Introduction: Simple Frequency Division Bat Detector (Ultrasound to Sound Converter)

A bat detector is a device that detects ultrasound and converts it into an audible sound to allow for augumented expansion of the human hearing range.

There are many types of bat detector mechanisms, but frequency division and heterodyne are most commonly used for building simple circuits:

Heterodyne is based on the multiplication of sine waves to create an audible frequency. For example, we can multiply an input wave of 45kHz by a 40kHz (generated inside the device) to obtain a 85kHz and 5kHz wave, of which the latter is audible. This method is better at preserving amplitude variationsbut requires frequency tuning and only a narrow band of frequencies can be heard at one time.

Frequency Division uses a binary counter circuit to divide the frequency of the sound. This works because each increasingly significant bit takes 2 times longer to change between 1 and 0. As such consecutive pins (Q1,Q2,Q3) on the binary counter will result in a frequency division by 2,4,8,16. For example, a 40kHz sound can be divided by 16 to obtain an audible 2.5kHz wave. The binary counter requires all sound to be amplified to a square wave and only outputs two values - 0V and 5V, this means that most amplitude information is lost but no frequency tuning is required.

Supplies

Components:

1x Ultrasonic Microphone from Micbooster: https://micbooster.com/ultrasonic-microphones/146-ultrasonic-mic-board.html

1x Binary Counter (CD4040BE): https://www.mouser.co.uk/ProductDetail/Texas-Instruments/CD4044BE?qs=gqbMQSs93zOdbgCsHzcB2Q==&mgh=1&vip=1&gclid=CjwKCAjwk6-LBhBZEiwAOUUDpzfzMFqFg-ISnsXoAj8rlsVeEizpXDt1PjJP_fF8Xh3HrCAy583_EBoCvfoQAvD_BwE

3x Audio Amplifier (LM386N): https://www.mouser.co.uk/ProductDetail/Texas-Instruments/LM386N-4-NOPB?qs=QbsRYf82W3FsJPldcpZj5g%3D%3D&mgh=1&vip=1&gclid=CjwKCAjwk6-LBhBZEiwAOUUDp2UN7liwiVYV9dgxu1_1MOPkAPElD6mvy_6bjBbk1rbn5S3znzgAMxoC_CMQAvD_BwE

1x 3.5mmAudio Jack

4x 10 microfarad capacitor

1x 0.1 microfarad capacitor

3x 1 nanofarad capacitor*

1x 7.5k resistor*

1x 1.5k resistor*

1x Switch

1x 3AA battery holder (4.5 Volts)

1x Variable Resistor (For volume control - optional)

You may want to get more to duplicate high and low pass filters for better frequency rejection.

WARNING: The microphone is expensive and reverse voltage or voltage above 5V will kill it - be extra careful when wiring your circuit and check all connections before powering.

Step 1: Circuitry

The Bat detector has the following stages:

1- Microphone interface unit - this involves the microphone itself and 2 capacitors (10microfarad for removing dc offset and 0.1 to stabilise the microphone)

2- 20x Amplification - an LM386N amplifier with no connection between pins 1 and 8. These pins are there to set the gain and frequency rejection; there are 2 options, a 10microfarad capacitor give and gain of 46 and a max frequency of 20kHz and no connection giving a gain of 20 and a max frequency of 110kHz. Although the gain is lowered, we must choose the second option for ultrasonic amplification.

3- 20x Amplification - same as step 2 (total gain 400)

4- Passive and Active Low Pass filters - these are designed to make the circuit less responsive to audible sound (<20kHz or so) and remove electrical noise beyond the limit of the microphone (above 120kHz). As such the two cutoff frequencies were selected to be 21.2kHz (7.5k resistor) and 106.1kHz (1.5k resistor) giving a first-order filter with -6dB per octave which is significant to attenuate the unwanted signals. It should be noted that some high-frequency audible sounds will still be audible through the device if loud enough - to minimize this effect, duplicate the filters.

5- 20x Amplification (total gain - 8000) - same as step 2 - by now most sounds are amplified to a square wave.

6- Binary Frequency Division - the square wave sound is divided by 16 and output to the audio jack/speaker.


WARNING - without the volume limiting variable resistor (built-in in headphones or added to the circuit) the output through the speakers could be VERY loud - first switch on the device and only then put on the headphones to prevent hearing damage!


Step 2: Limitations

The circuit comes with 2 main limitations:

-The nature of a frequency division bat detector makes it that amplitude variation is lost (i.e. a loud ultrasound and a quieter ultrasound may often sound the same. Some amplitude variation is detectable as certain quiet sounds will not be fully amplified to a square wave and will sound more distorted and white. Thus you can still figure how loud the sound is by moving the microphone around.

The microphone reacts to sound like a normal microphone would so audible sound can still pass through if it is loud enough. Although filters have been added, certain high-pitched sounds (from about 8-10kHz for moderate volume and 4-8kHz for loud volume) will still make it through the device. This is not necessarily a bad thing, as people often lose high-frequency hearing with age, however, sounds like an ambulance siren or loud music will saturate the device and mask all other sounds. This can be overcome by duplicating filters or listening out to the frequency of the sounds - if you can hear a low pitch tone (below 1kHz) from the device and possibly an audible sound in your environment you will know it is not ultrasonic. Conversely, if things sound high pitched (1kHz- 5kHz) and the sound is not audible without the device, then you have detected ultrasounds.

Step 3: Enjoy!

Step 4: Common Ultrasounds

This device will reveal a hidden world of sound normally inaudible to human hearing. Listen out for key clicks, car breaks and sensors, rustling leaves, ultrasonic sensors, running water, rubbing fingers, and much much more!

Step 5: Sound Recordings