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signal from the wiper goes through a multi-stage RC low-pass filter. The setting of the potentiometer will vary the source impedance seen by the filter and thus slightly alter the corner frequency. Still, since the square wave harmonics start at three times the fundamental frequency, this won’t materially affect its ability to filter them out. The fourth-order filters roll off at 24dB per decade, so the third harmonic will be attenuated by well over 30dB. The filters each have four passive stages, each with the same corner frequency but ten times the impedance of the last, so as to not overly load the previous stage. The output of each set of filters is AC-coupled to an op amp two-times gain stage, DC biased to the 2.5V half-supply rail generated using a pair of 10kW resistors and filtered by 100µF and 100nF capacitors. These stages not only apply gain to make up for signal lost in the filters, but also convert the high-impedance output of the filters into a low imped- WiFi Snooping with a Raspberry Pi Smartphones send out WiFi “probe requests” to see what access points are close by. These requests contain the MAC address of the WiFi module in the smartphone, which is a unique identifier for that phone. The first three bytes of the MAC address contain the OUI (Organisational Unique Identifier), sometimes called the Vendor ID. The IEEE assigns OUIs to vendors. The last three bytes are the unique device serial number assigned by the vendor. A monitoring device can silently capture probe requests, collecting information such as the date and time, MAC address and the signal strength, which indicates how close the phone is. Several devices can be strategically placed to triangulate the signal levels and pinpoint the location of the phone. Shopping centres, train stations, airports etc are already using systems like this. You can use a Raspberry Pi as a silent monitoring device by running a Python program called “probemon”. Probemon captures all the data mentioned above. Also, the probe request sometimes contains the Access Point details that the phone was last connected to. When that happens, it is also captured by probemon. To use this software, you will need a USB WiFi adaptor that supports “monitor” mode (the internal WiFi on the Pi does not). I bought a RaLink RT5370 via eBay for less than $10. First, install Raspbian Buster on the Pi. Then plug in the USB WiFi adaptor and check it with the following command: lsusb Note the WLAN number of the USB WiFi (probably wlan1). Unplug and replug the USB adaptor, and check again siliconchip.com.au to be sure. Then install aircrack-ng: sudo apt-get install aircrack-ng Run airmon-ng (a part of aircrackng) to kill processes that will conflict with Monitor Mode: sudo airmon-ng check kill Put the WiFi adaptor into monitoring mode: sudo airmon-ng start wlan1 Check that you now have a virtual adaptor (wlan1mon): ifconfig Install netaddr, scapy and probemon: mkdir python cd python git clone https://github.com/ drkjam/netaddr cd netaddr sudo python setup.py install cd ~/python git clone https://github.com/ secdev/scapy.git cd scapy sudo python setup.py install cd ~/python git clone https://github.com/ nikharris0/probemon.git cd probemon Now test probemon: sudo python probemon.py –i wlan1mon –f –s –r –l It will take a few minutes before you see any results. You will likely get some errors that need fixing by editing the file “/home/pi/python/probemon/ probemon.py”. If you encounter the error type object ‘datetime.datetime’ has no attribute ‘datetime’, change line 36 of that file from: Australia’s electronics magazine ance signal, suitable for driving other equipment. These signals are again AC-coupled to remove the 2.5V DC bias and then fed to the output terminal pairs via 10W isolating resistors. The remaining six square wave signals are simply fed to a separate set of outputs via 47W isolating resistors. The whole thing is powered from a 5V USB supply, with LED1 lighting to indicate the presence of power. Petre Petrov, Sofia, Bulgaria. ($80) log_time = datetime.datetime. now().isoformat() to: log_time = datetime.now(). isoformat() The RSSI value doesn’t work, so change line 56 from: rssi_val = -(256-ord( packet.notdecoded[-4:-3])) to: rssi_val = packet.dBm_AntSignal Now the list of approved MAC Address Vendors has to be updated: curl http://standards-oui.ieee. org/oui.txt --output /home/ pi/python/netaddr/netaddr/ eui/oui.txt cd ~/python/netaddr/netaddr/eui python ieee.py cd /home/pi/python/netaddr sudo python setup.py install Rerun the capture program, and it should be fully working: cd ~/python/probemon $ sudo python probemon.py –i wlan1mon –f –s –r –l Captured data is stored in probemon.log. iPhones use MAC address randomisation, so the only time the correct MAC address is sent in a packet is when it is connected to a WiFi Access Point. Other times, it is recorded in the log file as “UNKNOWN”. When previously connected SSIDs are captured, you can search www. wigle.net which has a vast number of Access Points in its worldwide database. You can also enter your home address into www.wigle.net and see what Access Points are near you. Sid Lonsdale Cairns, Qld. ($80) February 2021  73