Raspberry Pi basics

Raspberry Pi is a very useful small computer that is well suited for DIY projects of various kinds.

In this case it is used to study bats by recording ultrasound. Then there is another parallel system for recording infrared video. The system for ultrasound is called WURB, Wireless Ultrasonic Recorder for Bats. The system for infrared video is called WIRC, Wireless InfraRed Camera.

Software and installation instructions for WURB and WIRC can be found here:

• https://github.com/cloudedbats/wurb_2026
• https://github.com/cloudedbats/wirc_2026

In this document I have collected some basic information about the hardware parts used in the WURB detector, experiences about what I think works well and recommended components/accessories.

What is Raspberry Pi?

Image: Three models of Raspberry Pi. From left: Raspberry Pi 3B+ inside a case, Raspberry Pi 4 on top of a transparent case, and finally Raspberry Pi 5 inside a case and with heat sinks attached.

Raspberry Pi is a small (size as a credit card) single-board computer that has become very popular among people who build their own systems. Although it is small and cheap, it contains all the capabilities of a much more expensive desktop or server computer, but with a limited computing speed.

In addition to the computer itself, a large market has emerged for additional accessories that are specifically aimed at DIY projects. In parallel with this, there is of course also a very large and committed community.

If you want to read more about Raspberry Pi, you can read here:

• https://www.raspberrypi.com/documentation/
• https://www.raspberrypi.com/documentation/computers/raspberry-pi.html
• https://www.raspberrypi.com/documentation/accessories/

If you are not familiar with the Linux operating system and the basics of how to use a terminal window, I recommend that you ask for help from someone with this knowledge. This knowledge is only necessary when installing the software on your detector, when troubleshooting, and if you want to update the program code to the latest version.

Why Raspberry Pi for bat monitoring?

For this purpose, the Raspberry Pi has exactly what is needed to build a bat detector.

There are four USB ports where you can connect the ultrasonic microphone, a GPS dongle, an external USB memory card or SSD, or an USB 4G/LTE modem when needed. The microphone is the only mandatory part, the rest is optional.

Then there is a wireless network (WiFi) built in, a modem cable connector (Ethernet). The power supply is via 5V USB connectors in the same way as you charge a mobile phone. This allows you to use a regular power bank for power supply when needed, and adapters from 12V to 5V are easy to find if needed.

In this project, one goal has been to not require soldering or connections via the 40-pin header when building your own detector. USB-connected devices are prioritized and these should be automatically identified by the software when connected. An exception is the camera sensors used in the WIRC system because they are connected with a flat cable (called "FPC Camera Cable") to special camera connectors on the Raspberry Pi board.

Recommendations

Raspberry Pi Models

Raspberry Pi 4 is the model that I use most and recommend if anyone asks. I have had these both as mobile detectors and as stationary detectors where they have been up all year round without problems. However, the microphones can be sensitive to damp weather and other damage. Raspberry Pi 4 also handles variations in power supply significantly better than its successor RPi 5.

Raspberry Pi 3B also works but the processor is slower and you may encounter gaps in the recordings if the processor is busy with other things such as creating a spectrogram. In addition, installation and other maintenance take more time. Apart from that, it is definitely a model that you can use. Some advantages are that it is often cheaper and draws a little less power.

The Raspberry Pi 5 is a significantly faster computer, it has an on/off button and if you connect a special battery it also contains a real-time-clock (RTC). Unfortunately, there are a couple of disadvantages that make it less suitable for WURB and WIRC. It is very sensitive to incorrect power supply and USB cables used and it goes into sleep mode if it does not receive enough power. This is especially a problem if you are in the field and do not have sufficient power supply or when they are remotely controlled and you don't easily can just restart them.

But if you use the recommended power adapter from Raspberry Pi or find another acceptable solution, it is a fantastically fast little computer. Also check the section "power supply" below.

Raspberry Pi Zero 2W can maybe be used as long as you do not stress it with anything other than recording audio continuously (don't activate FFT). But there is only one microUSB available for attached accessories.

About Internal memory. Since Raspberry Pi 3 works, the size of the internal memory is not critical, but if you can afford to go up a bit in memory size it is preferable. The size of the internal memory largely determines the price of the model and memory capsules are something that is expensive right now. Personally I mostly use the 4GB versions of Raspberry Pi 4.

Micro SD memory card

In these computers, micro SD cards are normally used instead of a hard drive or SSD disk. The micro SD card is used both for installing the operating system, various software such as WURB and WIRC and for user data. Most relatively fast micro SD cards should work. I usually use SanDisk Extreme PRO microSDXC. The size 64 GB is enough for most applications, even if you save the recorded audio files on the micro SD card. Be careful when buying these because the price can vary greatly.

Power Supply

Raspberry Pi 3B+ and older models use micro-USB for power supply. Raspberry Pi 4 and later use USB-C.

For Raspberry Pi 5, it should be a USB-C PD so that it can automatically figure out how much the power supply can deliver. At least USB-C PD with 5A at 5V is needed.

