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

sarahtelayrii

Joined May 2, 2009
12
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 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.national.com/ds/LM/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:

bertus

Joined Apr 5, 2008
21,449
Hello,

What voltage is the reciever working?
The 74HC04 will work between 2 and 6 Volts.

Greetings,
Bertus

sarahtelayrii

Joined May 2, 2009
12
5V.

I'm not too sure if it were my connections or not but the Vcc is at 5V and the GND pin is on ground. I'm sure all the other wires are fine, but as the result was before when its all connected, the output from the schmitt trigger pin 4 on the oscilloscope, theres only sine waves and even in that, there's no response on the screen when I wave my hand or move it up and down for instance across the transmitter. BTW I should mention the transmitter is at 40KHz.

CDRIVE

Joined Jul 1, 2008
2,219
If I'm reading your post correctly you're on the right track. If you're saying that you need pre-amplification prior to the Inverter, you're correct.

bertus

Joined Apr 5, 2008
21,449
Hello,

What happens when you put a capacitor of 0.1 μF between the 1K resistor and your sensor?
This will help to keep the DC at the input of the 74HC04 better in the middle.

Greetings,
Bertus

sarahtelayrii

Joined May 2, 2009
12
If I'm reading your post correctly you're on the right track. If you're saying that you need pre-amplification prior to the Inverter, you're correct.
But shouldn't it already be amplifying? It is a gain of 100 afterall before being sent to the schmitt trigger.

also how are my other 2 proposals?

bertus

Joined Apr 5, 2008
21,449
Last edited:

CDRIVE

Joined Jul 1, 2008
2,219
But shouldn't it already be amplifying? It is a gain of 100 afterall before being sent to the schmitt trigger.

also how are my other 2 proposals?
I don't consider logic blocks such as the 74H04 as an amplifier. Logic components (whether TTL or CMOS) are not linear components nor are they intended to handle small perturbations. A FET or OpAmp front end would be nice!

Audioguru

Joined Dec 20, 2007
11,249
The ultrasonic transducer is probably a piezo type. It needs a 10k load, not the almost dead-short of the 1k input resistor of the inverter. So it is loaded down and has a low output level. It is like a capacitor so an additional coupling capacitor is not needed.

An inverting amplifier has a certain gain that results when it is fed from an extremely low impedance source which the transducer does not provide.
The "HC" inverter has a very high idle current which results in a gain that is far less than 100.

Use a non-inverting (for high input impedance) high frequency opamp like an OPA134 that can have a gain as high as 200 at 40kHz.

sarahtelayrii

Joined May 2, 2009
12
My local supplier doesn't have the OPA134.
http://www.jaycar.com.au/semiconductor.asp

What about the LM833? In regards to the OpAmps for voltage gain I don't have to worry about the V+, V- right? Or I need the V+ as my reference voltage just like I needed it in the LM311?

sarahtelayrii

Joined May 2, 2009
12
hello just an update..

I modified the circuit so now I have a dedicated +6.7V rail and a dedicated -9V 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)

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:

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??

thanks guys

eblc1388

Joined Nov 28, 2008
1,542
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.