Group 6 project: myDAQ acoustics – myBAT

Mohammed Hassan Miah and Kamil Ropiak
 

BATECTOR

By considering how waves travel and bounce back once they have hit an object will help with understanding how bats hunt using echolocation techniques.  Take radar for example, here radio waves are emitted from a transmitter into a ‘target’ area and any waves that have reflected upon contact from an object will scatter into many directions.  Radar receivers (antennas) capture the returning waves and thus a ‘picture’ of what is in the target area can be seen.  Bats use the same principle but instead use ultrasonic waves.  They are advanced species and can see where their prey by determine the length of time taken for the reflected signal to return and can determine the direction of the prey by calculating the direction the signal returned from.  In simple terms, when the returning signal hits them; if the right side of the body comes into contact with the returning signal first then the prey is to the right and vice versa.  Bats can also see whether their prey is moving away from them or coming toward them because of a slight change in frequency caused by the Doppler Effect. 

Our aim is to capture a bat call, something that is in the ultrasonic range greater than 20 kHz and less than 65 kHz using a special high frequency microphone.  Upon capturing the signal, we would like to shift the signal toward the audible frequency range which is greater than 40 Hz and less than 20 kHz.  

There are many methods on how to shift these signals but the one we are considering is the Heterodyne method.  This method basically combines the bat call with a constant internal frequency and the sum and the difference of the frequencies will shift the resulting frequency into the audible range.  For example, a bat call at 42 kHz and a constant internal frequency of 46 kHz produces output frequencies of 4 kHz and 88 kHz.  The 88 kHz is inaudible and filtered out leaving the 4 kHz which is fed into a speaker.

Above is the first part of our project which is mainly research based.  The second part is more experimental based.  Here we must now consider subsonic waves.  There are many methods of capturing subsonic waves but we have decided to use a method which uses a spring, a rare earth magnet and a Hall Effect sensor.  The spring will be connected vertically to a fixed surface on one side and on the other side it will be connected to the magnet.  Subsonic waves (below 40 Hz) will push the air surrounding the magnet causing it to move, the Hall Effect sensor having high sensitivity will collect the data.

                                                                                             

To date we have spent a lot of time researching bat detectors.  At the start of this week we decided to find a bat detector that someone else had already built, on the internet.  This is so we could build a design that we know should definitely work.  From this we hope to gain a better understanding of how the hardware works.  However whilst we waited for our components to arrive we decided to create a simulation on labVIEW of how the detector will work.  This is a basic version and will continue to add to it as we plan to make a simulation of our final design before we build it.

Our simulation is shown in screenshots below.  Using the Get Waveform Components function, we could retrieve useful information about waves, in this case sine waves.  From there we could modify dt which is the time interval measured in seconds between data points in the waveform.  This increased the time constant thus decreasing the frequency.

1.      On the first picture we can see Generated Signal of frequency 71 kHz.  The user is able to change the frequency within a range of 20kHz to 100kHz.  The time interval is pldisplayed and to the right of the top graph are the values of Y which are used to plot the graph.  The graph below is generated by the Spectral Measurements function which is configured to show the frequencies within the waveform. This is the original signal which we hope to shift toward audible frequency.  The STOP button terminates program. 

 2. The graph below shows the waveform at frequency of about 2.2 kHz.  We have been successful in shifting the signal to an audible frequency.  This is now the modified signal.  Like before, Dt, Y, and Waveform Graph can be seen by the user. The frequency still can be adjusted and the effects will be updated real time.

  

3. Now we used the DAQ assistant to send the modified signal to one of the analogue outputs on the myDAQ.  As we could not get a speaker we decided to use an oscilloscope instead to test if the waveform is at right frequency.  The picture to the left shows the modified signal displayed on the Oscilloscope – showing frequency of 2.247 kHz. 

So far the program is operational with simple waveforms of a constant frequency.  Our next aim is to connect a microphone and a speaker to the myDAQ breadboard and capture real audible frequencies and see how we can shift the signal about the audible range thus allowing us to playback the sounds via the speaker.

As stated earlier we found a bat detector online that somebody had already made.  On software called Altium we have taken the design and made some slight modifications to the schematics but the layout is still similar.  This design consists of two microphones which will be connected via the Molex’s shown at the top left of the schematic.  The microphones will be connected to two separate preamplifiers with a gain of 50 each.  The chip 74HC4046 contains the phase-locked loop and using the voltage-controlled oscillator output we can generate the digital clock for the switching circuit.  The switching circuit is where the bat signals will be shifted.  Finally the headphones are connected at the right side of the board, this is where the shifted signal will be played back.

After the schematics were created, we needed to put it onto a virtual PCB also on Altium, as this would be sent off to be built.  After carefully placing all the components onto a 10cm x 10cm PCB, we auto routed and this is our first design ready to be built.

Our next is aim to get the PCB built which should take one day and then the next steps are to solder the components and then test the board to see if we can capture signals and shift them.