In cases where you want to supply power from 12V, there are two options. One way is to use a 12V to 5V adapter that you normally use in your car. Another is to get a HAT (Hardware-At-Top) which can often convert an input voltage between 6V and 30V to 5V for the Raspberry Pi.

Another option is PoE, Power-over-Ethernet. One advantage is that you get both network and power in the same cable and these usually work over long distances, such as 50-100m without a repeater. Then you need one PoE injector and one PoE splitter. For the PoE splitter part there are also HAT cards for Raspberry Pi available.

This calculation shows approximately for how long a detector can be powered by a power bank. A good rule of thumb is that a Raspberry Pi draws 5W of power, which is equal to 1A at 5V. A 20000 mAh power bank contains energy according to this calculation 20Ah * 3.6V = 72Wh. So 72Wh / 5W = 14.4 hours.

Ultrasonic microphones

When choosing an ultrasonic microphone, it is good to be aware of Nyquist's theorem. It states that the sampling rate of a microphone should be twice as high as the actual sound you want to record. For European species, a sampling rate of 384 kHz is sufficient, and a sampling rate of 192 kHz can work for a large number of species that rarely have parts of their sound pulses above 90 kHz. If there are real sounds above the Nyquist frequency (half the sampling frequency), an "anti aliasing" filter is needed to handle this. Unfortunately, not all ultrasonic microphones on the market have this type of filter.

The ultrasonic microphones that are automatically identified by the WURB software are those shown in the image. From left they are:

1. Pettersson M500 This is the only 500 kHz that is supported by WURB. The communication protocol is specially designed for this model and it will not be recognized as a normal sound device when connected to other devices.
2. Pettersson M500-384 Acoustically this model is similar to M500, but it is running at 384 kHz and uses the standard sound card protocol for communication.
3. Pettersson u384 and u256 Older versions have micro-USB and newer ones have USB-C.
4. AudioMoth with the microphone firmware installed (https://www.openacousticdevices.info/usb-microphone) is supported.
5. Some models in the Pipistrelle family (https://www.pippyg.com) are supported. For example "Griff" and "Griff Mini".
6. Dodotronic, various models from Dodotronic are directly supported and this is an example of a 384 kHz microphone.
7. Various experimental microphones can also be used as long as they can be recognized as a sound card. In this example an adapter from a 3.5 mm headset connector to USB is used together with an attached MEMS microphone.

Other sound cards with a USB connector can be used, but then you have to add them to the system's configuration file. See the user manuals advanced section for more information.

Optional accessories

Image, from left to right. Two GPS units, a USB memory stick, an extra WiFi dongle and a 4G/LTE modem.

GPS units

The most important metadata for a recorded audio file is when and where. Without this information, the audio file is basically useless in many contexts. WURB therefore supports GPS devices and GPS can deliver both time and position. These values ​​are then included in the filename of each recorded audio file.

This is especially useful when the detector is used for transect inventory or when the detector is deployed at new locations each night. Since Raspberry Pi does not normally have a built-in RTC, Real-time-clock, using a GPS device is an option to automatically set the time as soon as the device has found a sufficient number of satellites.

These USB devices will hopefully be automatically recognized when plugged in. If not, there is an instruction in the user manuals advanced section.

Sometimes it can take a very long time for a GPS device to lock onto the satellites, especially if the devices have not been used for a while. There are two things that can be recommended if this is a problem. Use an USB extender so that the GPS device is not disturbed by signals from the Raspberry Pi or other electronics. The second trick is to leave them on for a few days before using them. It is fine to connect them to a power source of your choice with 5V USB, but note that a power bank may be automatically be turned off as these devices draw very little power.

USB memory devices

In principle, any type of USB memory device can be connected. If it is formatted as FAT32, it should work regardless of whether it is a regular USB memory stick, a hard drive or SSD. Note that there is a limit of 2 TB for FAT32 that can be tricky to exceed.

The software in WURB supports two USB devices and detects when these are full and then switches to the next one. If they are full or not connected, the files will then be stored on the detector's micro SD card.

The audio files from monitored nights always end up in a directory named "wurb_recordings". If WIRC is used, the video files will end up in a corresponding directory named "wirc_recordings".

There is a possibility to customize this via configuration files, more information can be found in the advanced section of the user manual.

Communication

Although a Raspberry Pi is a small computer, there are many possible ways to communicate with it remotely or as a part of your local network.

Both WURB and WIRC run in what is called "headless mode", which means that there is no graphic user interface in the same way as for a desktop/laptop computer. For maintenance you have to use SSH and connect from a terminal window. However, the detector contain a web server and the detectors user interface can be accessed via regular web browsers. This works the same way regardless of whether the detector is locally connected to your own home network, if the detector shares it's own WiFi network as a hotspot, or if it is placed somewhere out in the field with internet access.

WiFi and local network

When installing the operating system via "Raspberry Pi Imager", you have probably entered the name and password for your local WiFi network. This WiFi connection works both when installing the system and when you later want to run your detector.

