designing an ultrasonic reciever

hi !
I found this circuit on ECElab.com , but I dont know what the variable resisters R2 and R8 do. can someone explain me that?
also I understand that 741 is an old am and i should use TL071?
Also are there any low power, cmos ultrasonic circuits available that can run on battery power say around 5-6V?

I found this circuit on ECElab.com , but I dont know what the variable resisters R2 and R8 do. can someone explain me that?

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R2 sets the gain of the first stage of amplification. R8 sets the center frequency of the tone decoder.

also I understand that 741 is an old am and i should use TL071?

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The fact that a product like the 741 has been around for a long time is no reason to avoid using it as long as it does the job. A TL071 is not specified for operation at low supply voltages.

Also are there any low power, cmos ultrasonic circuits available that can run on battery power say around 5-6V?

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There are several opamps that can operate at those voltages. An LM358 is one common example. I'm not sure how useful it will be at ultrasonic frequencies, but its bandwidth is in the same range as that of a 741. And then there are others like the TLC227x series which are faster and can operate at low voltages.

The fact that a product like the 741 has been around for a long time is no reason to avoid using it as long as it does the job. A TL071 is not specified for operation at low supply voltages.

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Minimum output swing datasheet specifications are for supply voltages of +/-3.5 V with TL071 and +/-5V with LM741. Also TL071 has about doubled bandwidth, in so far it's better suited for ultrasonic applications.

For 6V battery operation a low voltage op should be used. LM567C can operate down to 5V.

so in the above circuit i should replace 741s with TL071 and NE567 with LM567C ?

is the above circuit a good working circuit as ultrasonic reciever ?

---------- Post added at 17:18 ---------- Previous post was at 17:16 ----------

also without an oscilloscope how can i adjust R2 and R8 ?

is the above circuit a good working circuit as ultrasonic reciever ?

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Good for which ultrasonic application? In my opinion it's a bad idea to operate U2 as comparator with a threshold depending on accidental OP offset voltages, not utilizing the high sensitivity of U3.

iam trying to build a circuit that will be mounted in front of my car and if the car is moved it will raise alarm . so basically i want to built an alarm for large object removal detection .
if this is not a good a circuit can you point me one that is good ?