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Generating 100mV to a few volts via amplifier or some sort?

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sarahtelayrii

New Member
Hi all,

I'm working on an ultrasonic rangefinder and I have the transmitter working just fine and my receiver circuit looks something similar to the below:


On the oscilloscope it's receiving (the ultrasonic receiver between 50-115mV, that is, when I connect the ends of receiver to the oscilloscope and it being placed about an inch away from the transmitter (at 40KHz 5V p-p) both facing directly at me, I was able to see sine wave responses on the oscilloscope via holding a chair and moving it forwards and backwards about 2-2.5 meters away.

The thing is the receiver circuit is supposed to amplify the received signal to a few volts and then use a schmitt trigger to smooth out the sine wave with noise) to a square wave as much as possible.

It worked for a few moments but not at the voltage I expected (still was in very low volts), with alot of noise being detecting (thick line on the top of the received square wave) however since pressing auto-set on the oscilloscope the whole thing just went berserk. Now, there's no received square wave and all I get at the end of the schmitt trigger is sine waves that doesn;t responsd to an object moving at all. Even after re-trying several times.

So any ideas? I know it's hard to say without looking at a screen shot (i have no access to oscilloscope until monday) however I'm thinking the cirucit I have above (im using the same components) might not be suitable in my case.

So I was thinking of one of these 2 solutions:

a)
Use the above concept (which is basically my transmitter circuit) but have V+ (pin 8) at 5V and on Pin 3 have the resistors instead of at 1K each have R1 at 10K and R2 at 1M Ohm, so the reference shall be instead of 2.5V it would be at about 50mV. The output of Pin 7 can then go through a schmitt trigger to convert the sine wave to an approximate square wave via upper/lower Threshold voltages. This would then be fed back into the microcontroller pin to be read by the software to calculate distance etc...OR

b) Use a "LM8272 is a Rail-to-Rail input and output" OpAmp.
http://www.electro-tech-online.com/custompdfs/2009/05/LM8272.pdf
The pin 2, inverting Input A shall receive the main signals from the ultrasonic receiver, V+ reference I guess at 5V, V- at GND. Correct me here if I'm wrong I don't have much practice with OpAmps here. The main Output A can then go through a Schmitt trigger and then to micro...

Thanks everyone, I'm getting progress in this project albeit Im working on all of it my own.. and am happy with that but bit by bit.


PS: I have the transmitter working after testing on the oscilloscope, it's sending to about approximately 2-2.5 meters there's a sine wave response on the receiver transducer. I switched to the Murata ultrasonic transmitter (the MA40B8R) and the distance increased by a lot.
 
Last edited:

Willbe

New Member
The thing is the receiver circuit is supposed to amplify the received signal to a few volts and then use a schmitt trigger to smooth out the sine wave with noise) to a square wave as much as possible.
You're supposed to have a voltage gain of 100 with that inverter.
 

audioguru

Well-Known Member
Most Helpful Member
The input impedance of the inverter is much too low to be the amplifier for an ultrasonic transducer. Then the transducer is loaded down so that its output level is too low.

The 74HC04 has an idle current that is too high to be a linear amplifier. Then its gain will be too low and it might smoke too much.

Instead of the inverter you need an opamp that can amplify 40kHz and has an input impedance of at least 10k ohms. An OPA134 opamp has a max gain of 200 at 40kHz. When it is non-inverting then its input impedance can be as high as you want.
 

audioguru

Well-Known Member
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Farnell in Australia has 150 OPA134 opamps in stock. But their rip-off price is more than double the cost in America. Its minimum total supply is 5.0V.
The LM833 is a dual opamp with a minimum total supply of 10.0V.
A TL071 has a minimum total supply of 7.0V. It can have a max gain of 80 at 40kHz.

If you don't bias the inputs of an opamp properly when it has a single-polarity supply then it will not work.
 

sarahtelayrii

New Member
Ok I've been going through the tutorials over OpAmps...now I understand that the V+ acts as a reference voltage right? This is the voltage that the amplifier will try to amplify to - am I correct?

If this is the case V- must be to ground.
The 74HC04 has an idle current that is too high to be a linear amplifier. Then its gain will be too low and it might smoke too much.
Could you please explain what you mean by idle current? is that the input clamp current in the datasheet??
Oh BTW in regards to the 74HC04 I don't need to provide it Vcc right? Stupid question I know, just asking since I still saw a square wave (not how I want it...not amplified but still in mV) even when Vcc wasn't connected to the +5V rail.
 
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audioguru

Well-Known Member
Most Helpful Member
V+ is the positive power supply pin for an opamp.
V- is the negative power supply pin.
+in is the non-inverting input.
-in is the inverting input.
Then learn about how to bias and how to set the amount of DC and AC gain of an opamp.
 

sarahtelayrii

New Member
So I take it V+ is not the level of voltage that it will amplify to? As was the case with the LM311, thanks for the help btw
 
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Hero999

Banned
I'd be tempted to use the CD4069.