In the following examples we assume that the hostname of your detector is "wurb01". If you are on your local network, add ".local" after the hostname. Use this command to connect with SSH:

ssh wurb@wurb01.local

The WURB software use port 8080 and WIRC use 8082 by default. Use a web browser to access the user interface for WURB and WIRC:

http://wurb01.local:8080
http://wurb01.local:8082

Note that if you reinstall the system and keep the same hostname, you may have to clear a file containing known hosts. This is part of a security system that is present on most computers. On macOS, this file is located in a hidden folder and is called ".ssh/known_hosts".

Modem cable (Ethernet)

I there are problems with the WiFi solution above, then you can use a modem cable to connect the Rasperry Pi to an internet modem. The rest works as described above.

Note that this alterative can be combined with Power-over-Ethernet (PoE).

Raspberry Pi as a hotspot

Many users use the WiFi unit in the Raspberry Pi to let it share it's own network as a WiFi hotspot. Then it is easy to connect to it from a mobile phone to run the user interface when out in the field.

In this case an IP address should be used. Examples for how to connect with SSH and to the web pages for WURB and WIRC:

ssh wurb@wurb01
http://wurb01:8080
http://wurb01:8082

There is an instruction for how to install the WiFi hotspot in the installation instruction.

Since the Raspberry Pi only have one internal WiFi unit it is not possible to use it to connect to your home network again with WiFi. One practical solution is to add a second WiFi unit as a USB connected one, or use a modem cable when you are back home.

One important thing to notice if you are running the detector as a hotspot without any connection to internet is that time is not set properly. The solution to this is that there is a button in the user interface called "Set time". Another solution is to attach a GPS unit. When it has been running for some time the GPS time will be used to set the detector time.

WiFi for internet access

When the detector is installed as a permanent station at a place where WiFi is available it can be used to remotely connect to it. There is a command line tool with as simple graphical user interface that can be used to configure WiFi connections:

sudo nmtui

It is recommended to use a "guest" network since there is no need for the detector to have access to someones local network.

To remotely connect to the detector with SSH, HTTP and SFTP I normally use Tailscale. More info below. For temporary use and if it is not set up as a hotspot you can share internet from your mobile phone and then use Tailscale to access it from your phone, or any other device with internet connection.

4G/LTE

If there is no WiFi available it is possible to use a 4G/LTE modem. The USB modem Huawei E3372 is often recommended for use with Raspberry Pi and I have used them a lot. It is also possible to use a normal internet modem with 4G/LTE and then connect to that one with a modem cable or WiFi.

Tailscale can also be used when 4G/LTE is used.

Bluetooth

Bluetooth is not used for WURB and WIRC, but it is a part of the Raspberry Pi hardware and can be used in the future.

File Access with SFTP

If the recorded files are stored at an external USB memory device, then that device can be moved to a desktop/laptop computer for post processing. If the files are stored on the micro SD card in the Raspberry Pi then it must be downloaded with SFTP. This is also a possibility if the detector is remotely deployed and connected to internet in some way.

The SFTP client software I mostly use is FileZilla and it is available for Windows, macOS and Linux. For Windows users there is a similar software called WinSCP.

Connect with:

Protocol: SFTP
Host: wurb01     (or "wurb01.local", "10.42.0.1")
User: wurb
Password: your-secret-password

Then the recorded sound files stored on the micro SD card can be found in this directory "/home/wurb/wurb_recordings". If they are stored on the USB memory devices they can be found "/media/USB-sda1/wurb_recordings" or here "/media/USB-sda2/wurb_recordings". In summary, look here for the recorded sound files:

/home/wurb/wurb_recordings
/media/USB-sda1/wurb_recordings
/media/USB-sda2/wurb_recordings

Tailscale, or similar tunneling techniques

The normal way to set up a computer as a server for external access is that you have either a dynamic or static IP address. Then you use, for example, nginx or Caddy to protect your server with logins etc.

This does not work if the detector is connected to internet via someones WiFi or if it is using 4G/LTE for internet access. Reverse SSH tunneling can be used to access a remotely deployed detector, but it is quite tricky to set up manually.

In recent years, I have used Tailscale for this. With the help of Tailscale, I can connect to my detectors with SSH, HTTP and SFTP as soon as they are connected to the internet. It looks like they are all parts in my home network and I can access them like this (example where the detector hostname is wurb01):

SSH: ssh wurb@wurb01
HTTP for WURB: http:wurb01:8080
SFTP: Protocol: SFTP, port: 22, host: wurb01, user: wurb, password: same-as-for-ssh

In some cases DNS for the device hostname does not work, but Tailscale also provide an internal IP number for access. It is also possible to share a detector with some other Tailscale users.

What you have to do is this. Get an account on Tailscale, the free account allows for up to 100 devices. Then download the software to your computer, mobile phone and other units that you want to include in your Tailscale network.

Then install Tailscale on each detector. You can do this by selecting "Add device/Linux server" which creates a script that you then should run on the detector.

Contact

Arnold Andreasson, Sweden.

info@cloudedbats.org