Hello! I am taking an electronics course through edx, in that course we have to build a circuit with a Wheatstone bridge and an amplifier (they use the OPA2344 but I am using the LM358), the instructors use the TI MSP430, but, we are free to choose the microcontroller we want, I am using ofcourse Arduino, there aren't much differences between the two, but I wanna be sure, I don't know to much about electronics and I don't want to burn my Arduino. I've attached an image with the circuit, as you can see, the Arduino has to read the output from the amplifier, and also has to feed the circuit with energy. Is this safe? Thanks
Its safe but you've set the thing up as a comparator, not an amplifier, so an LM339 would be a better choice.
You need to add decoupling to the amplifier or comparator.
If you mean to amplify, you would use a low voltage instrumentation amplifier like the INA333 which
doesn't load the bridge significantly.
It is a comparator, you are right, I am using the LM358 because it is pin compatible with the OPA2344 and that is what the instructors are using in the course
MarkT:
You need to add decoupling to the amplifier or comparator.
I don't know what did you mean by that. Can you please explain me better? Thank you very much for your answer.
You need the decoupling caps, 0.1uf ceramic cap between vcc and ground as close to chip as possible, one for each ic unless specified otherwise in datasheet. Basic stuff. How are you in a circuit design class and not familiar with this?
DrAzzy:
How are you in a circuit design class and not familiar with this?
This is a very basic course, and like a I said before I don't know very much about electronics, I am new on this.
I've attached an image of the schematic of the course and it doesn't have the capacitors. Do I need these because I am using Arduino? or it doesn't have anything to do with it
Is it safe to do it without the capacitors?
Thanks for your answer
Safety is not why they are there. It is to remove noise that can interfere with the wanted signal.
You can leave them out but you may also get inconsistent readings due to noise pickup.
Weedpharma
DrAzzy:
You need the decoupling caps, 0.1uf ceramic cap between vcc and ground as close to chip as possible, one for each ic unless specified otherwise in datasheet. Basic stuff. How are you in a circuit design class and not familiar with this?
That's for logic chips, opamps want much bigger caps (when used as amps), 47uF or so is a good
starting point, although the need for decoupling depends on the amount of gain and the stiiffness
of the supply. Logic chips switch in a few ns, opamps run at timescales of us, hence the widely
different requirements.
A comparator chip is much much faster than standard opamps and more like a logic chip, so 0.1uF
would be OK there.
Quick question, I made the connection with the Arduino, without using the input pin, I am just feeding the circuit with the Arduino, and then I've measured the output with a multimeter and I got a little bit above 43 milliamps, I believe that the limit on the Arduino pins is about 40 milliamps, so before I try to read the output of the comparator from the Arduino, I should use a resistor to protect my Arduino. Am I right?
Arduino inputs take no(*) current, its only outputs that this is an issue.
(*) More precisely they take incredible small currents, measured in picoamps or nanoamps.
FYI, since you said you are new to electronics and also that you are in a class, I am going to take a big leap and assume you want to learn as much as possible about electronics. (not always the case here on the forum. Often a poster just wants to get one project working and could care less about everything else). I would recommend you stop using Fritzing and download ExpressSCH. The only way to download it is to download ExpressPCB, and ExpressSCH is automatically installed simultaneously. There is no option to choose only one or the other. ExpressSCH is schematic capture and ExpressPCB is PCB board layout CAD. Also, learn electronic symbols and start drawing schematics with a dark ballpoint pen and photographing them with your cellphone and upload the file. Before you upload it open it with Windows Paint and select "resize" (upper left corner) and enter "50" where you see the "100" (%). Then select
"File/Save As" and choose JPG. Upload the photo of your hand drawn schematic instead of the Fritzing.
WHY ? Because we absolutely HATE FRITZING here on the forum. If you want to piss us off that's a great way to do it. We would rather look at the crappiest handdrawn schematic than look at a Fritzing.
Also, if you haven't already, learn Ohm's Law. Your questions about capacitors and what they are for reveals a basic lack of understanding of capacitance and inductance and the fact that these devices (capacitors and inductors) are essentially storage devices. The equations for capacitive and inductive reactance contain "f" (for frequency) . The question about capacitors indicates you are not aware that the purpose of decoupling caps on the power pins of ANY ic are to prevent glitches in the circuit by
shunting high frequency spikes to GND. If you take the equation for capacitive reactance:
Xc= 1/(2Pif*C)
and you "Let C = 0.01 uF, and plug in different values for frequency , starting at 1000 hz and incrementing in 5000 hz steps you will see that the higher the frequency, the lower the reactance of the cap, which means (in plain english) that the higher the frequency of the spike the more likely it will be shunted directly to ground by the cap.
ergo the name:
decoupling capacitor
You could , if you were so inclined, think of a cap as a frequency dependent resistor. if you keep in mind that it is an energy storage device, which is why equipment with large capacitors continues to work for some seconds after you turn off the power .(but don't tell anyone I told you that)
I think it's about 170 nA for the ATmega328