Hello, I'm doing a project where I have to create ultrasonic transmitters/receiver circuits. I've done the transmitter part and it works well, but the receiver circuit isn't working no matter which one I try to use. Right now I'm trying to use the circuit from this website: https://turnelltech.wordpress.com/2013/01/11/building-an-ultrasonic-sensor/.
All of the circuits I've done have a similar problem: I connected a LED to the output pin to see when the output would be LOW/HIGH, and the LED is usually always on. It doesn't really matter if the receiver is receiving a soundwave or not, it's always on. It also suffers variations when I touch the resistors on the circuit; which I would think it's a poor connection problem, but the resistors can't really go lower in the breadboard or they'll pierce through it. I'm using the murata sensors, therefore why I'm using the above circuit for my receiver, but I've tried several other circuits and they end up having the same problem. I have an oscilloscope connected to my receiver, so I know it's receiving the ultrasounds, but when it comes to treating the signal so I can have LOW and HIGH outputs (or a non oscillating signal), it doesn't really work. I'm using the LM324 AmpOp and a power supply of 5V.

 

Hi didifi.
Welcome to AAC.
Where is the circuit diagram showing how the LED is connected to the receiver.?
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Hi didifi.
Welcome to AAC.
Where is the circuit diagram showing how the LED is connected to the receiver.?
E

The LED is connected to the second output (pin 7) and GND

 

Hi,
Does the LED have a series current limiting resistor.?
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hi didif,
An LTSpice simulation shows that the Vout {LED} will always be ON, ie: Lit.
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Note: the pin marked Output will also be High.

 

What frequency does the ultrasonic transducer operate at?
20kHz is a bit of a struggle for an LM324, so ultrasonics may be beyond its capabilities!

 

hi didif,
An LTSpice simulation shows that the Vout {LED} will always be ON, ie: Lit.
E
View attachment 293711

With no signal it will either be permanently on or permanently off depending on the size and polarity of the offset voltage. As soon as there is a signal on the input, the output should change state.

 

Hi Ian,
Why are you telling me that information.?

I already have answered his query, why the LED will always be ON with no Sense input.
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hi didif,
This is how the circuits functions with a simulated 40kHz pulsed input, note the voltage swing on Vo and Output.
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hi didif,
An LTSpice simulation shows that the Vout {LED} will always be ON, ie: Lit.
E

Note: the pin marked Output will also be High.

View attachment 293711

Hello thank you, do you know why it's always HIGH? I've tried a couple of similar circuits (some didn't have a comparator though) and it's always the same problem although people that tried those circuits said it worked. I'm confused on how to get an OUTPUT that will get me HIGH/LOW depending on whether the receiver is receiving an ultrasound or not. I thought a comparator would work as it could alternate between the two states and I don't really understand why the circuit is still HIGH even when pin 3 is LOW due to the receiver not receiving any ultrasound

 

hi didif,
This is how the circuits functions with a simulated 40kHz pulsed input, note the voltage swing on Vo and Output.
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View attachment 293714

So on both V(output) and VO the voltage was already higher than 0? Assuming by the line on above 2.2V and the one at 3.6V... I don't know if I'm interpreting correctly.

 

What frequency does the ultrasonic transducer operate at?
20kHz is a bit of a struggle for an LM324, so ultrasonics may be beyond its capabilities!

40KHZ, same sensors used with the same ampOp by the person in the link I posted

 

So on both V(output) and VO the voltage was already higher than 0? Assuming by the line on above 2.2V and the one at 3.6V... I don't know if I'm interpreting correctly.

hi d,
That is correct.
One the printed circuit PCB thats shown in your link, Can you post a copy of the circuit showing how the designer has connected his LED.?
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hi d,
That is correct.
One the printed circuit PCB thats shown in your link, Can you post a copy of the circuit showing how the designer has connected his LED.?
E
View attachment 293717

I think this LED is only to indicate that the sensor is turned on, since it looks like it's connected to the red wire, which is usually the VCC. Also, I can't really post it because the files they put on the site are outdated I assume (since that post is from 2013). The LED I connected was so I could analyze the output without having to connect it to a microcontroller to debug it and see whether the output is HIGH or LOW. But the original person didn't put the LED in their schematic on the website.

