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Friday, April 27, 2018

Practical Sensing - RF

We, as humans, are so used to know who we are and where we are that we sometimes forget that it comes at a cost, a person sees where they are, they have an awareness (or general awareness) of their location in a room or on the street, they can also see a step (or feel if one is blind) and eventually know where obstacles are, get from a place to place and plan routes around obstacles.


What can computers do? 

Radio Sensing

Radio sensing has been invented 1904 and while only capable of detecting a presence it has since evolved into many other sensors.

RF Radar has a few implementations, among them is presence, distance and movement, the principle is the same, an RF wave is transmitted, it is then reflected (or not) and the returned radio waves are detected, in more advanced scenarios, the received signal goes through FFT to detect the reflected timing/phase, which is further processed to get distance and/or speed of one or more objects.

Another implementation of radio sensing is localization, in its simplest form triangulation and more advanced is GPS which uses clocks to indicate when the signal was sent to better localize the receiver. BLE have been used for indoor navigation as well.

A different method of obtaining general location as long as you have network connectivity is using a service such as google geolocate which uses your IP and near by WIFI networks to guess the location.

An attempt was made to discover the hackability of Bosch Radars (diydrones, mikrocontroller) but so far without success.

Another form of electromagnetic sensing is a Geiger counter, which can be used to detect radiation, very useful if you want to detect Radon in your basement or take a relatively safe hike near Chernobyl.

Lastly Radio can be used as a cheap way to find out if a certain device is near another device or even communicate a secret of some sort.


Doppler - Doppler type sensors detect change or movement, one such cheap sensor is the HB100.


Distance - Sensors such as FM24-NP100 ($110) provide distance to the biggest reflection but also spectrum data which can be used to monitor multiple objects. These type of sensors measure the phase difference between two wavelengths.

Presence - Modules like the HW-MS03 (about $2) are in essence a Doppler radar combined with a timer to switch a pin/relay on or off.

Some radar modules (such as CFK024-5A for about $50) have FMCW tuning capabilities, which is very useful if you want to do a sweep which can be used to detect distance of multiple objects but it requires more than basic knowledge.


GPS receivers determine location by triangulating the timestamps and signals received from satellites, the more satellites, the more accurate the location will be. but GPS technology is limited by atmospheric conditions, limiting the accuracy possible.

GPS receivers have advanced over the years, GPS L1 and L2, GloNass, Galileo, Beidou and more, but the accuracy stayed more or less the same at this moment, the peak is around 2.5 meters accuracy for private use.

To overcome the accuracy limitations, a few augmentations were developed, some are over the air such as SBAS and QZSS, some are based on static base stations like DGPS and RTK.

Commercial RTK solutions are provided by drotek and Emlid to name a few.


BLE beacons are low energy (hence LE) devices which transmit a message once in a while, by reading the received power level (RSSI), it is possible to estimate the distance to the beacon. By knowing where the beacons are, it is possible to triangulate (called trilateration) the location of the receiver.

proximi.io (no affiliation) is one of the companies that does that kind of indoor positioning.

Nuclear Radiation

Following Fukushima disaster and Chernobyl tourism, personal radiation detection devices became more and more popular. 

Dosimeters available in Film Badges, MOSFET and Geiger-Muler tubes to name a few.


Wireless identity devices are devices that contain a chip with a small coil, the coil is used to power up the chip and transmit data. two such devices are RFID and NFC devices.

Some devices have static data, some can store custom data, some can encrypt and authenticate but the principle is the same.

RFID more common types are the 125KHz and 13.56MHz and contain a 20 bytes ID and anywhere between 0 and 64 bytes of custom data.

13.56MHz module
125Khz module

One of the more interesting things about RFID is the tag sizes

NFC works much in the same way, memory capacity is between 48 bytes and 32kb.


While radio communication is not a sensor per say it is however a way to communicate and locate and can be used to sense location, state and various data.

Standard modules come in various frequencies and modulations, some even implement protocols, error correction and buffers.

Among the more popular ones are the 315Mhz, 433Mhz, 868Mhz, 915Mhz, depending on the country and local regulations and considered ISM band, which can be used for anything from car remotes, multirotor telemetry and various remote switches.

433Mhz RF transmitter and receiver 

XBEE, Bluetooth and WIFI are also considered ISM band but more robust implementations are available and are mostly used for higher bandwidth application.


Some of the more popular 2.4Ghz modules are the NRF24L01, A7105 and CC2500 which are used in RC Toys.

FrSky Taranis Q X7S

Lora, LoraWan, SigFox and NB-IOT are mostly used for smart appliances such as water meters, power meters and relatively long range and big coverage requirements and are very low bandwidth.

Cell (2G/3G and up) are used nowadays for anything from messaging to video playback.




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