already try a HX711 breakout board.
That seems much easier than fiddling with opamps.
A HX711 is designed for Wheatstone bridges, and has a 24-bit A/D that connects directly to a 5volt processor.
Leo..
mA?
The PT2257 is one of the many audio volume chips. They commonly have a log taper, which is more suitable for audio than a normal digital pot.
Most modern audio power amplifiers also have a mute pin, to completely silence it.
Maybe the tiny 2x3Watt PAM 8403 is enough for your application.
Leo..
Wawa, you have helped me more than anyone and I really appreciate it. I looked up the PT2257. I already use a Princeton Technology delay IC. A really great company with great products. I looked up the HX711, looks really interesting. Perfect for the Wheatstone bridge arrangement of the GMR AA005-02E
Wawam you are such great helpâThank You! Actually you are a bit of fresh air. I have sadly run into some jerks on the Forum in the past. I will check out your last suggestion, the 2X3 Watt PAM 8403. I have a lot of experience doing self taught analog electronics. I am pretty green doing digital and Arduino. Just learning C language on my own mostly trial and error. I was actually a high school math teacher in 1969 when I got drafted into the Marines and sent to Vietnam to kill people. When I came home as a disabled veteran no one would hire me. That is why I got into electronics. I am 100 percent self taught, and have made a living building analog electronics for many years. Should be retired, but have a strong interest in learning Arduino. I have built some simple things with Arduino. My most advanced Arduino project was a liquid crystal display, and blinking LED beats per minute controlled by a analog potentiometer. It is going to be a steep learning curve applying the PT2257 and HX711, but I can see that is the way to go. Thank you so much!
1 Like
Best to stick to the example in the datasheet of the PT2257, page 2.
Shouldn't be that hard to find an Arduino library with examples for that online.
I2C, so connects to Pin A4, A5 of the Uno R3.
The PAM is a bridge-mode digital (Class-D) amplifier (efficient at 5volt).
The sensor connects to E+/E- (power) and A+/A- (bridge output) of the HX711.
The HX711 spits out an int. value that you can use to map to a volume value for the audio pot.
Maybe wise to split up the project in two parts first. Sensor and sound, before trying to merge them. Have fun.
Leo..
1 Like
Thanks again!
I thought you wanted to stay with something low power so you could use a batery
jim-p Yes, that is true, I would like to run battery power. Wawa has really guided me in the right direction. The HX711 is designed for Wheatstone Bridges, and is a ADC That is what the GMR AA005-02E is, a Wheatstone Bridge sensor. HX711 hooks to 5 volt processor. Wawa has given me a couple of ideas for the audio amplifier. I see some samples on the internet of the HX711 using Arduino. I want to study that and the audio amplifier examples Wawa has presented. My fist thought about the HX711 is: The examples I see of the HX711 is for weight scales, not audio. That made me wonder if the examples are DC output, because I need AC for audio. Of course I am going to have questions as I work trying to hook up and code. What I am wonder now is if I need to start a new post concerning hooking up the GMR to the HX711 and the PT2257. The Forum Police often jump all over me if I start a new topic. Since the post has been confusing, I think it would benefit everyone if I started a new post concerning the parts mentioned. I need the Forum Police to let me know if it is OK to start a new topic? That is a questionâForum Police?
I am still confused where your sound is coming from.
Are you trying to use the magnetic sensor as volume control or are you expecting that the sensor itself could pick up sound (from what source).
Don't create a new thread. If you do then all the gathered info so far will be lost.
Leo..
Wawa, I will try to clear up the confusion. If you look at a diagram of the AA005-02E It has 4 GMR magnetic cells placed in the form of a Wheatstone Bridge. Same exact arrangement as a stain gage Wheatstone Bridge. When a magnet moves near the Wheatstone Bridge, it changes the GMR magnetic cells. When the magnet is closer, the Wheatstone Bridge outputs more signal, because of the imbalance within the Bridge. Move the magnet away from the Bridge and there is less output. In the case of an audio signal input, moving the magnet closer to the Bridge means there is more volume output. Move the magnet away from the Bridge means there is less volume output. Like the strain gage, the signal coming out is not a strong signal. GMRs, like TMRs are new magnetic technologyânothing like the old magnetic Hall Effect sensors. You can think of the 4 GRMs in the Wheatstone Bridge almost like little resistors, that are changed by moving a magnet closer or further away. You ask where the sound is coming from? The input sound to the GMR Wheatstone Bridge arrangement can be any sound signal. For example: Sine wave generator, output of a television or radio, the output of a guitar pickup, the output of a microphone. The audio volume output of the Wheatstone Bridge is determined just like the output of the strain gage. The volume is changed by the magnet. Move the magnet toward the Wheatstone Bridge and the volume gets louder. Move the magnet away and the sound gets less. Exactly like a strain gage, more strain more signal. Less strain less signal. So âYESâ the GMR is controlling the volume. I have tried my best to describe the GMR sensor. It is exactly like a strain gage Wheatstone Bridge. The only difference you use a magnet instead of a strain gage. It is just a Wheatstone Bridge whoâs out put is controlled by a magnet. From your post, I know you are thinking of the old technology âHall Effect. The GMR is nothing like a Hall Effect sensor. Wawa, here is something even better about the GMR sensors. They operate from such low power, you can use a really low level audio signal to power the GMR. Meaning no power needed. Sounds crazy but I have one working. My only problem is getting it to go totally off. The reason for it not going totally off is two foldâThe Wheatstone Bridge is never totally balanced, and the off set in the Op Amp or instrumentation amplifier used to amplify the output of the bridge. I have started looking into the HX711, and getting snowed pretty quickâbut I donât give up. I see how to hook the GMR to the HX711, and I think I can program it like the strain gage. At this point I am not seeing how to get the HX711 signal sent to the PT22 57 through Arduino Uno R3.
You lost me here.
Before that part I understood that you wanted to generate a volume level, depending on a magnetic field.
The sensor has nothing to do with sound. It just generates a volume value.
That volume value is then sent to the volume control chip, which processes the actual sound.
Leo..
They will probably say no but what you can do is change the title of this topic to something like "GMR, HX711 and Digital Pot"
Then be prepared to answer some of the same quesions all over again.
Sorry mate, but had to spread it out, it is easier to read than a single string of sentences one after the other.
Tom.... 
Wawa, I thought I might lose you when I said: âThey operate from such low power, you can use a really low level audio signal to power the GMRâ. I know donât believe me, but it works, I built it. This means the sensor is powered by the audio signalâthere is no power connected to the GMR sensor. The AA005-02E is a sensor in the form of a Wheatstone Bridge. Normal Wheatstone Bridges have 4 connection points. I hooked the low level audio signal generator to a Unity gain non inverting Op Amp, and feed the output of the Op Amp to two points on the Wheatstone Bridge. The other two points of connection on the Wheatstone Bridge are output connections. The two output connections on the Wheatstone Bridge then went to a Op Amp to amplify the output signal of the Bridge. A magnet controls the volume, by moving toward or away from the Bridge. The (Only) problem I have is the audio will not go all the way off. The reason it wonât go all the way off is because you can never get the Bridge totally balanced, plus the off set of the output Op Amp. I thought about trying to balance the GMR sensors but could not see how to do that. I suppose it could be balanced with a lot of work using resistors in parallel on the 4 sensors of the GMR.
Now back to hooking up the HX711 to the GMR. The example I have been looking at is hooking up a strain gage using the HX711. I would think the strain gage is sending a DC signal out. My GMR will be sending an AC signal out. How does that change the hookup to the Arduino, and how does that change the code being used with the strain gage?