One or two sections could be configured as amplifiers, another two buffers could be configured as a Schmitt trigger.

I don't what the gain of an CD4069 is, more than one stage may be required.
 

audioguru

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Most Helpful Member
The LM311 is a comparator, not an opamp. A comparator has an output that switches from low to high when its +in is at a higher voltage than its -in. Its output is 0V or the +DC supply voltage or is a square-wave.

An opamp can have negative feedback so that it can have a certain set amount of voltage gain and can produce a sine-wave or a triangle-wave if you want. Its output can be nearly any DC voltage.
 

Willbe

New Member
Opamps can be used as comparators; I've never tried doing the reverse.
 

Sceadwian

Banned
Opamps as comparators generally don't perform as well as dedicated comparators. I'm pretty sure you can't practically turn a comparator into an opamp.
 

Hero999

Banned

sarahtelayrii

New Member
Ok guys I'm going to start thinking of various ways to make this receiver via the appropriate circuit. I have 2 questions please, this should answer my main concerns.

1)
I wanna know if I use the LM741 (I'm not going to use it but just using it as an example opamp since that's the one i can think off the top off my head this Sunday morning :p )


Now looking at the data sheet the input voltage is 15V, so V+ would be 15. Now the inverting input will be connected to one of the receiver ultrasonic pins receiving at around 50-100mV. -V to the GND. The output would be a square wave right?

2)
Before posting this topic I was unsure if this time of circuit would work for amplification but an opamp circuit with negative feedback, using the same LM741 as the example:
For instance:

Say Rf is at 400k, and Rin is at 10k, giving Av = 40, V+ at 15V (or wopuld that be the 22V supply voltage?), V- tied to GND

Since the differential amplifier amplifies the difference in voltages of the inputs, if V2 from the picture was tied to GND, V1 was 0.05mV, the output would be with a gain of 40 at 1.1V.

Also just wanna know that there's a limit to I'm sure how much an OpAmp can amplify to.

3) The amount of voltage that can be sent to either V1 or V2 in an OpAmp is limited by the input voltage range, in the LM741's case it is +-13, so anything with a voltage > +13 or negative of less than -13 the LM741 would be damaged.

Sorry just confirming by doubts, there's lots of parameters in the datasheet.

Thanks everyone!
 

audioguru

Well-Known Member
Most Helpful Member
Since you have a single positive supply voltage then many opamps won't work unless their inputs are biased near half the supply voltage. The inputs work over a range of voltage called the "allowed common mode voltage" which is a couple of volts above ground and a couple of volts below the positive supply voltage.
Also, the outputs of most opamps cannot go less than a couple of volts above ground and cannot go higher than a couple of volts below the positive supply voltage.

The lousy old 741 opamp is 41 years old and was designed for a 30V total supply voltage. Many will not work with a supply of only 10V. The output level drops above only 9kHz.
 

Hero999

Banned
Here's how I would do it.
 

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sarahtelayrii

New Member
Hi, which type of a single-input opamp is that?

Also BTW I modified the circuit so now I have a dedicated +6.7V rail (for V+) and a dedicated -9V(for V-) rail supply.

I'm using the CA3130 opamp which has a maximum voltage rating at 16V p-p.

I'm using it in the non-inverting amplifier configuration with R2 at 2K and R1 at 330 Ohm, giving me a gain of about 7.

I was told by my friend that:
Vout = Vin (Av)

Now that's fine I already knew that however he adds:
Vin for formula = Vin (this is 0.1) - (-9V) / V+ - (-V-)

I can't remember if this is the formula from the top of my head but its like this.

I remember getting for this Vin as 0.68, thus Vout is at 4.76V. And it does show this voltage across the Vout pin on the opamp however when I hook up this output and the ground to the oscilloscope I get a waveform similar to this: untitled.JPG

It has V p-p that fluctuates somewhat between 8-11V. The frequency the receiver is picking up is just over 1MHz, with a period of around 800ns. When I move my hand up and down the waveform does not respond at all.

if I don't have the ultrasonic receiver connected to any opamp at all but simply via clips directly to the oscilloscope, when I move it I can clearly see the amplitude change in relation to the distance from the transmitter. Also it shows fluctuations from 40KHz A(above it and below). *But* when this is connected to the opamp I don't see these results (the waveform doesnt respond when I move my hands up and down - it just stays still) even though amplification has been made.

So what might be happening here?? :confused:

thanks guys
 
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audioguru

Well-Known Member
Most Helpful Member
The extremely old CA3130 opamp oscillates at about 1MHz when it has negative feedback and no phase compensation capacitor.

Maybe your wires at the inverting pin are too long and they pickup an AM radio station.
 
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