 

Hello thank you, do you know why it's always HIGH? I've tried a couple of similar circuits (some didn't have a comparator though) and it's always the same problem although people that tried those circuits said it worked. I'm confused on how to get an OUTPUT that will get me HIGH/LOW depending on whether the receiver is receiving an ultrasound or not. I thought a comparator would work as it could alternate between the two states and I don't really understand why the circuit is still HIGH even when pin 3 is LOW due to the receiver not receiving any ultrasound

For a perfect op-amp the output of U1 would be at the same voltage as C3, and therefore the two inputs of U2 would both be at the same level, so it is random whether the output of U2 would be low or high.
However, a LM324 has a bias current of typically 10nA (maximum 60nA). That current establishes a voltage of 1mV across R3. That should be compensated for by the current through R1, but the impedances are not equal, because of R4 and R5, making the impedance at the non-inverting input 150k, that will make the non-inverting input of U2 higher than the inverting input, so the output of U2 will go high.
Also, The LM324 has an offset voltage of ±600uV (maximum ±3mV). That adds a random element to the inputs of U2, so the output could be either high or low at rest, with a preference for high.

I'll just add - don't use that driver circuit with a 12V supply and a 0-5V square-wave input - it will blow up.

 

Hi did,
OK,
So the diode you are wanting to add, is to show a received pulse.?
Where are the the Vo and Outputs pins externally connected to some other circuit on your project.?

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For a perfect op-amp the output of U1 would be at the same voltage as C3, and therefore the two inputs of U2 would both be at the same level, so it is random whether the output of U2 would be low or high.
However, a LM324 has a bias current of typically 10nA (maximum 60nA). That current establishes a voltage of 1mV across R3. That should be compensated for by the current through R1, but the impedances are not equal, because of R4 and R5, making the impedance at the non-inverting input 150k, that will make the non-inverting input of U2 higher than the inverting input, so the output of U2 will go high.
Also, The LM324 has an offset voltage of ±600uV (maximum ±3mV). That adds a random element to the inputs of U2, so the output could be either high or low at rest, with a preference for high.

I'll just add - don't use that driver circuit with a 12V supply and a 0-5V square-wave input - it will blow up.

Would there be a chance of it working if the impedances were equal then? Of course, not counting with the offset voltage, because it could still go wrong because of it. But if the output was low at rest by chance, with equal impedances could it work?
Also, I didn't use that driver circuit, but thank you for the warning

 

Hi did,
OK,
So the diode you are wanting to add, is to show a received pulse.?
Where are the the Vo and Outputs pins externally connected to some other circuit on your project.?

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Yes! Ultimately I wanted to connect it to an arduino to process data, but for now I just wanted to see if it would work with HIGH/LOW outputs with receiving/not receiving states. The Output is only connect to pin 5 of the ampOp, and the Vo is connected to the LED, but ultimately it would be connected to an arduino pin, if it worked properly

 

hi.
Looking at post #9 simulation, it is working properly, the Output swings above and below +2.5v level.

You could write the Arduino sketch to detect a Vo going LOW on an Input pin, [or trigger an Interrupt].
The the Arduino could then flash an LED for any Period you decide.


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hi.
Looking at post #9 simulation, it is working properly, the Output swings above and below +2.5v level.

You could write the Arduino sketch to detect a Vo going LOW on an Input pin, [or trigger an Interrupt].
The the Arduino could then flash an LED for any Period you decide.


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I will try again and see what I get on an arduino analog pin, but last time I tried to see variations on voltage it would be very random :/ It would oscillate between 0 and 1023 even when there were no waves being received, aka no 40 kHz signal in input